THE 1 ATOMIC NATURE THE ATOMIC OF MATTER NATURE OF...

THE BIG

IDEA ......

.. Atoms are the building blocks of most matter.

THE ATOMICNATURE OF MATTER

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Suppose you break apart a large boul-der with a heavy sledgehammer. You break the boulder into rocks. Then you

break the rocks into stones, and the stones into gravel. You keep going and break the gravel into sand, and the sand into a powder of fine crystals. Each fine crystal is composed of many billions of smaller particles called atoms. Atoms are the building blocks of most matter. Everything you see, hear, taste, feel, or smell in the world around you is made of atoms. Shoes, ships, mice, lead, and people are all made of atoms.17.0

How Do We Know That Atoms Exist?1. Place a drop of water on a microscope slide.

2. Add a drop of whole milk to the water drop using a wire or needle and stir.

3. Place a cover slip on the slide.

4. Insert the slide into a microscope and wait a while for the milk to settle.

5. Focus on fat globules in the milk. Start with a low magnification and then switch to a higher one.

Analyze and Conclude1. Observing What did you see when

you viewed the fat droplets under the microscope?

2. Predicting What would happen if you replaced the fat globules with chalk dust?

3. Making Generalizations What does the motion of the milk fat droplets tell you about the surrounding water?

discover!

1

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THE ATOMIC NATURE OF MATTERObjectives• Describe the connection

between substances and elements. (17.1)

• Give examples that illustrate the small size of atoms. (17.2)

• Compare the ages of atoms to the ages of the materials they compose. (17.3)

• State evidence for the existence of atoms. (17.4)

• Describe molecules. (17.5)

• Describe how compounds are different from their component elements. (17.6)

• Describe the distribution of mass in an atom. (17.7)

• Explain the cause of an atom’s chemical properties. (17.8)

• Identify the four phases of matter. (17.9)

discover!

MATERIALS water, milk, needle, slide, cover slip, microscope

EXPECTED OUTCOME Fat globules will move randomly around.

ANALYZE AND CONCLUDE

The droplets moved around randomly.

The chalk dust would also move around randomly.

There are small particles in the water undergoing random motion, hitting the milk droplets.

TEACHING TIP The random motion of small particles in water is the direct consequence of the motion of neighboring atoms and molecules.

1.

2.

3.

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CHAPTER 17 THE ATOMIC NATURE OF MATTER 325

17.1 ElementsJust as dots of light of only three colors combine to form almost every conceivable color on a television screen, only about 100 distinct kinds of atoms combine to form all the materials we know about. Atoms are the building blocks of matter. A material composed of only one kind of atom is called an element.

To date about 115 elements are known. Of these, about 90 occur in nature. The others are made in the laboratory with high-energy atomic accelerators and nuclear reactors. These laboratory-produced elements are too unstable (radioactive) to occur naturally in appre-ciable amounts.

Every simple, complex, living, or nonliving substance in the known universe is put together from a pantry containing less than 100 elements. More than 99% of the material on Earth is formed from only about a dozen of the elements. The other elements are relatively rare. Living things, for example, are composed primarily of five elements: oxygen (O), carbon (C), hydrogen (H), nitrogen (N), and calcium (Ca). The letters in parentheses represent the chemical symbols for these elements. Table 17.1 lists the 16 most common ele-ments on Earth. Most of these elements, not just the five most com-mon ones, are critical for life.

The lightest element of all is hydrogen. In the universe at large, it is the most abundant element—over 90% of the atoms in the known universe are hydrogen. Helium, the second-lightest element, makes up most of the remaining atoms in the universe, although it is rare on Earth. The heavier, naturally formed atoms that we find around us were manufactured by fusion reactions in the hot, high-pressure cauldrons deep within stars. Elements heavier than iron are formed when huge stars implode and then explode—an event called a super-nova. The heaviest elements are formed when pairs of neutron stars, the super-dense cores of supernovas, collide. Nearly all the atoms on Earth are remnants of stars that exploded long before the solar system came into being.

Just as we don’t own the atoms in our bod-ies, we don’t own energy—we rent it. Much of the energy we receive from the sun is eventually radiated back into space.

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This chapter is the most important chapter in Unit II, and should not be skipped. It provides a good background for the chapters on heat (Chapters 21–24) and also provides the necessary background for Chapters 28, 32, and all of Unit VI.

17.1 Elements

Key Termsatoms, element

Common Misconception Material things are made of thousands of different kinds of atoms.

FACT To date, there are only about 120 known elements.

� Teaching Tip Begin by describing breaking a boulder into rocks, rocks into stones, stones into pebbles, pebbles into gravel, gravel into sand, sand into powder, and so forth until you get to the fundamental building block—the atom. Give examples to convey the idea of the smallness of the atom, i.e., an atom is as many orders of magnitude smaller than a person as an average star is larger than a person. The size of an atom is to the size of an apple as the size of an apple is to the size of Earth. An apple is full of as many atoms as Earth would have apples if it were packed solid with apples.


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How fast do atoms migrate?1. Put some drops of food coloring into a small container of water.

How long does it take for the colored drops to spread to all parts of the water?

2. Repeat, using hot water. What happened to the migration rate of the drops?

3. Think Why did the rate change?

discover!

All of the matter that we encounter in our daily lives, as well as matter in the sun and other stars, is made up of elements. But not all matter in the universe is composed of elements. In the closing years of the twentieth century, astrophysicists found that gravita-tional forces within galaxies were far greater than visible matter could account for. Only in this twenty-first century has it been confirmed that some twenty-three percent of the matter in the universe is com-posed of an unseen dark matter. Astrophysicists believe this dark matter is made up of particles not yet detected, and that much of the rest of the universe is dark energy (briefly mentioned in Chapter 9). Indeed, the nature of dark matter, which gravitationally affects ordi-nary matter, and dark energy, which pushes outward on the expand-ing universe, is the focus of enormous present-day research.

CONCEPTCHECK ...

... What do all substances have in common?

If a typical atom were expanded to a diameter of 3 km, about the size of a medium-sized air-port, the nucleus would be about the size of a basketball. Atoms are mostly empty space.

FIGURE 17.1 �Both Leslie and you are made of stardust—in the sense that the carbon, oxygen, nitrogen, and other atoms that make up your body originated in the deep interior of ancient stars, which have long since exploded.

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The lovely girl in Figure 17.1 is my daughter Leslie when she was 16. She is now a geologist and a teacher.

� Teaching Tip Stress that the fusing of hydrogen nuclei to form nuclei of other elements is very different from the formation of compounds as discussed in Section 17.6. In fusion, the nuclei actually merge and become one, whereas in compound formation, the individual nuclei do not change.

Every simple, complex, living, or

nonliving substance in the known universe is put together from a pantry containing less than 100 elements.

discover!

MATERIALS small container, water, food coloring

EXPECTED OUTCOME Students should observe that the rate of migration is higher in the hotter water.

THINK The molecules in the hot water move faster.

T e a c h i n g R e s o u r c e s

• Reading and Study Workbook

• PresentationEXPRESS

• Interactive Textbook

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17.2 Atoms Are SmallAtoms are so small that there are about 1023 atoms in a gram

of water (a thimbleful). The number 1023 is an enormous number, more than the number of drops of water in all the lakes and rivers of the world. So there are more atoms in a thimbleful of water than there are drops of water in the world’s lakes and rivers. Atoms are so small that there are about as many atoms in the air in your lungs at any moment as there are breathfuls of air in the atmosphere of the whole world.

Atoms are perpetually moving. They also migrate from one loca-tion to another. In solids the rate of migration is low, in liquids it is greater, and in gases migration is greatest. Drops of food coloring in a glass of water soon spread to color the entire glass of water. Likewise, a cupful of toxic material thrown into an ocean spreads around and is eventually found in every part of the world’s oceans. The same is true of materials released into the atmosphere.

It takes about six years for one of your exhaled breaths, such as the one in Figure 17.2, to become evenly mixed in the atmosphere. At that point, every person in the world inhales an average of one of your exhaled atoms in a single breath. And this occurs for eachbreath you exhale! When you take into account the many thousands of breaths that people exhale, at any time—like right now—you have hordes of atoms in your lungs that were once in the lungs of every person who ever lived. We are literally breathing one another’s breaths.

Atoms are too small to be seen—at least with visible light. You could connect an array of optical microscopes atop one another and never “see” an atom. This is because light is made up of waves, and atoms are smaller than the wavelengths of visible light. The size of a particle visible under the highest magnification must be larger than the wavelengths of visible light. This is better understood by an analogy with water waves. A ship is much larger than the water waves that roll on by it. As Figure 17.3 shows, water waves can reveal fea-tures of the ship. They diffract as they pass the ship. In contrast, dif-fraction is nil for waves that pass the anchor chain, revealing little or nothing about it. Similarly, waves of visible light are too coarse com-pared with the size of an atom to show details of the atom’s size and shape. Atoms are incredibly small. (More about this in Chapter 31.)

CONCEPTCHECK ...

... How small are atoms?

FIGURE 17.3 �Information about the ship is revealed by passing waves, because the dis-tance between wave crests is small compared with the size of the ship. The passing waves reveal nothing about the chain.

Does your brain contain atoms that were once part of Albert Einstein? Explain.Answer: 17.2

think!

FIGURE 17.2 �There are as many atoms in a normal breath of air as there are breathfuls of air in the atmosphere of the world.

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17.2 Atoms Are Small

� Teaching Tip On the board, write a 1 followed by 23 zeros to show the meaning of 1023. State that the number of molecules in a liter of air is about 1023. The number of liters of air in the atmosphere is about 1023. Hence, evenly mixed, after a single breathful of air spreads over the atmosphere (about 6 years), on average, one molecule of that breath will be in each liter of air in the atmosphere.

� Teaching Tip State that if you put a drop of ink in a bathtub full of water, you can very soon sample any part of the water and find ink in it. The atoms of ink spread out. There are more atoms in a thimbleful of ink than there are thimblesful of water in all the lakes and rivers of the world. That means if you throw a thimbleful of ink into one of the Great Lakes, eventually it will mix, and then if you dip anywhere in the lake with a thimble, you’ll have many atoms of ink in your sample.

� Teaching Tip Point out that atoms are so tiny that you inhale billions of trillions with each breath. Assuming that most of the atoms previously breathed by people are still part of the atmosphere, then you inhale billions of atoms exhaled by nearly every person who ever breathed! So in this sense we are all one!

Atoms are so small that there are about

1023 atoms in a gram of water (a thimbleful).


• Laboratory Manual 47

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The jittery motion of a huge balloon in the midst of a soccer field filled with jostling people would look like Brownian motion from a high-flying aircraft. The people may be too small to see, but not the larger balloon.

17.3 Atoms Are RecyclableAtoms are ageless and are much older than the materials they com-pose. Some atoms are nearly as old as the universe itself. Most atoms that make up our world are at least as old as the sun and Earth.

Atoms in your body have been around since long before the solar system came into existence, more than 4.6 billion years ago.They cycle and recycle among innumerable forms, both living and nonliving. Every time you breathe, for example, only some of the atoms that you inhale are exhaled in your next breath. The remain-ing atoms are taken into your body to become part of you, and most leave your body sooner or later—to become part of everything else.

Strictly speaking, you don’t “own” the atoms that make up your body—you’re simply their present caretaker. There will be many oth-ers who later will care for the atoms that presently compose you. We all share from the same atom pool, as atoms migrate around, within, and throughout us. So some of the atoms in the ear you scratch today may have been part of your neighbor’s breath yesterday!

Most people know we are all made of the same kinds of atoms. But what most people don’t know is that we are made of the sameatoms—atoms that cycle from person to person and creature to crea-ture as we breathe and perspire.

CONCEPTCHECK ...

... For how long have the atoms in your body been around?

17.4 Evidence for AtomsThe idea that matter is made of atoms goes back to the Greeks in the 400s B.C. It was revived in the early 1800s by an English meteorolo-gist and school teacher, John Dalton. He explained the nature of chemical reactions by proposing that all matter is made of atoms, but he had no direct evidence for their existence. The first fairly direct evidence for the existence of atoms was unknowingly dis-covered in 1827. A Scottish botanist, Robert Brown, was looking through a microscope to study pollen grains floating in water. He noticed that the grains were in a constant state of agitation, always jiggling about. At first, Brown thought that the grains were some sort of moving life forms. Later, he found that inanimate dust par-ticles and grains of soot floating in water also showed this kind of motion. Brownian motion is the perpetual jiggling of particles that are just large enough to be seen.

think!World population grows each year. Does this mean the mass of Earth increases each year? Explain.Answer: 17.3

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17.3 Atoms Are Recyclable

Common Misconceptions The atoms that make up a newborn baby were made in the mother’s womb.

FACT The atoms that make up a developing baby in the mother’s womb are supplied by the food the mother eats. These atoms were formed in stars that have long since exploded.

The age of atoms in a baby is less than the age of atoms in an old person.

FACT The atoms that make up a baby are the same age as those that make up everybody, no matter what their age.

Atoms in your body have been around

since long before the solar system came into existence, more than 4.6 billion years ago.





• Conceptual Physics Alive! DVDs Atoms

17.4 Evidence for Atoms

Key TermBrownian motion

Demonstrate Brownian motion by putting a tiny amount of lycopodium in some water and have students observe a drop under a microscope.

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Atoms are in a state of perpetual motion—moving all the time.

Brownian motion is evidence that atoms exist, as it results from the motion of neighboring atoms and molecules. They bump into the larger particles we can see. More direct evidence for the existence of atoms is available today. An image of individual atoms is shown in Figure 17.4. The image was made not with visible light but with an electron beam. A familiar example of an electron beam is the one that sprays the picture on some television screens. Although an electron beam is a stream of tiny particles (electrons), it has wave properties, with a wavelength more than a thousand times smaller than the wavelength of visible light. With such a beam, atomic detail can be seen. The historic (1970) image in Figure 17.4 was taken with a very thin electron beam in a scanning electron microscope. It is the first such image of clearly distinguishable atoms.

In the mid-1980s, researchers developed a different kind of micro-scope—the scanning tunneling microscope, small enough to be held in your hand. In Figure 17.5, you can see individual atoms. Even greater detail is possible with newer types of imaging devices that are presently revolutionizing microscopy.

We can’t see inside atoms, but images with today’s devices help us to construct better models of the atom. From these models we can make predictions about unseen portions of the natural world.

CONCEPTCHECK ...

... How does Brownian motion provide evidence for the existence of atoms?

� FIGURE 17.5A scanning tunneling microscope created this image of uranium atoms.

FIGURE 17.4 �The strings of dots are chains of thorium atoms imaged with a scanning electron microscope by researchers at the University of Chicago’s Enrico Fermi Institute.

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� Teaching Tip Tell students that the first real “proof” for atoms was given by Einstein in 1905, the same year he published his paper on relativity. He calculated what kind of motion there ought to be in Brownian motion, based on ideas such as energy and momentum conservation, and the idea of heat as atomic motion.

� Teaching Tip Ask what an atom would “look like” if viewed through a vertical bank of about 40 high-powered optical microscopes stacked one atop the other. Then explain that atoms don’t look like anything—they have no appearance in the range of frequencies we call light.

� Teaching Tip Discuss Figures 17.4 and 17.5. These techniques have opened new doors in fields such as medicine and biology. The positions of atoms in complex molecules are no longer guesswork.

Brownian motion is evidence that atoms

exist, as it results from the motion of neighboring atoms and molecules. They bump into the larger particles we can see.





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17.5 MoleculesAtoms can combine to form larger particles called molecules.A molecule is the smallest particle of a substance consisting of two or more atoms that bond together by sharing electrons. Molecules can be made up of atoms of the same element or of different elements. For example, two atoms of hydrogen (H) combine with a single atom of oxygen (O) to form a water molecule (H

2O). The gases

nitrogen and oxygen, which make up most of the atmosphere, are both made of simple two-atom molecules (N

2 and O

2). In contrast,

the double helix of deoxyribonucleic acid (DNA), the blueprint of life, is composed of millions of atoms.

Matter that is a gas or liquid at room temperature is usually made of molecules. Matter made of molecules may contain all the same kind of molecule, or it may be a mixture of different kinds of mol-ecules, as shown in Figure 17.6. Purified water contains almost entirely H

2O molecules, whereas clean air contains molecules belong-

ing to several different substances.But not all matter is made of molecules. Metals and crystalline

minerals (including common table salt) are made of atoms that are not joined in molecules.

Like atoms, individual molecules are too small to be seen with optical microscopes.17.5 More direct evidence of tiny molecules is seen in electron microscope photographs. The photograph in Figure 17.7 is of virus molecules, each composed of thousands of atoms. These giant molecules are visible with a short-wavelength electron beam, but are still too small to be seen with visible light.

We are able to detect some molecules through our sense of smell. Noxious gases such as sulfur dioxide, ammonia, and ether are clearly sensed by the organs in our nose. The smell of perfume is the result of molecules that rapidly evaporate from the liquid and jostle around completely haphazardly in the air until some of them accidentally get close enough to our noses to be inhaled. The perfume molecules are certainly not attracted to our noses! They wander aimlessly in all directions from the liquid perfume to become a small fraction of the randomly jostling molecules in the air.

CONCEPTCHECK ...

... What are molecules made of?

FIGURE 17.6 �

Models of the simple mol-ecules O2 (oxygen gas), NH3

(ammonia), and CH4 (meth-ane) show their structure. The atoms that compose a molecule are not just mixed together, but are bonded in a well-defined way.

FIGURE 17.7 �A scientist used an electron microscope to take this photograph of rubella virus molecules. The white dots are the virus erupting on the surface of an infected cell.

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17.5 Molecules

Key Termmolecule

� Teaching Tip Distinguish between atoms and molecules. There is a limited number of different atoms, but there are innumerable different molecules—and more are being discovered and constructed.

Ask How many elements are in a water molecule? Two: hydrogen and oxygen How many atoms are in a water molecule? Three: two of hydrogen and one of oxygen

� Teaching Tip Point out that whereas an individual atom cannot be seen by the naked eye, some molecules can. One such molecule, called a macro-molecule, is a diamond. A diamond is actually one big carbon molecule!

Open a bottle of strong perfume in one corner of the room. Ask students to raise their hands as soon as they smell it. This will give your class a good indication of the speed of dispersion of the perfume molecules.

Molecules can be made up of atoms of

the same element or of different elements.



• Laboratory Manual 48



DemonstrationDemonstration

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17.6 CompoundsA compound is a substance that is made of atoms of different ele-ments combined in a fixed proportion. The chemical formula of the compound tells the proportions of each kind of atom. For example, in the gas carbon dioxide, the formula CO

2 indicates that for every car-

bon (C) atom there are two oxygen (O) atoms. Water, table salt, and carbon dioxide are all compounds. Air, wood, and salty water are not compounds, because the proportions of their atoms vary.

A compound may or may not be made of molecules. Water and carbon dioxide are made of molecules. On the other hand, table salt (NaCl) is made of different kinds of atoms arranged in a regular pat-tern. (We’ll soon see that the atoms in NaCl are actually ions.) Every chlorine atom is surrounded by six sodium atoms, as shown in Figure 17.8. In turn, every sodium atom is surrounded by six chlorine atoms. As a whole, there is one sodium atom for each chlorine atom, but there are no separate groups that can be labeled molecules.

Compounds have properties different from those of the ele-ments of which they are made. At ordinary temperatures, water is a liquid, whereas hydrogen and oxygen are both gases. Table salt is an edible solid, whereas chlorine is a poisonous gas.

CONCEPTCHECK ...

... How are compounds different from their component elements?

17.7 The Atomic NucleusAn atom is mostly empty space. Almost all of an atom’s mass is packed into the dense central region called the nucleus. The New Zealander physicist Ernest Rutherford discovered this in 1911 in his now-famous gold foil experiment. Rutherford’s group shot a beam of charged particles (alpha particles) from a radioactive source through a thin gold foil. They measured the angles at which the particles were deflected from their straight-line paths when they emerged. This was accomplished by noting spots of light on a zinc-sulfide screen that nearly surrounded the gold foil as shown in Figure 17.9.

� FIGURE 17.9 The occasional large-angle scattering of alpha particles from the gold atoms led Rutherford to the discovery of the small, very massive nuclei at their centers.

FIGURE 17.8 �Table salt (NaCl) is a com-pound that is not made of molecules. The sodium and chlorine ions are arranged in a repeating pattern. Each ion is surrounded by six ions of the other kind.

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17.6 Compounds

Key Terms compound, chemical formula

Compounds have properties different

from those of the elements of which they are made.





17.7 The Atomic Nucleus

Key Termsnucleus, nucleons, neutrons, protons, isotopes, atomic number

� Teaching Tip Stress the emptiness of the atom and lead into the idea of solid matter being mostly empty space. State that our bodies are 99.999% empty spaces, and that a particle, if tiny and not affected by electrical forces, could be shot straight through us without even making a hole! Neutrons do just that. From a beam of neutrons, a few may make bull’s-eye collisions with some of our atomic nuclei—this can do damage, so we wouldn’t want to really do this. However, all but a minute fraction in a beam of neutrons would pass unhindered through the body.

� Teaching Tip Discuss Rutherford’s discovery of the nucleus (Figure 17.9), the Bohr model of the atom (Figure 17.10), and the electrical role of the nucleus and surrounding electrons.

CONCEPTCHECK ...

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Most particles continued in a more or less straight-line path through the thin foil. But, surprisingly, some particles were widely deflected. Some were even scattered back almost along their incoming path. It was as surprising, Rutherford said, as firing a 15-inch artillery shell at a piece of tissue paper and having it come back and hit you.

Rutherford reasoned that within the atom there had to be a positively charged object with two special properties. It had to be very small compared with the size of the atom, and it had to be mas-sive enough to resist being shoved aside by heavy alpha particles. Rutherford had discovered the atomic nucleus.

The mass of an atom is primarily concentrated in the nucleus. However, the nucleus occupies less than a trillionth of the volume of an atom. Atomic nuclei (plural of nucleus) are extremely compact and extremely dense. If bare atomic nuclei could be packed against one another into a lump 1 cm in diameter (about the size of a small grape), the lump would weigh about a billion tons!

Huge electrical forces of repulsion prevent such close packing of atomic nuclei because each nucleus is electrically charged and repels the other nuclei. Only under special circumstances are the nuclei of two or more atoms squashed into contact. When this happens, the violent reaction known as nuclear fusion takes place. Fusion occurs in the core of stars and in a hydrogen bomb.

Nucleons The principal building blocks of the nucleus are nucleons. 17.7 Nucleons in an electrically neutral state are neutrons. Nucleons in an electrically charged state are protons. All neutrons are identical; they are copies of one another. Similarly, all protons are identical. Atoms of various elements differ from one another by their numbers of protons. Atoms with the same number of protons all belong to the same element.

For:Visit:Web Code: –

Links on atoms www.SciLinks.org csn 1707

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Atoms were a philo-sophical concept with ancient Greeks and became a scientific con-cept with the experi-ments of the chemist John Dalton in the early 1800s. Atoms weren’t fully validated until the work of Albert Einstein in the early 1900s.

Water What’s in a glass of water? Tap water is far from being pure H2O,for it contains dissolved compounds of metals such as iron, potassium, and magnesium; dissolved gases such as oxygen and nitrogen; trace amounts of heavy metals and organic compounds; and other chemical compounds such as calcium fluoride and chlorine disinfectants. Now don’t panic and go thirsty. You probably wouldn’t like the taste of pure water, for some dissolved substances give water a pleasing taste and promote good health. As much as 10% of our daily requirement of iron, potassium, calciuum, and magnesium is obtained from ordinary drinking water. Bottoms up!

Link to CHEMISTRY

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� Teaching Tip Discuss the role of electrical forces in preventing us from oozing into our chairs and so forth. Stand up on your table. Ask your students to imagine that it is a large magnet, and that you wear magnetic shoes that are repelled by the table you “stand” on. Then state that on the submicroscopic scale that this is indeed what happens when you walk on any solid surface. Only the repelling force isn’t magnetic, it’s electric! Discuss the submicroscopic notion of things touching. Acknowledge that under very special circumstances the nucleus of one atom can actually touch the nucleus of another atom—that this is what happens in a thermonuclear reaction.

Ask True or False? There exists a large air gap between the nucleus of an atom and the orbiting electrons. False, there is a void, but it is not an air gap. Air is far from being a void, and is a substance that consists principally of nitrogen and oxygen molecules—much too big to fit between a nucleus and its orbiting electrons.



Chemist Many of the products you use every day—from shampoo to vitamins to some foods—were developed by chemists. A chemist uses an understanding of atoms and elements to isolate and identify unknown chemicals, synthesize chemicals in the laboratory, and perform tests to maintain quality standards in industrial processes. Chemists work for government and university laboratories as well as for research departments of private corporations. Most chemists hold undergraduate degrees in chemistry along with advanced degrees in more specific fields such as biochemistry, nuclear chemistry, or analytical chemistry.

Isotopes For a given element, however, the number of neutrons will vary. Atoms of the same element having different numbers of neutrons are called isotopes of that element. The nucleus of the common hydrogen atom has a single proton. When this proton is accompanied by a neutron, we have deuterium, an isotope of hydro-gen. When two neutrons are in a hydrogen nucleus, we have the iso-tope tritium. Every element has a variety of isotopes. Lighter elements usually have an equal number of protons and neutrons, and heavier elements usually have more neutrons than protons.

Atomic Number Atoms are classified by their atomic number, which is the number of protons in the nucleus. The nucleus of a hydrogen atom has one proton, so its atomic number is 1. Helium has two protons, so its atomic number is 2. Lithium has three pro-tons, so its atomic number is 3, and so on, in sequence up to the heaviest elements.

Electric Charge Electric charge comes in two kinds, positive and negative. Protons in the atom’s nucleus are positive, and electrons orbiting the nucleus are negative. Positive and negative refer to a basic property of matter—electric charge. (Much more about electric charge in Unit V.) Like kinds of charge repel one another and unlike kinds attract one another. Protons repel protons but attract electrons. Electrons repel electrons but attract protons. Inside the nucleus, protons are held to one another by a strong nuclear force. This force is extremely intense but acts only across tiny distances. (More about the strong nuclear force in Chapter 39.)

CONCEPTCHECK ...

... Where is the mass of an atom primarily concen-trated?

How long would it take to count to one million (106)? If each count takes one second, counting nonstop to a million would take 11.6 days. Counting to a bil-lion (109) would take 31.7 years. Counting to a trillion (1012) would take 31,700 years, and to a trillion trillion (1024)would take about 2 mil-lion times the estimated age of the universe.

Physics on the Job

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� Teaching Tip Tell students that each of the common elements carbon, oxygen, and nitrogen has two or three isotopes that are found in nature. The carbon-14 isotope is used in radioactive dating, as will be described in Chapter 39.

� Teaching Tip Van der Waals force (the relatively weak attractive force between non-polar molecules) mainly accounts for the adhesion of the many ridges in the feet of a gecko.

The mass of an atom is primarily

concentrated in the nucleus.



• Transparency 26



• Next-Time Questions 17-1, 17-2

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17.8 Electrons in the AtomElectrons that orbit the atomic nucleus are identical to the electrons that flow in the wires of electric circuits. They are negatively charged subatomic particles. The electron’s mass is less than

11800 the mass of a

proton or neutron, so electrons do not significantly contribute to the atom’s overall mass.

In an electrically neutral atom, such as the one shown in Figure 17.10, the number of negatively charged electrons always equals the number of positively charged protons in the nucleus. When the num-ber of electrons in an atom differs from the number of protons, the atom is no longer neutral and has a net charge. An atom with a net charge is an ion.

Attraction between a proton and an electron can cause a bondbetween atoms to form a molecule. For example, two atoms can be held together by the sharing of electrons (a covalent bond). Atoms also stick to each other when ions of opposite charge are formed, and these ions are held together by simple electric forces (an ionic bond).

Just like our solar system, the atom is mostly empty space. The nucleus and surrounding electrons occupy only a tiny fraction of the atomic volume. Yet the electrons, because of their wave nature, form a kind of cloud around the nucleus. Compressing this electron cloud takes great energy and means that when two atoms come close together, they repel each other. If it were not for this repulsive force between atoms, solid matter would be much more dense than it is.

FIGURE 17.10 �The classic model of the atom consists of a tiny nucleus surrounded by orbiting electrons.

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17.8 Electrons in the Atom

Key Terms ion, shell model of the atom, periodic table

� Teaching Tip Schematically show the hydrogen atom, and add a proton and neutrons to build a helium atom, then a lithium atom, and so on. Discuss atomic number, and the role that the number of protons plays in the nucleus in dictating the surrounding electron configuration.

� Teaching Tip Call attention to and briefly discuss the periodic table on page 336. Point out that the atomic configurations depicted in Figure 17.11 are simply models. Models are not complete or accurate. For example, if the nuclei were drawn to scale they would be scarcely visible specks. And the electrons don’t actually orbit like planets as the drawings suggest—such terms don’t have much meaning at the atomic level.


11/13/07 10:21:32 AM


We and the solid floor upon which we stand are mostly empty space, because the atoms making up these and all materials are them-selves mostly empty space. But we don’t fall through the floor. The forces of repulsion keep atoms from caving in on one another under pressure.

Scientists use a model to explain how atoms of different elements interact to form compounds. In the shell model of the atom, elec-trons are pictured as orbiting in spherical shells around the nucleus, as shown in Figure 17.11. There are seven different shells, and each shell has its own capacity for electrons. The arrangement of elec-trons in the shells around the atomic nucleus dictates the atom’s chemical properties. These properties include melting and freezing temperatures, electrical conductivity, and the taste, texture, appear-ance, and color of substances. The arrangement of electrons quite literally gives life and color to the world.

The periodic table is a chart that lists atoms by their atomic number and by their electron arrangements as shown in Figure 17.12 on the next page. As you read across from left to right, each element has one more proton and electron than the preceding element. As you go down, each element has one more shell filled to its capacity than the element above.

Elements in the same column have similar chemical properties, reacting with other elements in similar ways to form new compounds and materials. Elements in the same column are said to belong to the same group or family of elements. Elements of the same group have similar chemical properties because their outermost electrons are arranged in a similar fashion.

CONCEPTCHECK ...

... What does the arrangement of electrons around the nucleus determine?

The periodic table is a chemist’s road map.

FIGURE 17.11 �The shell model of the atom pictures the electrons orbit-ing in concentric, spherical shells around the nucleus.

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335

Electrons don’t exactly orbit because we lose the distinction between particle and wave. Can we say they “swarm” or “smear”? For your students that continue studying physics, they will later be up against the concept that something can be both a particle and a wave.

� Teaching Tip Tell students that if the nucleus of the atom were a peanut on second base in Yankee Stadium, the electron cloud would extend 200 m in every direction, encompassing the whole stadium and reaching as high as a 50-story building.

The arrangement of electrons in the shells

around the atomic nucleus dictates the atom’s chemical properties.

CONCEPTCHECK ...

...CONCEPTCHECK ...

...


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0324_CP09_SE_CH17.indd 336 1/4/08 2:30:58 PM

336

� Teaching Tip Emphasize the “orderliness” of all the elements. There is a pattern and relationship between each element and those surrounding it in the periodic table.

� Teaching Tip State that the configuration of electrons and their interactions with each other are basically what chemistry is about.



• Concept-Development Practice Book 17-1

• Transparencies 27, 28





17.9 The Phases of MatterMatter exists in four phases: solid, liquid, gaseous, and plasma.

In the plasma phase, matter consists of positive ions and free electrons. These charged particles make plasma a great conductor of electricity. The plasma phase exists only at high temperatures. Although plasma is less common to our everyday experience, it is the predominant phase of matter in the universe. The sun and other stars as well as much of the intergalactic matter are in the plasma phase. Closer to home, the glowing gas in a fluorescent tube is a plasma, as are the gases of the aurora borealis, shown in Figure 17.13.

In all phases of matter, the atoms are constantly in motion. In the solid phase, the atoms and molecules vibrate about fixed positions. If the rate of molecular vibration is increased enough, molecules will shake apart and wander throughout the material, jostling in nonfixed positions. The shape of the material is no longer fixed but takes the shape of its container. This is the liquid phase. If more energy is put into the material so that the molecules move about at even greater rates, they may break away from one another and become a gas.

All substances can be transformed from one phase to another. We often observe this changing of phase in the compound H

2O. When

solid, it is ice. If we heat the ice, the increased molecular motion jiggles the molecules out of their fixed positions, and we have liquid water. If we heat the water, we can reach a stage where the continued increase in molecular motion results in a separation between water molecules, and we have steam. Continued heating causes the mol-ecules to separate into atoms. If we heat these to temperatures exceeding 2000°C, the atoms themselves will be shaken apart, making a gas of ions and free electrons. Then we have a plasma.

CONCEPTCHECK ...

... What are the four phases of matter?

Watch for superheated plasma torches that create more electricity than they consume as they incinerate trash, making today’s landfills history.

� FIGURE 17.13The aurora borealis is light given off by glowing plasma. High-altitude gases in the northern sky are transformed into glowing plasmas by the bombard-ment of charged particles from the sun. Less spec-tacular plasmas are found in glowing fluorescent tubes and advertising signs.

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17.9 The Phases of Matter

Key Term plasma

� Teaching Tip Briefly discuss the phases of matter, and how different molecular speeds account for the solid, liquid, gaseous, and plasma phases.

In Chapter 23, students will learn how matter changes from one phase to another.

Matter exists in four phases: solid, liquid,

gaseous, and plasma.





CONCEPTCHECK ...

...CONCEPTCHECK ...

...


For:Visit:Web Code: –

Self-Assessment PHSchool.com csa 1700

338

Concept Summary ••••••

• Every simple, complex, living, or nonliv-ing substance is put together from a pantry containing less than 100 elements.

• Atoms are so small that there are about 1023

in a gram of water (a thimbleful).

• Atoms in your body have been around since long before the solar system came into exis-tence, more than 4.6 billion years ago.

• Brownian motion is evidence that atoms exist, as it results from the motion of neigh-boring atoms and molecules. They bump into the larger particles we can see.

• Molecules can be made up of atoms of the same elements or of different elements.

• Compounds have properties different from those of the elements of which they are made.

• The mass of an atom is primarily concen-trated in the nucleus.

• The arrangement of electrons in the shells around the atomic nucleus dictates the atom’s chemical properties.

• Matter exists in four phases: solid, liquid, gaseous, and plasma.

atoms (p. 325)

element (p. 325)

Brownian motion (p. 328)

molecule (p. 330)

compound (p. 331)

chemical formula (p. 331)

nucleus (p. 331)

nucleons (p. 332)

neutrons (p. 332)

protons (p. 332)

isotopes (p. 333)

atomic number (p. 333)

ion (p. 334)

shell model of the atom (p. 335)

periodic table (p. 335)

plasma (p. 337)

17.2 Yes, and of physicist Richard Feynman too. However, these atoms are combined differ-ently than they were before. The next time you feel insignificant, take comfort in the thought that many of the atoms that compose you will be part of the bodies of all the people on Earth who are yet to be! In this sense, our atoms at least, are immortal.

17.3 The mass of Earth does increase by the addi-tion of roughly 40,000 tons of interplanetary dust each year. But the increasing number of people does not increase the mass of the Earth. The atoms that make up our body are the same atoms that were here before we were born. The atoms that make up a baby forming in the mother’s womb must be sup-plied by the food she eats. And those atoms were formed in the stars that have long since exploded.

think! Answers

1 REVIEW

Key Terms ••••••

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REVIEW


• TeacherEXPRESS

• Virtual Physics Lab 18

• Conceptual Physics Alive! DVDs Atoms


11/13/07 10:21:44 AM


Check Concepts ••••••

Section 17.1 1. Approximately how many elements are

known today?

2. Which element has the lightest atoms?

Section 17.2 3. How does the approximate number of

atoms in the air in your lungs compare with the number of breaths of air in the atmosphere of the whole world?

4. From where did the heaviest elements origi-nate?

5. How do the sizes of atoms compare with the wavelengths of visible light?

Section 17.3 6. How does the age of most atoms compare

with the age of the solar system?

7. What is meant by the statement that you don’t “own” the atoms that make up your body?

Section 17.4 8. What causes dust particles to move with

Brownian motion?

9. Individual atoms cannot be seen with visible light; yet there is an image of indi-vidual atoms in Figure 17.4. Explain.

10. What is the purpose of a model in science?

Section 17.5

11. Distinguish between an atom and a molecule.

12. a. How many elements compose pure water? b. How many individual atoms are there in a

water molecule?

13. a. Cite an example of a substance that is made of molecules.

b. Cite a substance that is made of atoms rather than molecules.

14. True or false: We smell things because certain molecules are attracted to our noses.

Section 17.6 15. a. What is a compound? b. Cite the chemical formulas for at least

three compounds.

ASSESS1

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ASSESS

Check Concepts 1. About 100

2. Hydrogen

3. About the same

4. From exploding stars (supernovae)

5. Atoms are much smaller.

6. Most atoms are older.

7. Atoms migrate and spend little time within us.

8. They are bombarded by moving molecules.

9. The image was made using electrons, not light.

10. To make predictions

11. A molecule is a specific combination of atoms.

12. a. Two; hydrogen and oxygen b. Three; two hydrogen and one oxygen

13. a. Water (Check students’ work for other examples.) b. Metals

14. False; there is no attraction.

15. a. A substance made of two or more elements chemically combined in a fixed proportion

b. Some examples: CO2, H2O, NaCl (Check students’ work for more examples.)


ASSESS (continued)

340

Section 17.7 16. What did Rutherford discover when his

group bombarded a thin foil of gold with subatomic particles?

17. How does the mass of an atomic nucleus compare with the mass of the whole atom?

18. How does the size of an atomic nucleus compare with the size of the whole atom?

19. What are the two kinds of nucleons?

20. a. What is an isotope? b. Give two examples of isotopes.

21. How does the atomic number of an element compare with the number of protons in its nucleus?

22. How does the atomic number of an element compare with the number of electrons that normally surround the nucleus?

Section 17.8

23. How does the mass of an electron compare with the mass of a nucleon?

24. a. What is an ion? b. Give two examples of ions.

25. At the atomic level, a solid block of iron is mostly empty space. Explain.

26. What is the periodic table of the elements?

27. What does the atomic number of an ele-ment tell you about the element?

28. According to the shell model of the atom, how many electron shells are there in the hydrogen atom? The lithium atom? The aluminum atom?

Section 17.9

29. What are the four phases of matter?

30. In terms of electrical conduction, how does a plasma differ from a gas?

31. How many types of atoms can you expect to find in a pure sample of any element?

32. How many individual atoms are in a water molecule?

Think and Explain ••••••

33. Which of these formulas represent pure ele-ments? H

2, H

2O, He, Na, NaCl, Au, U

34. Which are older, the atoms in the body of an elderly person, or those in a baby?

35. A cat strolls across your backyard. An hour later, a dog with his nose to the ground fol-lows the trail of the cat. Explain this occur-rence from a molecular point of view.

36. Suppose you smell the shaving lotion your brother is wearing almost immediately after he walks into the room. From an atomic point of view, exactly what is happening?

37. If no molecules in a body could escape, would the body have any odor?

1

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16. Scattering that suggested the existence of the nucleus

17. Nearly all the atom’s mass is in the nucleus.

18. The nucleus is extremely small compared to the atom.

19. Protons and neutrons

20. a. An atom with a specific number of neutrons in the nucleus

b. carbon-14, carbon-12

21. Same

22. Same

23. Electron’s mass is about 1/1800 of the nucleon’s mass.

24. a. Atom that has lost or gained one or more electrons b. Na1, Cl2 (Check students’ work for more examples.)

25. Atoms are themselves mostly empty space.

26. A chart showing the elements organized according to the properties of their atoms

27. Tells you the number of protons in its nucleus and its place in the table

28. 1; 2; 3

29. Solid, liquid, gas, plasma

30. Plasma conducts electricity, gases normally don’t.

31. One

32. Three: two hydrogens, one oxygen

Think and Explain 33. H2, He, Na, Au, and U are

pure elements. H2O and NaCl are compounds.

34. No substantial difference

35. Cat leaves a trail of molecules that mix with air and enter nose.

36. Molecules evaporate from shaving lotion and mix with molecules of the air; some get into the nose.

37. No. A body has odor only if it sheds some of its molecules.


11/13/07 10:21:48 AM


ASSESS (continued)1


44. If two protons and two neutrons are re-moved from the nucleus of an oxygen atom, what nucleus remains?

45. What element results if you add a pair of protons to the nucleus of mercury? (See periodic table.)

46. What element results if one of the neutrons in a nitrogen nucleus is converted by radio-active decay into a proton?

47. What element will result if two protons and two neutrons are ejected from a uranium nucleus?

48. In what way does the number of protons in an atomic nucleus dictate the chemical properties of the element?

49. What element results if two protons and two neutrons are ejected from a radium nucleus?

50. You could swallow a capsule of the element germanium without harm. But if a proton were added to each of the germanium nuclei, you would not want to swallow the capsule. Why?

38. A kitten will add several kilograms to its mass as it grows into a full-sized cat. From where do the atoms that make up this added mass originate?

39. Where were the atoms that make up a new-born baby manufactured?

40. Although you can’t see an atom through a microscope, at some point a clump of atoms is large enough to see as a “dot” through a microscope. What determines when the clump of atoms is big enough to be seen?

41. Why is Brownian motion apparent only for microscopic particles?

42. Atoms are mostly empty space, and struc-tures such as a floor are composed of atoms and are therefore also mostly empty space. Why don’t you fall through the floor?

43. What element will result if a proton is added to the nucleus of carbon? (See periodic table.)

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38. They come from the food that the cat eats and air it breathes.

39. In ancient stars that long ago exploded

40. Clump must be as large as wavelength of light used to view it.

41. Brownian motion depends on more bumps on one side of a particle than on the other. The bigger the particle, the more the bumps even out.

42. There are electrical repulsion forces between atoms in the floor surface and our surface. Whirling electrons “fill up” the space within an atom.

43. Nitrogen (7 protons)

44. Beryllium

45. Lead

46. Oxygen

47. Thorium (90 protons)

48. It dictates the number of electrons about the nucleus.

49. Radon

50. It is arsenic (33 protons).


ASSESS (continued)1

342

51. A particular atom contains 29 electrons, 34 neutrons, and 29 protons. What is the identity of this element and what is its atomic number?

52. The atomic masses of two isotopes of cobalt are 59 and 60.

a. What is the number of protons and neutrons in each?

b. What is the number of orbiting electrons in each when the isotopes are electrically neutral?

53. When an atom loses an electron and be-comes a positively charged ion, how signifi-cant is the change in the atom’s mass?

54. One isotope of lead has 82 protons and 124 neutrons in its nucleus. What can you say about the number of protons in the nucleus of any other isotope of lead?

55. Which contributes more to an atom’s mass—electrons or protons? Which contrib-utes more to an atom’s size?

56. An ozone molecule and an oxygen mol-ecule are pure oxygen. How are they different?

57. Is it possible to have a molecule that isn’t a compound? Give an example.

58. Is it possible to have a compound that isn’t made up of molecules? Give an example.

59. If you eat metallic sodium or inhale chlorine gas, you run a great risk of dying. When these two elements combine, how-ever, you can safely sprinkle the resulting compound on your popcorn for better taste. What is going on?

60. To become a negative ion, does an atom lose or gain an electron?

61. To become a positive ion, does an atom lose or gain an electron?

62. Helium is an inert gas, meaning that it doesn’t readily combine with other ele-ments. What five other elements would you also expect to be inert gases? (See the periodic table.)

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51. Copper, atomic number 29; any atom with 29 protons is copper.

52. a. Both have 27 protons. Cobalt-59 has 32 neutrons, and cobalt-60 has 33 neutrons.

b. 27 electrons in each

53. Almost no significance; electron has 1/1800 mass of proton.

54. The same; all have 82 protons.

55. Protons contribute more to mass, electrons more to size.

56. They contain different numbers of oxygen atoms. Ozone molecule has 3 oxygen atoms; oxygen molecule has 2 oxygen atoms.

57. Yes; example is oxygen gas, O2, or ozone, O3.

58. Yes; salt (NaCl) is crystal array of ions, not molecules.

59. When combined they form harmless table salt.

60. Gains an electron

61. Loses an electron

62. Neon, argon, krypton, xenon, and radon (the noble gases)


11/19/07 2:19:01 PM


ASSESS (continued)1


63. Why don’t equal masses of golf balls and table-tennis balls contain the same number of balls? Also, why don’t equal masses of pure carbon and oxygen contain the same number of atoms?

64. Which contains more atoms: 1 kg of lead or 1 kg of aluminum?

65. In a gaseous mixture of hydrogen and oxygen molecules, both with the same aver-age kinetic energy, which molecules move faster on average?

66. A hydrogen atom and a carbon atom have the same speed. Which has the greater kinetic energy?

67. In what sense is it correct to say that much of a tree is solidified air?

68. The phases of matter are solid, liquid, gas, and plasma. What does the addition or subtraction of heat have to do with changes of phase?

69. Write a letter to your grandparents that discusses the importance of the periodic table of elements. Also tell them what you’ve learned about the differences among atoms, elements, and molecules.

Think and Solve ••••••

70. Show that there are 16 grams of oxygen in 18 grams of water.

71. Show that there are 4 grams of hydrogen in 16 grams of methane gas. (The chemical formula for methane is CH

4.)

72. A typical atom is around 2 × 10�10 m in diameter, while a baby’s hair is about 2 × 10�5 m in thickness. How many atoms thick is a typical baby’s hair?

73. A typical atom is around 2 × 10�10 m in diameter, while a typical bacterium is about 10�6 m in diameter. How many atoms thick is the typical bacterium?

74. Gas A is composed of diatomic molecules (two atoms to a molecule) of a pure element. Gas B is composed of monatomic molecules (one atom to a molecule) of another pure element. Gas A has three times the mass of an equal volume of gas B at the same tempera-ture and pressure. How do the atomic masses of elements A and B compare?

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343

63. Table-tennis balls have lower mass—so greater number. Similarly, carbon has lower mass—so greater number.

64. Al; less massive atoms so more in 1 kg

65. H2; has less mass, so greater speed for same KE

66. Carbon, due to greater mass (KE = 1/2 mv2)

67. Its carbon comes from CO2 absorbed from air.

68. Adding heat to a solid changes it to liquid; add more heat and it changes to gas; add more and it changes to plasma. Subtract heat and change is in opposite direction.

69. Open to many answers

Think and Solve 70. Twice as many H atoms

(atomic mass 1) as O atoms (atomic mass 16)

71. For every 4 g of H, there will be 12 g of C.

72. Hair is (2 3 1025 m/2 3 10210 m) 5 105 atoms thick—about 100,000 atoms thick.

73. Bacterium is about (1026 m/2 3 10210 m) 5 about 5,000 atoms thick.

74. Gas A has three times the mass of Gas B. There are twice as many atoms in A, so the mass of each atom must be half of three times as much, 3/2.


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