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1 Atoms and Elements Chapter 2. 2 Atomic theory John Dalton, 1808: matter is made of tiny indestructible particles called atoms What evidence persuaded.

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Presentation on theme: "1 Atoms and Elements Chapter 2. 2 Atomic theory John Dalton, 1808: matter is made of tiny indestructible particles called atoms What evidence persuaded."— Presentation transcript:

1 1 Atoms and Elements Chapter 2

2 2 Atomic theory John Dalton, 1808: matter is made of tiny indestructible particles called atoms What evidence persuaded John Dalton that matter was made of atoms?

3 3 Evidence for Atoms Boyles Law (1660s) Gases can be compressed (PV = constant) Suggests that a gas is made of particles with space between Conservation of mass (Lavoisier, 1789) In a chemical reaction, matter is neither created nor destroyed

4 4 Evidence for Atoms Law of Definite Proportions (Proust, 1797) All samples of a compound, regardless of source or how prepared, have the same proportions of their constituent elements (constant composition) Law of Multiple Proportions (Dalton, 1803) When two elements (A and B) form two different compounds, the masses of B that combine with 1 g of A can be expressed as a ratio of small whole numbers.

5 5 Law of Multiple Proportions

6 6 Evidence for Atoms Combining Volumes of Gases When two gases combine to form a new compound, the volumes that combine will be in a ratio of small whole numbers.

7 7 A New System of Chemical Philosophy: John Dalton, 1808 Each element is composed of tiny indestructible particles called atoms. All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements. Atoms combine in simple, whole-number ratios to form compounds. Atoms of one element cannot change into atoms of another element. In a chemical reaction, atoms change the way that they are bound together with other atoms to form a new substance.

8 8 Cathode ray tube

9 9 Discovery of the electron J.J. Thompson (1897) Cathode rays are deflected by an electric or magnetic field Cathode ray = beam of negatively charged particles (electrons) coming out of cathode metal atoms Determined electron mass-to-charge ratio = – x 10 –9 g/C

10 10 Millikan oil-drop experiment

11 11 Charge of the electron Robert Millikan (1909) Drop charges are integer multiples of 1.60 x 10 –19 Coulomb

12 12 Radioactivity Some elements spontaneously emit radiation called radioactivity Some elements spontaneously emit radiation called radioactivity Three types of radioactivity: Three types of radioactivity: Alpha ( ) = heavy, +2 charge Alpha ( ) = heavy, +2 charge Beta ( ) = light, –1 charge Beta ( ) = light, –1 charge Gamma = high-energy photon Gamma = high-energy photon

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14 14 The gold foil experiment Ernest Rutherford (1910) Bombarded gold foil with alpha particles Most went straight through Some slightly deflected A few strongly deflected about as credible as if you had fired a 15-inch shell at a piece of tissue paper, and it came back and hit you

15 15 Alpha particle scattering explained

16 16 Rutherfords nuclear model Mass & positive charge concentrated in nucleus Most particles miss the nucleus & are not deflected: most of the atom is empty space Some particles come near the nucleus & are deflected: nucleus is positively charged and very dense Tiny, lightweight electrons circle the nucleus, like planets around the sun m m

17 17 Composition of the atom Protons Protons +1 charge +1 charge x 10 –24 g = amu x 10 –24 g = amu Neutrons Neutrons 0 charge 0 charge x 10 –24 g = amu x 10 –24 g = amu Electrons Electrons –1 charge –1 charge x 10 –24 g = amu x 10 –24 g = amu

18 18 Atomic number and mass number Atomic number (Z) = protons Atomic number (Z) = protons Mass number (A) = protons + neutrons Mass number (A) = protons + neutrons A 35 has 17 protons and 18 neutrons A 35 Cl atom has 17 protons and 18 neutrons Also shown as chlorine-35 Also shown as chlorine-35

19 19 Ion charge When an atom loses or gains an electron, it becomes an ion When an atom loses or gains an electron, it becomes an ion Ion charge is shown in the upper right corner of the atomic symbol Ion charge is shown in the upper right corner of the atomic symbol If no charge is shown, the charge is zero If no charge is shown, the charge is zero2311 Na 1+ mass number, A protons + neutrons atomic number, Z protons ion charge ion charge protons – electrons

20 20 Give the structure (p +, n 0, e – ) of these atoms 9 p +, 10 n 0, 9 e – 19 p +, 21 n 0, 18 e – 92 p +, 143 n 0, 92 e – 16 p +, 18 n 0, 18 e – 19 F 40 K U 34 S 2–

21 21 Give the symbol of each atom or ion 35 p +, 44 n 0, 36 e – l 79 Br 1– l 47 p +, 62 n 0, 46 e – l 109 Ag 1+ l 26 p +, 28 n 0, 26 e – l 54 Fe

22 22 Isotopes Atoms with the same number of protons but different numbers of neutrons are isotopes of the same element 6 Li and 7 Li are isotopes of lithium l Both are the element lithium l 6 Li has 3 protons, 3 neutrons l 7 Li has 3 protons, 4 neutrons

23 23 A Mass Spectrometer

24 24 Mass Spectrum of Neon 20 Ne90.48% 21 Ne 0.27% 22 Ne 9.26%

25 25 Average atomic mass Elements atomic mass = weighted average of masses of all naturally-occurring isotopes of that element

26 26 Average Atomic Mass Calculate the average atomic mass of Ne: ISOTOPE ISOTOPIC MASS (amu) ABUNDANCE 20 Ne amu 90.48% 21 Ne amu 0.27% 22 Ne amu 9.26%

27 27 Average Atomic Mass Calculate the average atomic mass of Ne: ISOTOPE ISOTOPIC MASS (amu) ABUNDANCE 20 Ne amu 90.48% 21 Ne amu 0.27% 22 Ne amu 9.26% (0.9048)( amu) = amu (0.0027)( amu) = amu (0.0926)( amu) = 2.04 amu amu

28 28 How to calculate the average atomic mass Magnesium occurs as three isotopes: 24 Mg, mass amu (78.99%) 25 Mg, mass amu (10.00%) 26 Mg, mass amu (11.01%) The average atomic mass of Mg is ( x ) + ( x ) + ( x ) = amu Be careful about sigfigs!

29 29 Atomic mass Two natural isotopes of antimony exist. 57.3% exists as 121 Sb (mass amu), and the rest is 123 Sb (mass amu). What is the atomic mass of antimony? The abundances must total 100%, so abundance of 123 Sb = 100 – 57.3 = 42.7% (0.573 x ) + (0.427 x ) = = amu

30 30 Percent abundance Two natural isotopes of copper exist, 63 Cu ( amu) and 65 Cu ( amu). What is the abundance of each isotope? The abundances must total 100% l If x = 63 Cu abundance, then 65 Cu = 1–x Average mass (from periodic table) = amu

31 31 Percent abundance x = 63 Cu ( amu) & 1–x = 65 Cu ( amu) Average mass = amu 63 Cu = 69.15% and 65 Cu = 30.85%

32 32 A unit for counting atoms We have units for describing mass, volume, temperature, and so forth, but we need a unit for counting the number of items For large items like eggs or donuts, we can use dozen: one dozen = 12 items For tiny items like atoms or molecules, we need... the mole!

33 33 What is a mole? One mole is defined as the number of atoms in exactly 12 grams of 12 C One mole contains x particles This number is called Avogadros number (N A ) any substance There is Avogadros number of particles in a mole of any substance

34 34 Calculations with Avogadro How many atoms of gold are present in mol Au? How many moles of Pb atoms are 8.27 x atoms of Pb?

35 35 Mass on the periodic table One atom of Cl weighs amu OR One mole of Cl atoms weighs grams The molar mass of Cl is g/mol

36 36 Calculations with molar mass What is the mass of 1.38 mol Al? How many moles are in 35 g of Zn?

37 37 Chemistry toolbox You now have 2 mole tools in your toolbox: Avogadros number 6.02 x particles = 1mol Relationship between particles (atoms/molecules) & moles Molar mass The atomic mass on the periodic table, in grams = 1 mol Relationship between grams & moles

38 38 Avogadro and molar mass What is the mass of 2.35 x atoms of Cu?

39 39 Avogadro and molar mass How many He atoms are present in a 22.6 g sample of He gas?

40 40 Mole concept Examples 2-7A, 2-7B, 2-8A, 2-8B How many atoms of gold are present in 5.07 x 10 –3 mol Au? How many 206 Pb atoms are present in 8.27 x 10 –3 mol of Pb? (Pb contains 24.1% 206 Pb ) What is the mass of 2.35 x atoms of Cu? How many He atoms are present in a 22.6 g sample of He gas?

41 41 Mole concept Examples 2-9A and 2-9 B How many Pb atoms are in a piece of lead with volume cm 3 ? Density of Pb = g/cm mg of Re contains 2.02 x atoms of 187 Re. What is the percent abundance of 187 Re in this sample?

42 42 Exercise 30 Show that mass is conserved within experimental error in this experiment: g calcium carbonate was dissolved in mL hydrochloric acid (d = g/mL). The products were g solution and 2.22 L carbon dioxide gas (d = g/L)

43 43 Exercise 31 In one experiment, 2.18 g sodium were reacted with g chlorine. All the sodium was used up, and 5.54 g sodium chloride were produced. In a second experiment, 2.10 g chlorine were reacted with g sodium. All the chlorine was consumed, and 3.46 g sodium chloride were produced. Are these results consistent with the law of constant composition?

44 44 Exercise 38 In an experiment like the oil drop experiment, drop 1 carried a charge of 6.41 x 10 –19 C drop 2 had 1/2 the charge of drop 1 drop 3 had twice the charge of drop 1 drop 4 had a charge of 1.44 x 10 –18 C drop 5 had 1/3 the charge of drop 4 Do these data support the value of x 10 –19 C for the electron charge? Could Millikan have inferred the electron charge from these data?

45 45 Exercise 42 For the ion 228 Ra 2+, with a mass of u, determine The numbers of protons, neutrons, and electrons The ratio of the mass of this atom to that of an atom of 16 O ( u)

46 46 Exercise 49 There are three naturally occurring isotopes of magnesium: u78.99% u10.00% u11.01% Calculate the weighted average atomic mass of magnesium.

47 47 Exercise 54 Boron has a weighted average atomic mass of u. What are the percent natural abundances of its two isotopes, 10 B ( u) and 11 B ( u)?

48 48 Exercise 58 Determine The number of Ar atoms in 5.25 mg argon The molar mass of an element if the mass of 2.80 x atoms of that element is 4.24 g The mass of a sample of aluminum that contains the same number of atoms as g zinc

49 49 Exercise 60 What is the total number of atoms present in a 75.0 cm 3 sample of plumbers solder, an alloy containing 67% Pb and 33% Sn by mass and having a density of 9.4 g/cm 3 ?

50 50 Early Observations of Mass 1770s Lavoisier - Conservation of Matter Careful measurements showed that mass is neither created nor destroyed in a chemical reaction. The total mass before and after reaction is the same

51 51 Conservation of mass Examples 2-1A and 2-1B A g sample of magnesium reacts with g nitrogen gas. The sole product is magnesium nitride. After the reaction the mass of unreacted nitrogen is g. What mass of magnesium nitride was produced? A 7.12g sample of magnesium is heated with 1.80 g of bromine. All the bromine is consumed and 2.07 g of magnesium bromide is the only product. What mass of magnesium is left over?

52 52 Early Observations of Matter 1799 Proust - Constant Composition All samples of a given compound contain exactly the same proportion of elements by mass. This is the Law of Definite Proportions

53 53 Constant composition Examples 2-2A and 2-2B This information was provided in the example: A g sample of magnesium combines with oxygen to yield g magnesium oxide What is the mass of magnesium in g of magnesium oxide? You wish to make exactly 2.00 g of magnesium oxide. What masses of magnesium and oxygen must you combine?

54 54 Early Observations of Matter 1803 John Dalton - Law of Multiple Proportions When the same elements form different compounds, the combining ratios in the compounds are always in a whole number relationship. Compound II has exactly twice as much O per C as compound I, by mass mass of Oxygen reacting w/ 1 g of Carbon cmpd I 1.33 g O/g C cmpd II 2.66 g O/g C

55 55 Definite and Multiple Proportions 3:2 3:2:3

56 56 The Electron: Part I J.J. Thompson Cathode rays can be deflected by electric and magnetic fields The charge-to-mass ratio of an electron can be determined:1.76 x 10 8 C/g

57 57 Thomson (1897) Cathode Rays Deflected by Electric Field

58 58 Thomson (1897) Cathode Rays Deflected by Magnetic Field

59 59 Thomson (1897) Balancing Deflections: Measures the e/m Ratio

60 60 Millikans Oil-drop Experiment Determines the charge of one electron

61 61 RadioactivityRadioactivity æ Some elements spontaneously emit radiation called radioactivity æ Three types of radioactivity: Alpha ( )= He 2+ Alpha ( )= He 2+ Beta ( )= e 1– Beta ( )= e 1– Gamma = no mass or charge Gamma = no mass or charge Marie Curie 1876–1934

62 62 Alpha Particle Scatter Explained Rutherfords Nuclear Model

63 63 Atomic composition Examples 2-3A and 2-3B Write an appropriate symbol for the species with 47 protons, 61 neutrons, and 47 electrons Determine the numbers of protons, neutrons, and electrons in an ion of sulfur-35 that carries the charge 2–

64 64 Atomic Masses Calculate the atomic mass of copper, Cu, from the following data : ISOTOPE ISOTOPIC MASS (amu) ABUNDANCE 63 Cu amu 69.2% 65 Cu amu 30.8%

65 65 Atomic Masses Calculate the atomic mass of copper, Cu, from the following data : ISOTOPE ISOTOPIC MASS (amu) ABUNDANCE 63 Cu amu 69.2% 65 Cu amu 30.8% 63 Cu: x = Cu: x = the average atomic mass of Cu is 63.5 amu

66 66 Periodic Table Dmitri Mendeleev developed the modern periodic table in 1869,based on his belief that element properties are periodic functions of their atomic masses Dmitri Mendeleev developed the modern periodic table in 1869,based on his belief that element properties are periodic functions of their atomic masses We now know that element properties are periodic functions of their ATOMIC NUMBERS We now know that element properties are periodic functions of their ATOMIC NUMBERS

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