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11
AtomsAtomsandand
ElementsElements
Chapter 2
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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
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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.
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Law of Multiple ProportionsLaw of Multiple Proportions
2.67 gOgC
1.33 gOgC
2
1
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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
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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.
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Cathode ray tubeCathode ray tube
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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
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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”
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Alpha particle scattering explainedAlpha particle scattering explained
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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
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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%
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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%
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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