Chapter 3

Atomic Structure

Objective:• You will explain why the model of

the atom changed throughout history.

What is a model?

• Models are used to help us understand things that cannot be seen directly

• Models are used when something is too large, too small, or too dangerous to be studied directly.

Examples of Scientific Models

This model shows the alignment between the sun, moon, & earth. As it rotates, it shows the phases of the moon and how we measure a year.

This model is a mathematical representation of a sound wave. You cannot see sound, but you can see how it affects other objects with its vibrations.

Examples of Scientific Models

• What other examples can you think of?

• Are there other models present in this room?

A good model…

• Must be based on observations and indirect experimentation.

• Must explain as many characteristics of the original object as possible.

• Should be as simple as possible.

When do you change a model?

• All models have limitations —No model has ever been totally complete.

• A model changes when observations of a new situation do not agree with the current model.

Creating a Model

• The “Think Tube” is also a model for something you cannot see directly.

THINK TUBE ?

Creating a Model

• How many strings are on the inside?

• Make your own model showing how

the “Think Tube” works.

Atoms

• The atom has not changed over time, but our idea and model of the atom has.

Definition of Atom:

•the smallest particle of an element that retains the chemical properties of that element.

Democritus 400 BC

By convention there is color,By convention sweetness,By convention bitterness,But in reality there are atoms and space.   -Democritus (c. 400 BCE)

Convention means because we said so- doesn’t really exist

Democritus 400 BC• Democritus was smashing up sea shells one day and thought that you can break down the shell to tiny pieces, but it can not be completely destroyed.

Democritus 400 BCDemocritus 400 BC Looked at sand on the beach. Cut Looked at sand on the beach. Cut

sand in half and got fewer and fewer sand in half and got fewer and fewer grains of sand.grains of sand.

What was the smallest piece?What was the smallest piece?

He called it atomos = He called it atomos = greek word meaning cannot be greek word meaning cannot be

cutcut

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Democritus 400 BC According to Democritus atoms are:

• Invisible• Indivisible• Solid• Eternal • Surrounded by an empty space

Democritus 400 BC continued:

• Have an infinite number of possible shapes. • Each type of atom had a different size.

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Democritus 400 BC Atoms -

Aristotle’s Idea 300 BCAristotle’s Idea 300 BC

All substances are made of 4 All substances are made of 4 elements: Fire, Air, Earth, and elements: Fire, Air, Earth, and WaterWater

Aristotle’s Idea 300 BC

• There were also four qualities: dryness, hotness, coldness, and moistness.

• Fire was dry and hot, while water was moist and cold, etc.

• Each of these elements move naturally in a line to their "proper place," where it will be at rest.

Aristotle 300 BC

• Water sits on top of the earth, he explained, because it is lighter, yet air floats above the water because it is lighter still—and fire, lightest of all, rises highest. Furthermore, he claimed that the planets beyond Earth were made up of a "fifth element," or quintessence, of which little could be known.

Democritus vs Aristotle

• Ancient Greeks accepted Aristotle’s ideas and rejected Democritus.

• What holds the particles together?• Democritus could not answer this

question • Remained that way until the 17th

century

• Law of Conservation of Mass • Law of Definite Proportions • Law of Multiple Proportions

Important DiscoveriesImportant Discoveries

Law of Conservation of Mass (Matter)

• In a chemical reaction, matter is neither created nor destroyed.

•Lavosier measured the mass of chemicals before and after a chemical reaction and found that the weight did not change.

Law of Definite Proportions

•Also called Law of Constant Composition

•Proposed by Joseph Proust•Elements always react and

combine with one another in the same proportions.

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Law of Definite Proportions

A chemical compound is always composed of the same combination of atoms -

copper carbonate CuCO3

Law of Definite Proportions

• Water has the formula H2O. This means that water in the ocean, lakes, or in our sinks always contains 2 atoms of Hydrogen for every 1 atom of Oxygen. What percent of water is Hydrogen and what percent is Oxygen?

H: 2g/18g = 11% O: 16g/18g = 89%

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Law of Multiple Proportions

• If two elements form more than one compound between them, then the ratios of the weights of the two atoms will be ratios that can be reduced to small whole numbers.

Law of Multiple Proportions

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John Dalton 1766-1844

• English school teacher and public lecturer by the age of 12.

• As a Quaker, Dalton led a modest existence, although he received many honors later in life. In tribute, more than 40,000 people marched in his funeral procession.

John Dalton’s Atomic Theory

• Dalton’s Theory was a return to the ideas of Democritus

• Dalton turned the idea into a scientific theory that could be tested

• Not all of Daltons ideas are still true today. Some ideas were modified.

John Dalton’s Atomic Theory:

1. All matter is composed of atoms.2. Atoms of a particular element have

identical properties. Elements of a different element have different properties.

3. Atoms cannot be divided or destroyed.

4. Atoms combine to form compounds.5. During a chemical reaction atoms

are rearranged.

John Dalton’s Atomic Theory:

1. All matter is composed of atoms.2. Atoms of a particular element have

identical properties. Elements of a different element have different properties.

3. Atoms cannot be divided or destroyed.

4. Atoms combine to form compounds.5. During a chemical reaction atoms

are rearranged.

JJ Thomson 1856-1940

Excuse me... how can you discover a particle so small that nobody has ever seen one?

http://www.aip.org/history/electron/jjsound.htm

J.J. Thompson’s Model

_

__

_

_

+ ++

+

+

Discovery of the Electron

JJ Thomson determined that:

• Atom is a sphere of positive matter that holds electrons in it.

• Also called the plum pudding model or the raisin dough model.

His experiment

http://www.sciencemuseum.org.uk/on-line/electron/section2/shockwave2.asp

Thomson’s Experiment

Voltage source

+-

Vacuum tube

Metal Disks

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

• The cathode ray travels from the cathode to the anode when current was passed through the tube.

Voltage source

+-

J.J. Thompson’s Cathode Ray Tube

Cathode Anode

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field he found By adding an electric field he found that the moving pieces were negativethat the moving pieces were negative

+

-

Maltese Cross Tube

•Using a Maltese cross as the anode, this produced a shadow that glowed at the end of the tube. This showed that the cathode rays traveled in straight lines.

Paddle Wheel Discharge Tube

A paddle wheel placed in the path of the cathode rays turned. This proved that the cathode rays contained mass, and that they might be made of particles.

Thomson's “Plum Pudding Model"

http://www.broadeducation.com/htmlDemos/AbsorbChem/HistoryAtom/page.htmt

Thomson's “Plum Pudding Model"

http://molaire1.club.fr/e_histoire.html

Electrons are red.

Negative electrons

Are embedded in a

Blue positive atom.

J.J. Thompson’s Model

1.Cathode rays are beams of negatively-charged particles called electrons.

2.All atoms contain electrons.3.Atoms also contain an equal

and opposite positive charge.

Ernest Rutherford 1871-1937

• "All science is either physics or stamp collecting."

http://www.pbs.org/wgbh/aso/databank/entries/bpruth.html

Ernest Rutherford’s Model

• He became a student of a teacher named Professor J J Thomson at Cambridge University in England

Ernest Rutherford’s Model

•Rutherford studied how gold atoms interacted with radioactivity.

Radioactivity

•Radioactivity is the processes by which

unstable atoms emit subatomic particles (radiation).

• Made of 2 protons and 2 neutrons.• Charge of +2, and a mass of 4• Relatively slow and heavy.

• Charge of minus 1• Mass is very small. • They are the same as an electron.• They are fast, and light.

• Gamma rays are waves, not particles. • They have no mass and no charge.

Penetrating Power

•Alpha particles are easy to stop, gamma rays are hard to stop.

Lead block

Uranium

Gold Foil

Fluorescent Screen

Here’s how it looked.

What he expected

Rutherford Expected:

• The alpha particles to pass through without changing direction.

• Because…?• …the positive charges were

thought to be spread out evenly. Alone they were not enough to stop the large alpha particles.

What was expected:

What he got

Rutherford’s Gold Foil Results

Rutherford said that having the alpha particle bounce off of the foil was equivalent to seeing a cannon ball bounce off of a piece of tissue paper.

What Actually Happened…

+

-

--

--

-

How he explained it:

+

• Atom is mostly empty.• It has a small • dense, positive piece

at center.• Alpha particles

are deflected bythe nucleus if they

get close enough.

---

-- -

Ernest Rutherford’s Results:

• Atom is mostly empty space.

• Suggests that an atom has a nucleus that holds most of

the mass of the atom.

+

---

-- -

What Actually Happened

Rutherford’s Atom:A sea of electrons that surrounded a positively charged nucleus.

              

 

Rutherford’s Atom• His model of the atom was similar

to the solar system. Like planets, electrons orbited a central, sun-like nucleus.

http://www.broadeducation.com/htmlDemos/AbsorbChem/HistoryAtom/page.htmt

Rutherford’s Atom

              

 

PROTONS

+

Rutherford’s Atom

1. Most of the mass of an atom must be located in a small volume at the center of the atom (the nucleus).

2. The nucleus is made of positively charged particles called protons.

3. The electrons move in a large volume which is mostly empty space.

Problems with Rutherford’s Atom

• According to "classical" theory the electrons should lose energy by radiating electromagnetic radiation, as they are accelerated electric charges.

• They should spiral into the nucleus.

Problems with Rutherford’s Atom

2 years after Rutherford…. Neils Bohr

1885 - 1962

Bohr Model of the Atom:

Bohr Model of the Atom:

• Electrons are a particular distance from the nucleus

• The energy of each electron is not the same

• Electrons close to center = low Energy

• Electrons farther away = high Energy

Neils Bohr’s Model• There are 2 electrons in the lowest

energy level, 8 electrons in the second energy level, and 18 in the third…

…this is the model of the atom we will use!

James Chadwick

Chadwick

• In 1932, Chadwick proved the existence of neutrons - elementary particles devoid of any electrical charge.

• Located in the nucleus

(Rutherford also put out the idea that there could be a particle with mass but no charge)

Chadwicks Atom

Chadwick’s Atom Modern Atom

1. Most of the atom's volume is occupied by electrons.

2. The number and arrangement of electrons in an atom determine its chemical properties. 

Chadwick’s Atom

3. The identity of an element is determined by the number of protons in the nucleus.

4. Different isotopes of elements exist and differ only in the number of neutrons and hence the mass of the atom.

Modern Atom

• Electrons• Protons• Neutrons

Alpha & Beta particles

http://wps.prenhall.com/esm_brown_chemistry_9/0,4647,169289-,00.html

Electrons

•  Electrons are tiny particles which behave like clouds

• Electrons carry something called a negative electric charge.

• Electrons are responsible for the chemistry of the atom.

Electronshttp://education.jlab.org/atomtour/listofparticles.html

Atoms

- Atomic diameter ~ 1 to 5x10-8 cm

- Are about 10,000,000 in 1 mm- An H atom weighs 1.67x10-24 g- Note: (6.02x1023) x (1.67x10-24 g) = 1.00 g

- 6.02x1023 = Avogadro’s Number

Atoms are made up of three major parts:Part Found Mass Charge

Electrons Outside 9.1x10-28g (small) -1

Protons Nucleus 1.7x10-24g (1.0 AMU)+1

Neutrons Nucleus 1.7x10-24g (1.0 AMU) 0

Atomic Structure

1) Neutral atoms contain equal number of electrons and protons.

Atomic Structure

2) Atoms can loose or gain electrons to become charged = ions

Atomic Structure

3) Number protons determines the identity of the atom or ion.

Atomic Number

Atomic Structure

4) Mass Number = # Protons + # Neutrons

• APE MANAtomic Number =Proton # =Electron #

Mass # -Atomic Number

= Neutron #

SymbolMass Number

Atomic Number

Charge (if ion)

Atomic Mass-

An isotopes contribution is determined by its relative abundance.

EXAMPLE

How many protons, neutrons and electrons are found in an atom of

13355 Cs

Atomic number = protons and electronsThere are 55 protons and 55 electrons

Mass number = sum of protons and neutrons133 – 55 = 78There are 78 neutrons

HHydrogenHydrogen

1

1Atomic Mass

Atomic Number

Symbol

HHydrogenHydrogen

1

1

Protons:Electrons:Neutrons:

HHydrogenHydrogen

1

1

Protons: 1Electrons:Neutrons:

HHydrogenHydrogen

1

1

Protons: 1Electrons:1Neutrons:

HHydrogenHydrogen

1

1

Protons: 1Electrons:1Neutrons:0

NaSodiumSodium

23

11

Protons:Neutrons: Electrons:

NaSodiumSodium

23

11

Protons: 11Neutrons:Electrons:

NaSodiumSodium

23

11

Protons: 11Neutrons:Electrons: 11

NaSodiumSodium

23

11

Protons: 11Neutrons:Electrons: 11

12

12

RheniumRhenium

Re186

75

Protons: Neutrons:Electrons:

75

75

111

111

Rhenium IsotopeRhenium Isotope

Re187

75

Protons: Neutrons:Electrons:

75

75

112

112

Re-111

C-12C

12

6

Isotopes

• Atoms of a given element with differing numbers of neutrons are called isotopes.

Isotopes

• An atom is still the same element if it is missing an electron. The same goes for isotopes. They are still the same element. They are just a little different from every other atom of the same element.

Atomic Weight-

An isotopes contribution is determined by its relative abundance.

Atomic numbers are whole numbers

Mass numbers are whole numbers

The atomic mass is not a whole number.

Calculating Atomic Mass

atomic mass = (% abundance of isotope 1)(mass of isotope 1) +(% abundance of isotope 2)(mass of isotope 2) +

(% abundance of isotope 3)(mass of isotope 3)

+ ...

Calculating Atomic Mass

Answer the following questions:

"How many naturally occurring isotopes does carbon have?"

"What is the abundance of each of the isotopes?"

Isotope Atomic Mass Relative Abundance

C-12 12.00 98.93

C-13 13.00 1.07

The sum of all the fractions of abundance The sum of all the fractions of abundance of each naturally occurring isotopes of each naturally occurring isotopes

should equal 1.00 or 100%.should equal 1.00 or 100%.

atomic mass of carbon =

(0.9893)(12.000 amu)

+(0.0107)(13.00 amu)

= 11.868 amu + 0.1391 amu

= 12.0107 amu

What is the atomic mass of Lithium

Isotope Atomic Relative Mass Abundance

Li-6 6.015 7.59Li-7 7.016 92.41

What is the atomic mass of Lithium

0.0759*6.015 = + 0.9241* 7.016 =

0.45650.45656.48286.4828

6.93936.9393

What is the atomic mass of Oxygen?

Isotope Atomic Mass Relative Abundance

O-16 16 99.757

O-17 17 0.038

O-18 18 0.205

What is the atomic mass of Oxygen 0.99757*16 =

+ 0.00038*17= + 0.00205*18=

15.9611215.96112

0.006460.00646+0.0369+0.0369

16.0044816.00448