11

AtomsAtomsandand

ElementsElements

Chapter 2

22

Atomic theoryAtomic theory John Dalton, 1808: matter is made of tiny

indestructible particles called atoms What evidence persuaded John Dalton that

matter was made of atoms?

33

Evidence for AtomsEvidence for Atoms Boyle’s 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

44

Evidence for AtomsEvidence 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.

55

Law of Multiple ProportionsLaw of Multiple Proportions

2.67 gOgC

1.33 gOgC

2

1

66

Evidence for AtomsEvidence 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.

hydrogen

oxygen

2

1hydrogen

chlorine

1

1

hydrogen

nitrogen

3

1

77

A New System of Chemical A New System of Chemical Philosophy: John Dalton, 1808Philosophy: 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.

88

Cathode ray tubeCathode ray tube

99

Discovery of the Discovery of the electronelectron

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 = –5.6857 x 10–9 g/C

1010

Millikan oil-drop experimentMillikan oil-drop experiment

1111

Charge of the Charge of the electronelectron

Robert Millikan (1909)

Drop charges are integer multiples of 1.60 x 10–19 Coulomb

1212

RadioactivityRadioactivity

Some elements spontaneously emit radiation Some elements spontaneously emit radiation

called called radioactivityradioactivity

Three types of radioactivity:Three types of radioactivity:

• Alpha (Alpha () = heavy, +2 charge ) = heavy, +2 charge

• Beta (Beta () = light, –1 charge) = light, –1 charge

• Gamma Gamma = high-energy photon= high-energy photon

1313

1414

The gold foil experimentThe 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”

1515

Alpha particle scattering explainedAlpha particle scattering explained

1616

Rutherford’s Rutherford’s nuclear modelnuclear 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

10-15 m10-10 m

1717

Composition of the atomComposition of the atomComposition of the atomComposition of the atom ProtonsProtons

• +1 charge+1 charge• 1.67262 x 101.67262 x 10–24–24 g = 1.0073 amu g = 1.0073 amu

NeutronsNeutrons• 0 charge0 charge• 1.67493 x 101.67493 x 10–24–24 g = 1.0087 amu g = 1.0087 amu

ElectronsElectrons• ––1 charge1 charge• 0.00091 x 100.00091 x 10–24–24 g = 0.00055 amu g = 0.00055 amu

1818

Atomic number and mass numberAtomic number and mass numberAtomic number and mass numberAtomic number and mass number Atomic number (Atomic number (ZZ) = protons) = protons Mass number (Mass number (AA) = protons + neutrons) = protons + neutrons

A A 3535Cl atom has 17 protons and 18 neutrons has 17 protons and 18 neutrons Also shown as chlorine-35Also shown as chlorine-35

1919

Ion chargeIon chargeIon chargeIon chargeWhen an atom loses or gains an When an atom loses or gains an

electron, it becomes anelectron, it becomes an ionion Ion charge is shown in the upper right Ion charge is shown in the upper right

corner of the atomic symbolcorner of the atomic symbol If no charge is shown, the charge is zeroIf no charge is shown, the charge is zero

23231111 NaNa1+1+

mass number, Amass number, Aprotons + neutrons protons + neutrons

atomic number, Z atomic number, Z protonsprotons

ion chargeion chargeprotons – electronsprotons – electrons

2020

Give the structure (pGive the structure (p++, n, n00, e, e––))of these atomsof these atoms

9 p+, 10 n0, 9 e–

19 p+, 21 n0, 18 e–

92 p+, 143 n0, 92 e–

16 p+, 18 n0, 18 e–

19F

40K1+

235U

34S2–

2121

Give the symbol of each atom or ionGive the symbol of each atom or ion

35 p+, 44 n0, 36 e–

79Br1–

47 p+, 62 n0, 46 e–

109Ag1+

26 p+, 28 n0, 26 e–

54Fe

2222

IsotopesIsotopesAtoms with the same number of protons but

different numbers of neutrons are isotopes of the same element

6Li and 7Li are isotopes of lithium Both are the element lithium 6Li has 3 protons, 3 neutrons 7Li has 3 protons, 4 neutrons

2323

A Mass SpectrometerA Mass Spectrometer

2424

02_12

18 19 20 21 22

Atomic mass (amu)

Fractionalabundance

Mass Spectrum of NeonMass Spectrum of Neon

2020NeNe 90.48%90.48%

2121NeNe 0.27% 0.27%

2222NeNe 9.26% 9.26%

2525

Average atomic massAverage atomic mass

Element’s atomic mass = weighted average of masses of all naturally-occurring isotopes of that element

2626

Average Atomic MassAverage Atomic MassCalculate the average atomic mass of Ne: ISOTOPE ISOTOPIC MASS (amu) ABUNDANCE

20Ne 19.99244 amu 90.48% 21Ne 20.99395 amu 0.27%

22Ne 21.99138 amu 9.26%

2727

Average Atomic MassAverage Atomic MassCalculate the average atomic mass of Ne: ISOTOPE ISOTOPIC MASS (amu) ABUNDANCE

20Ne 19.99244 amu 90.48% 21Ne 20.99395 amu 0.27%

22Ne 21.99138 amu 9.26%

(0.9048)(19.99244 amu) = 18.09 amu(0.0027)(20.99395 amu) = 0.057 amu(0.0926)(21.99138 amu) = 2.04 amu

20.18 amu

2929

Atomic massAtomic mass

Two natural isotopes of antimony exist. 57.3% exists as 121Sb (mass 120.9038 amu), and the rest is 123Sb (mass 122.9041 amu). What is the atomic mass of antimony?

The abundances must total 100%, so abundance of 123Sb = 100 – 57.3 = 42.7%

(0.573 x 120.9038) + (0.427 x 122.9041)= 69.3 + 52.5 = 121.8 amu

3030

Percent abundancePercent abundance Two natural isotopes of copper exist, 63Cu

(62.9296 amu) and 65Cu (64.9278 amu). What is the abundance of each isotope?

The abundances must total 100% If x = 63Cu abundance, then 65Cu = 1–x

Average mass (from periodic table) = 63.546 amu

3131

Percent abundancePercent abundance x = 63Cu (62.9296 amu) & 1–x = 65Cu (64.9278 amu) Average mass = 63.546 amu

63Cu = 69.15% and 65Cu = 30.85%

x 62.9296amu 1 x 64.9278amu 63.546amu

62.9296x 64.9278 64.9278x 63.546

1.9982x 1.382

x 1.382

1.99820.6915               1 x 0.3085

3232

A unit for counting atomsA 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!

3333

What is a mole?What is a mole?

One mole is defined as the number of atoms in exactly 12 grams of 12C

One mole contains 6.0221421 x 1023 particles This number is called Avogadro’s number (NA)

There is Avogadro’s number of particles in a mole of any substanceany substanceThere is Avogadro’s number of particles in a mole of any substanceany substance

3434

Calculations with AvogadroCalculations with Avogadro

How many atoms of gold are present in 0.0507 mol Au?

How many moles of Pb atoms are 8.27 x 1022 atoms of Pb?

0.0507 mol 6.022 1023atoms

1 mol3.05 1022  atoms

8.27 1022 atoms 1 mol

6.022 1023atoms0.137 mol

3535

Mass on the periodic tableMass on the periodic table

One atom of ClOne atom of Clweighsweighs

35.45 amu35.45 amuOROR

One mole of Cl One mole of Cl atoms weighs 35.45 atoms weighs 35.45

gramsgrams

The molar mass of The molar mass of Cl is 35.45 g/molCl is 35.45 g/mol

3636

Calculations with molar massCalculations with molar mass

What is the mass of 1.38 mol Al?

How many moles are in 35 g of Zn?

1.38 mol 26.98g

1 mol37.2 g

35 g 1 mol

65.39 g0.54 mol

3838

Avogadro and molar massAvogadro and molar mass

What is the mass of 2.35 x 1024 atoms of Cu?

2.35 1024 atoms 1mol

6.02 1023atoms3.90mol  Cu

3.90mol  Cu 63.55g

1mol248g Cu

3939

Avogadro and molar massAvogadro and molar mass

How many He atoms are present in a 22.6 g sample of He gas?

22.6g He 1mol

4.0026g5.65mol  He

5.65mol 6.02 1023atoms

1mol3.40 1024 atoms