Unit 1 Matter

CH185 class notesChapter 2 the discovery of the atom and the periodic table1

Discovery of the elements and the atom500 BC Greek philosophers believed that matter was made up of tiny particlesatomos atomsElements known since ancient timesCarbon, sulfur, iron, copper, silver, gold, mercury, leadThe term atom that we use today started with ancient Greek philosophers who believed that matter was made up of tiny particles. They had no scientific proof for such thinking.

Nine elements have been known and used since ancient times: carbon, sulfur, iron, copper, silver, gold, mercury, lead. 2Discovery of the elements and the atom1661 - Robert Boyle (English) definedan element as a substance that cantbe broken down any fartherCompounds elementsAlso interested in theology

It wasnt until the 17th century when scientific discovery starting proving some beliefs about matter. In 1661 Robert Boyle distinguished the difference between compounds and elements. Elements cant be broken down to simpler substances where as compounds can be. Elements must be a primary material that makes up substances and compounds are made up of more than one element. As with many early scientists he was also interested in religion3Discovery of the elements and the atomElements discovered between the middle ages and 1700Phosphorus (1669), zinc, arsenic, antimony, platinum, and bismuth

Around the time Boyle was working other elements were being discovered. 15 elements were known around 1700. Phosphorus was the first element to be chemically isolated. It was taken from urine.41789 - Antoine Laurent de Lavoisier (French) chemical textbook contained a list of 33 elements known at that timeClassified elements as metals and nonmetalsAlso described the law of conservation of massKilled with the guillotine during the French Revolution

Much later in 1789 Antoine Lavoisier wrote one of the first chemical textbooks. At the time there were only 33 elements that were known. He was the first person to start to classify elements as metals and nonmetals. In the textbook he also described the law of conservation of mass. This law shows that during a chemical reaction mass or the starting material is equal to the mass of the final material. Matter doesnt magically appear or disappear. Lavoisier found himself on the wrong political side of the fence and is one of the few famous scientists to be executed.51799 Joseph Proust (French) described the law of definite proportions9 g water = 1 g hydrogen and 8 g of oxygen

Joseph Proust was the first to figure out the substance made of more than one element always contained the elements in the same proportions. 9 g of water always broke down to 1 g hydrogen and 8 g oxygen. It didnt matter if the water was obtained from rain or from any other source, its elemental proportions were the same.61807 John Dalton (English) used the concept of atoms to explain conservation of mass and definite proportionsHis papers survived until 1940 when German bombers destroyed them

Dalton was the first modern scientist to describe matter as small indivisible particles called atoms. While we now know that atoms can be broken up into smaller particles, the idea of describing atoms in this way helped our discovery of atomic structure of matter.71817 - Johann Wolfgang Dbereiner (German) classified elements based on properties (triads)chlorine, bromine, and iodine calcium, strontium, and barium sulfur, selenium, and tellurium lithium, sodium, and potassium He also invented a lighter based on zinc and sulfuric acid

Dobereiner realized that groups of elements had similar properties and classified the elements based on these properties.8Discovery of the elements and the atomElements discovered between 1735 and 1843

During the period from 1735 to 1843 there were a lot of elements discovered. Many of the metals and some nonmetals were discovered during this time period.91834 Michael Faraday (English) discovered that electrical current can cause chemical reactionsTurned down knighthood later in life

As more elements were discovered, scientists realized that the elements would react and form new substances. The next step came in 1834 when Faraday discovered that electricity can influence the outcome and speed of a reaction. This was the first evidence that electrons were associated with atoms.101865 John Newlands (English) classified 56 elements into 11 groupsLaw of octaves Compared elements to musical octave of eight notes

John Newlands realized that the element properties repeated, but put together that there seemed to be a pattern of associated with the repeating properties. As a result of his musical background it made sense to him to have a repeating pattern, much like the scales, same note, but higher frequency.111871 Dmitri Mendeleev (Russian) classified elements based on propertiesArranged by atomic weightLeft blank spaces where no element was knownThreatened suicide if his proposal of marriage was not accepted

Mendeleev was the first to arrange the elements much like we do today. He arranged them by weight and properties and left spaces for elements not known at the time, but important12Discovery of the elements and the atomElements discovered between 1843 and 1886

By 1886 most of the elements through element 92 were discovered131896 Henry Becquerel (French) discovered radioactivityPhotographic plates were exposed when near uranium even with careful emission of light

Becquerel discovered that uranium would expose photographic plates that had been kept in the dark. This was the first discovery of radioactivity, elements changed. Marie Curie also became famous for early work with radioactivity.141897 JJ Thompson (English) discovered that electrons were part of atoms

Cathode ray tubes had been discovered earlier, but JJ Thompson was the first person to put together the idea that electrons were part of atoms and that cathode rays were electrons.151897 JJ Thompson (English) discovered that electrons were part of atomsUsed magnetic and electric fields to manipulate cathode raysBending based on ________ and __________e/m=-1.76 x 108 C/g

The bending of the cathode rays by magnetic and electric fields is based on the electrons charge and mass. Thus JJ Thompson was able to measure the charge to mass ratio as -1.76 x 108 C/g.

The picture in the slide shows how magnetic and electrical fields can change the direction of a cathode ray. The fluorescent screen lights up when the ray strikes that portion. This is how the first TVs worked. They would send thousands of cathode rays to red, green, or blue pixels to make up a complete picture.161899 Ernest Rutherford (New Zealand) determined three types of radioactivityAlpha, beta, and gamma raysDied from a hernia in 1937 because a titled doctor couldnt operate in time

Rutherford discovered that there were actually three types of radiation. Electrically charged plates affected each type differently. Alpha rays are positive and deflected from the positive plate. Beta rays are negative and deflected from the negative plate. Gamma rays are not affected by a charged plate and are considered charge neutral17

1909 Robert Millikan (American) measured the charge of an electronX-rays removed electrons from air and were picked up by oil dropsRate of fall based on potential of plates and charge of oil dropsMillikan set up a special experiment to measure the charge of an electron. First oil drops were allowed to fall through a small hole. X-rays removed electrons from air, which were picked up by the falling oil drops. The drops which picked up more charged fell more slowly (negative plate on the bottom). 181909 Robert Millikan (American) measured the charge of an electronValues of charge were whole number multiples of -1.60 x 10-19 CEach electron has a charge of -1.60 x 10-19 CUsing e/m and e means m = 9.09 x 10-28 g USPS made a 37 cent postage stamp with his picture on it

Millikan figured out that all the data indicated charges in whole number multiples of 1.60 x 10-19 C. He determined that this was the charge of one electron. After knowing the charge along with the charge to mass ratio from JJ Thompson, he could calculate the mass of an electron to be 9.09 x 10-28 g.191911 Ernest Rutherford (New Zealand) determined atoms have dense nucleus"It was as incredible as if you had fired a 15 inch shell at a piece of tissue paper and it came back and hit you."

Rutherford used alpha particles to bombard a thin gold sheet. He expected the alpha particles to go right through, which most did. However, some of the particles were deflected backwards. Rutherford interpreted this result to mean that the atom had a dense nucleus surrounded by mostly empty space.201914 Henry Moseley (Australian) used the frequencies of x-rays emitted from atoms after bombardment with high-energy electrons to determine the atomic number Periodic table could then be arranged by atomic number instead of atomic weightPredicted undiscovered elements hafnium and rheniumProved no other elements would be discovered between Al and AuKilled by a snipers rifle during a World War I battle.

Moseley was able to measure the atomic number of each element. He predicted the existence of some elements and proved that no other elements between Al and Au would be discovered211917 Ernest Rutherford (New Zealand) proved protons existed in an atoms nucleusN + a O + p

In 1920 Rutherford knew there was a disparity between the number of protons in an atom and the mass of an atomHe proposed another particle in the nucleus

Rutherfords work with radioactivity allowed him to discover the proton. He also proposed another particle present in the nucleus221932 James Chadwick (English) proved neutrons existed in an atoms nucleusSet up a lab in a German POW camp, where he was a prisoner, during WWIProved that particles emitted from polonium irradiated with alpha particles were neutral and about the mass of a protonNeutrons were critical to the discovery of nuclear fission and the atomic bomb

Chadwick proved the existence of neutrons.23Discovery of the elements and the atom

The last remaining elements were discovered after 1923. Many as a result of work with atomic nuclei and atomic energy/weapons24Modern Atomic view

The modern view of the atom has a small nucleus containing protons and neutrons. The protons and neutrons have >99% of the mass of the atom. The electrons are small and occupy the otherwise empty space outside the nucleus. The electron has a charge of -1, proton +1, and the neutron is neutral.

Read more in Burdge and Overby pages 35-3925Atomic sizesAtoms have diameters 1-5 x 10-10 m = 100-500 pmChemists also use Angstroms (), 1 = 10-10 mAtoms have diameters of 1-5 Nuclei are 10-4 If atom is the size of a football stadium, the nucleus is the size of a marble on the field.Mass of atom in small volume = large density 1013-1014 g/cm3, A matchbox full of material with that density would weigh over 2.5 billion tons26Atoms are really small (hundreds of picometers). Chemists often use Angstroms (10-10m). Even though an atom might be 1-5 , the nuclei are only 10-4 .1964 Murray Gell-Mann (American) and George Zweig (Russian) introduced quarksProtons and neutrons are made of quarksGell-Mann started at Yale when 15 years oldGell-Mann came up with the name quark from the novel Finnegans Wake by James Joyce

Protons and Neutrons are made of quarks.27How can the nucleus of large atoms contain such large positive charge?Quarks make up protons and neutronsProton = Up, Up, DownNeutron = Up, Down, DownUp quark = +2/3Down quark = -1/3Color force holds quarks together (very strong)Strong nuclear force is an extension of the color force between quarks of different nucleons

28

Modern Periodic TableImportance of Periodic tableMetals, Nonmetals, MetalloidsPeriods and groupsElemental formsPhysical state of elemental formOur modern periodic table tells us a lot about the elements and their properties. Understanding the shape of the table and the positioning of the elements will allow you to predict chemical and physical properties.

The periodic table is broken up into three sections, metals, nonmetals, and metalloids. Metals are good conductors of heat and electricity, whereas nonmetals are not. Metalloids have properties in between metals and nonmetals.

The periodic table is broken up into periods (rows). Each row in a numbered period. The table is also broken up into groups (columns). There are a number of group numbering systems, we will use the 1A, 2A, 3B numbering system shown in the top row in the slide. Some groups have special names: Group 1A = alkali metals, Group 2A = alkaline earth metals, Group 7A = halogens, Group 8A = noble gases, Groups 3B-2B = transition metals.

Another important thing to know about the elements is the elemental forms. All elements except H, N, O, F, Cl, Br, and I exist in elemental states as single atoms. The exceptions exist in diatomic form: H2, N2, O2, F2, Cl2, Br2, and I2. For example if we are discussing elemental iron, we just use the symbol Fe. If we are discussing elemental oxygen we use O2.

It is important to know the physical state of the elemental forms (an elemental form is the form when it is not combined with any other elements) of the element at room temperature.All metals and metalloids, except Hg (liquid), are solids at room temperature in their elemental forms. The following nonmetals are solid in their elemental form at room temperature: C, P, S, Se, and I. Br is the only nonmetal that is a liquid at room temperature in its elemental form. The other nonmetals, H, N, O, F, Cl, He, Ne, Ar, Kr, Xe, Rn are gases in their elemental forms at room temperature.

Read more in Burdge and Overby pages 43-44.

29

Memorize names and symbols of these elementsOf the modern periodic table you should memorize the elements not blacked out 1-38, 44-48, 50-51, 53-56, 78-80, 82-83, 86, 92, 94-95. You should memorize names and symbols for these elements. You dont need to know atomic number or atomic weight. 30Chemical symbols

31The chemical symbol and periodic table tell us a lot of information. The symbol obviously tells us what element it is. The subscript in front of the symbol is the atomic number (found on the periodic table). The superscript in front of the symbol is the atomic weight (protons + neutrons). There are 6 protons in carbon, and 6 neutrons in carbon-12. Read more in Burdge and Overby pages 39-40.Isotopes:116C126C136C146C32Isotopes are atoms that have the same number of protons, thus they are the same elements, but different numbers of neutrons. Carbon always has 6 protons, but carbon-11 has 5 neutrons, carbon-12 has 6 neutrons, carbon-13 has 7 neutrons, and carbon-14 has 8 neutrons.

Read more in Burdge and Overby pages 39-40.

How many protons, neutrons, and electrons are in (a) an atom of 197Au (b) an atom of strontium-90?33Gold (Au) is atomic number 79. Therefore it has 79 protons. Gold-197 has 197-79 = 118 neutrons. There are 79 electrons in a neutral atom of gold.

Strontium-90 (Sr) is atomic number 38. Therefore it has 38 protons. Strontium-90 has 90-38=52 neutrons. There are 38 electrons in a neutral atom of strontium.

Do Section 2.3 Review problems on page 40 of Burdge and Overby for more practice in this area.Race Track for measuring vehicle mass

If we had a semi truck and a car take a turn we could determine their mass, based on the maximum speed they could take the curve. We will use this analogy in the next slide when discussing mass spectrometers and isotopes.34Mass spectrometer

35A mass spectrometer takes a sample and removes electrons from the atoms. The positively charged particle is accelerated and force to curve in a magnetic field. Only those particles with the proper mass make the proper turn to reach the slit prior to the detector. The magnetic field can be varied to vary the mass of particle that reaches the detector. By measuring the masses of each elemental sample we can determine which isotopes are present.

In the slide above only two isotopes of chlorine are present, chlorine-35 and chlorine-37. You can see that chlorine 35 is higher intensity, thus the average mass of chlorine in the sample is greater than 35, but less than 37 and probably closer to 35.

Read more about mass spectrometers in Burdge and Overby page 42.Naturally occurring chlorine is 75.78% 35Cl, which has an atomic mass of 34.969 amu, and 24.22% 37Cl, which has an atomic mass of 36.966 amu. Calculate the average atomic massof chlorine.36In this problem we are asked to calculate the average mass of chlorine based on the abundance of each isotope. Average mass = sum of the abundance fraction of each isotope times its mass.

Chlorine average mass = 0.7578*34.969amu + 0.2422*36.966amu = 35.45 amuIn this case the abundance percentage will determine the correct number of significant figures.

Try the worked example and Practice Problems A & B on page 41 of Burdge and Overby for more practice in these types of problems.

Average MassSignificant figuresThis periodic table from the inside front cover of your textbook that has 4 significant figures for most elements. 37

Counting Eggs vs Counting Atoms

We use many methods for counting eggs. You might say that you want 4 eggs for breakfast. You might also say that you need to buy 3 dz eggs at the store. If you work in a restaurant you might use 7 cases of eggs a week. Likewise in chemistry we have different ways of counting atoms which we will discus on the next slide.38Counting Atoms with Avogadro's Number1 mole atoms = 6.0221418 x 1023 atoms~6.022 x 1023 atoms1 mole of 12C has a mass of 12 gSpreading 6.022 x 1023 marbles over the entire surface of the earth would produce a layer about 3 miles thick.

39In chemistry we use the term mole to refer to counting numbers of atoms. 1 mole of atoms = 6.0221418 x 1023 atoms. You can remember 6.022 x 1023, as it has four significant figures and should be all that is needed in most cases. A mole is defined as the number of atoms in 1 g of carbon-12.

Shown in the figure on the slide is one mole of aluminum foil in a ball, one mol of copper BBs in a beaker, 1 mole of water in a 25 mL graduated cylinder, one mole of salt in the beaker, one mole of sugar in cubes, and one mole of helium in the balloon.

Read more in Burdge and Overby page 45.Mole Day1 mole = 6.02 x 1023 particlesMole Day celebrated on October 23 (10/23) starting at 6:02 AM and ending at 6:02 PMCheck out more at moleday.orgFrom the mole day dictionary:molearchy - government in which moles are in complete control; under this government Mole Day is celebrated three times a year and chemistry is the only scientific subject taught in school

Chemists get excited over the mole and have a National Mole Day. We celebrate mole day on October 23 because of the 1023 power in Avogadros number. Celebrations start at 6:02 AM and end at 6:02 PM. 40Calcium is the most abundant metal in the human body. A typical human body contains roughly 30 moles of calcium. Determine (a) the number of Ca atoms in 30.00 moles of calcium and (b) the number of moles of calcium in a sample containing 1.00 x 1020 Ca atoms.41In part a we are asked to convert moles of Ca into atoms of Ca. We need to use Avogadros number as a conversion factor with moles on the bottom and atoms on top, see work above and in page 45 of Burdge and Overby.

In part b we are asked to convert atoms of Ca into moles of Ca. We need to use Avogadros number as a conversion factor with atoms on the bottom and moleson top, see work above and in page 45 of Burdge and Overby.

Try practice problems A & B on page 45 in Burdge and Overby for more practice with this type of problem.

Weighing Nails

How many pounds of 4d common nails should you buy if you need 1000 nails for a project?Here we are using an analogy with nails, leading into how we treat atoms and mass.

If we need 1000 4d nails for a project and we know that there are 316 4d common nails per pound we can calculate using dimensional analysis the number of pounds needed for the project. See work in figure above. When done you can see that we can convert number of nails into weight of nails. Likewise by using the periodic table we can convert numbers of moles of atoms into grams of atoms.42

The average atomic weight of each element is the amu of one atom of that element. For example an average iron atom has a mass of 55.85 amu. The number also represents the average number of grams of one mole of that element. For example 1 mole of Fe atoms has a mass of 55.85g. This is known as the molar mass (the mass of one mole). We now have a conversion factor between moles and grams that we can use to work problems.43Molar MassCalculate the number of moles of carbon atoms in 25.00 g of carbon.Calculate the mass of 0.561 moles of sodium atom.44In the first problem we need to find the mass of 0.561 moles of Na atoms. From the periodic table we know that the molar mass of Na is 22.99 g/mol. See figure above for work using dimensional analysis.

In the second problem we need to find the number of moles of C atoms in 25 g of C. We need to use the reciprocal of the molar mass to get the units to cancel. See figure above for work.

Do the practice problems A&B on page 47 of Burdge and Overby for more practice with these kinds of problems.

45Imagine that you have just moved to a new city, Mole City. Since you dont know your way around you buy a map of the city. You find out that Mole City has three streets and 2 avenues. In order to get from one street to another you need to take an avenue.

This roadmap can be used when converting between moles, atoms, and grams. If you have grams and you want moles you need to use the molar mass. If you have atoms and you want moles you need Avogadros number. If you have atoms and you want grams you need to go to moles first.Determine the number of C atoms in 0.515 g of carbon.Determine the mass of helium that contains 6.89 x 1018 He atoms.46In the first problem we are asked to find carbon atoms in 0.515 g of carbon. Using the mole city map we first need the molar mass to get to mole street, then we need Avogadros number to get to atoms. See figure in notes section or the worked example on page 48 in Burdge and Overby for this calculation.

In the second problem we are given the number of He atoms and are asked to find the mass. Using mole city map we know that we first need to go from atoms to mole street using Avogadros number, then we need the molar mass to get to gram street. See figure in notes section or the worked example on page 48 in Burdge and Overby for this calculation.

For more practice with these kinds of calculations do Practice Problems A&B on page 48 of Burdge and Overby.