Chemistry: The Molecular Science
Moore, Stanitski and Jurs
Chapter 2: Atoms and Elements
© 2008 Brooks/Cole
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Atomic Structure and Subatomic Particles
Atoms are composed of subatomic particles:
• electron (e-), proton (p+) and neutron (n0).
Key discoveries:
Radioactivity
Becquerel (1896)
• U ore emits rays that “fog” a photographic plate.
Marie and Pierre Curie (1898)
• Isolated new elements (Po & Ra) that did the same.
• Marie Curie called the phenomenon radioactivity.
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Radioactivity
Electrical behavior: “+” attracts “-” but like charges repel
Radioactive material
β-particles (“–”)
Beam
of , ,
and 
Gamma ray (γ)
No charge,
no deflection
Electrically
charged plates
α-particle (“+” )
Heavier, deflected
less than β
Atoms must contain smaller sub-units.
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Electrons
Thomson (1897) discovered the e-:
“Cathode rays”
– high voltage +
fluorescent
screen
cathode ray
• Travel from cathode (-) to anode (+).
• Negative charge (e−).
• Emitted by cathode metal atoms.
Electric and magnetic fields deflect the beam.
• Gives mass/charge of e- = −5.60 x 10-9 g/C
• Coulomb (C) = SI unit of charge
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Electrons
Millikan (1911) studied electrically-charged oil drops.
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Electrons
Charge on each droplet was:
n (−1.60 x 10-19 C) with n = 1, 2, 3,…
n (e- charge)
Modern value = −1.60217653 x 10-19 C.
= −1 “atomic units”.
These experiments give:
mass
me = charge x
charge
= (-1.60 x 10-19 C)(-5.60 x 10-9 g/C) = 8.96 x 10-28 g
Modern value = 9.1093826 x 10-28 g
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Protons
Atoms gain a positive charge when e- are lost.
Implies a positive fundamental particle.
Hydrogen ions had the lowest mass.
• Hydrogen nuclei assumed to have “unit mass”
• Called protons.
Modern science: mp = 1.67262129 x 10-24 g
mp ≈ 1800 x me.
Charge = -1 x (e- charge).
= +1.602176462 x 10-19 C = +1 atomic units
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The Nuclear Atom
How were p+ and e- arranged?
Thompson:
• Ball of uniform positive charge, with small
negative dots (e-) stuck in it.
• The “plum-pudding” model.
1910 Rutherford fired α-particles at thin metal foils.
Expected them to pass through with minor
deflections.
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The Nuclear Atom
But … some had large deflections.
α particles
Rutherford
“It was about as credible as if you had fired a 15-inch
shell at a piece of paper and it came back and hit you.”
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The Nucleus
Most of the mass and all “+” charge is concentrated
in a small core, the nucleus.
≈10,000 times smaller diameter than the entire atom.
e- occupy the remaining space.
α particles
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Neutrons
Atomic mass > mass of all p+ and e- in an atom.
Rutherford proposed a neutral particle.
Chadwick (1932) fired -particles at Be atoms.
Neutral particles, neutrons, were ejected:
mn ≈ mp (0.1% larger).
mn = 1.67492728 x 10-24 g.
Present in all atoms (except normal H).
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The Nuclear Atom
Nucleus
• Contains p+ and n0
• Most of the atomic mass.
• Small (~10,000x smaller diameter than the atom).
• Positive (each p+ has +1 charge).
Electrons
• Small light particles surrounding the nucleus.
• Occupy most of the volume.
• Charge = -1.
Atoms are neutral. Number of e− = Number of p+
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Scanning Tunneling Microscopy
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Scanning Tunneling Microscopy
Fe atoms arranged on
Cu.
“Atom”
(Chinese characters)
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Sizes of Atoms and Units
Atoms are very small.
• 1 tsp of water contains 3x as many atoms as there
are tsp of water in the Atlantic Ocean!
Impractical to use pounds and inches...
Need a universal unit system
• The metric system.
• The SI system (Systeme International) - derived
from the metric system.
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Metric Units
• A decimal system.
• Prefixes multiply or divide a unit by multiples of ten.
Prefix
mega
kilo
deci
centi
milli
micro
nano
pico
femto
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M
k
d
c
m
μ
n
p
f
Factor
Example
106
1 megaton = 1 x 106 tons
103
1 kilometer (km) = 1 x 103 meter (m)
10-1 1 deciliter (dL) = 1 x 10-1 liter (L)
10-2 1 centimeter (cm) = 1 x 10-2 m
10-3 1 milligram (mg) = 1 x 10-3 gram (g)
10-6 1 micrometer (μm) = 1 x 10-6 m
10-9 1 nanogram (ng) = 1 x 10-9 g
10-12 1 picometer (pm) = 1 x 10-12 m
10-15 1 femtogram (fg) = 1 x 10-15 g
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Metric Units
How many copper atoms lie across the diameter of a
penny? A penny has a diameter of 1.90 cm, and a
copper atom has a diameter of 256 pm.
1 pm = 1 x 10-12 m
;
1 cm = 1 x 10-2 m
-2 m
1 pm
1
x
10
1.90 x 1010 pm
=
1.90 cm x
x
1 x 10-12 m
1 cm
Number of atoms across the diameter:
1.90 x 1010 pm x 1 Cu atom = 7.42 x 107 Cu atoms
256 pm
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Some Common Unit Equalities
Length
1 kilometer
= 0.62137 mile
1 inch
= 2.54 cm (exactly)
1 angstrom (Å) = 1 x 10-10 m
1 gallon
= 1000 cm3 = 1000 mL
= 1.056710 quarts
= 4 quarts = 8 pints
1 amu
1 pound
1 ton (metric)
1 ton (US)
= 1.66054 x 10-24 g
= 453.59237 g = 16 ounces
= 1000 kg
= 2000 pounds
Volume 1 liter (L)
Mass
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Some Common Unit Equalities
Report the mass of a 5.0 lb bag of sugar in
kilograms.
1 lb = 453. g
5.0 lb x 453. g
1 lb
= 2265 g
= 2.3 x 103 g
= 2.3 kg
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Some Common Unit Equalities
A patient’s blood cholesterol level measured 165
mg/dL. Express this value in g/L
1 mg = 1 x 10-3 g
;
1 dL = 1 x 10-1 L
-3 g
mg
1
x10
= 1.65 g/L
165
x 1 dL
x
1 x10-1 L
dL
1 mg
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Uncertainty and Significant Figures
All measurements involve some uncertainty.
Reported numbers include one uncertain digit.
Consider a reported mass of 6.3492 g
• Last digit (“2”) is uncertain
• Close to 2, but may be 4, 1, 0 …
• Five significant figures in this number.
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Uncertainty and Significant Figures
Read numbers from left to right.
Count all digits, starting with the 1st non-zero digit.
All digits are significant except zeros used to
position a decimal point (“placeholders”).
0.00024030
placeholders
significant
significant
5 sig. figs.
(2.4030 x 10-4)
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Uncertainty and Significant Figures
Number Sig. figs.
2.12
3
4.500
4
0.002541
4
0.00100
3
500
1, 2, 3 ?
Comment on Zeros
Not placeholders. Significant.
Placeholders (not significant).
Only the last two are significant.
Ambiguous. May be placeholders or
may be significant.
500.
3
Add a decimal point to show they are
significant.
5.0 x 102
2
No ambiguity.
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Significant Figures in Calculations
Addition and subtraction
Find the decimal places (dp) in each number.
answer dp = smallest input dp.
Add:
17.245
+ 0.1001
17.3451
dp = 3
dp = 4
Rounds to: 17.345
(dp = 3)
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Significant Figures in Calculations
Subtract 6.72 x 10-1 from 5.00 x 101
Use equal powers of 10:
5.00 x 101
– 0.0672 x 101
4.9328 x 101
Rounds to: 4.93 x 101
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dp = 2
dp = 4
dp = 2
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Significant Figures in Calculations
Multiplication and Division
Answer sig. fig = smallest input sig. fig.
17.245
x 0.1001
1.7262245
Rounds to: 1.726
sig. fig. = 5
sig. fig. = 4
sig. fig. = 4
Multiply 2.346, 12.1 and 500.99 = 14,221.402734
Rounds to:
1.42 x 104 (3 sig. fig.)
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Rules for Rounding
Examine the 1st non-significant digit. If it:
• > 5, round up.
• < 5, round down.
• = 5, check the 2nd non-significant digit.
 round up if absent or odd; round down if even.
Round 37.663147 to 3 significant figures.
last retained
digit
2nd nonsignificant
digit
Rounds up to 37.7
1st nonsignificant digit
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Rules for Rounding
Round the following numbers to 3 sig. figs.
Number
2.123
51.372
131.5
24.752
24.751
0.06744
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1st non-sig.
digit
2.123
51.372
131.5
24.752
24.751
0.06744
2nd non-sig.
digit
51.372
24.752
24.751
-
Rounded
Number
2.12
51.4
132.
24.7
24.8
0.0674
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Rules for Rounding
dp = 5
dp = 3
Answer dp = 3.
92.803 is the significant result.
(5 sig. figs).
92.80344
99.12444 – 6.321
=
= 3.37153195571
27.5256
27.5256
6 sig. figs.
Significant figures?
© 2008 Brooks/Cole
= 3.3715 (5 sig. figs.)
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Rules for Rounding
To avoid rounding errors
Carry additional digits through a calculation.
Use the correct number of places in the final answer.
Note
Exact conversion factors:
(100 cm / 1 m) or (2H / 1 H2O)
Have an infinite number of sig. figs.
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Atomic Numbers & Mass Numbers
Same element - same number of p+
Atomic number (Z) = number of p+
Atomic mass unit (amu) =
contains 6 p+ and 6 n0.
1
12 (mass
of C atom) that
1 amu = 1.66054 x 10-24 g
Particle
e−
p+
n0
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Mass
(g)
9.1093826 x 10-28
1.67262129 x 10-24
1.67492728 x 10-24
Mass
(amu)
0.000548579
1.00728
1.00866
Charge
(atomic units)
−1
+1
0
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Atomic Numbers & Mass Numbers
The sum of the number of p+ and n0 in an atom is:
A = mass number
A ≈ mass (in amu) of an atom
For element X, write:
or
or
A
ZX
A
X
X-A
e.g.
e.g.
12
6
12
C
C
e.g. carbon-12
(Z is constant for a given element)
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Atomic Numbers & Mass Numbers
How many p+, n0 and e- are in the following elements:
63
29 Cu
25
27
Mg
Al
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29 p+ = 29 e- (neutral atom: e- = p+)
63−29 = 34 n0
12 p+ = 12 e- (periodic table; neutral)
25−12 = 13 n0
13 p+ = 13 e27−13 = 14 n0
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Mass Spectrometer
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Isotopes and Atomic Weight
Isotopes
Atoms of the same element with different A.
• equal numbers of p+
• different numbers of n0
Hydrogen isotopes:
deuterium (D)
tritium (T)
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H
1
1 p+, 0 n0
2
H
1
3
H
1
1 p+, 1 n0
1 p+, 2 n0
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Isotopes and Atomic Weight
Most elements occur as a mixture of isotopes.
Magnesium is a mixture of:
24Mg
number of p+
number of n0
mass / amu
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12
23.985
25Mg
12
13
24.986
26Mg
12
14
25.982
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Isotopes and Atomic Weight
For most elements, the percent abundance of its
isotopes are constant (everywhere on earth).
The periodic table lists an average atomic weight.
Example
Boron occurs as a mixture of 2 isotopes, 10B and 11B.
The abundance of 10B is 19.91%. Calculate the
atomic weight of boron.
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Isotopes and Atomic Weights
Boron occurs as a mixture of 2 isotopes, 10B and 11B. The abundance of 10B is
19.91%. Calculate the atomic weight of boron.
Atomic mass = Σ(fractional abundance)(isotope mass)
10B
19.91 (10.0129 amu) = 1.994 amu
100
% abundance of 11B = 100% - 19.91% = 80.09%
11B
80.09 (11.0093 amu) = 8.817 amu
100
Atomic weight for B = 1.994 + 8.817 amu
= 10.811 amu
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Isotopes and Atomic Weight
Periodic table:
5
B
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Atomic number (Z)
Symbol
Boron
Name
10.811
Atomic weight
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Amounts of Substances: The Mole
A counting unit – a familiar counting unit is a “dozen”:
1 dozen eggs
= 12 eggs
1 dozen donuts = 12 donuts
1 dozen apples = 12 apples
1 mole (mol) = Number of atoms in 12 g of 12C
• Latin for “heap” or “pile”
• 1 mol = 6.02214199 x 1023 “units”
• Avogadro’s number
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Amounts of Substances: The Mole
A green pea has a ¼-inch diameter. 48 peas/foot.
(48)3 / ft3 ≈ 1 x 105 peas/ft3.
V of 1 mol ≈ (6.0 x 1023 peas)/(1x 105 peas/ft3)
≈ 6.0 x 1018 ft3
U.S. surface area = 3.0 x 106 mi2
= 8.4 x 1013 ft2
height = V / area, 1 mol would cover the U.S. to:
6.0 x 1018 ft3
8.4 x 1013 ft2
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=7.1 x 104 ft = 14 miles !
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Amounts of Substances: The Mole
1 mole of an atom = atomic weight in grams.
1 Xe atom has mass = 131.29 amu
1 mol of Xe atoms has mass = 131.29 g
1 He atom has mass = 4.0026 amu
1 mol of He has mass = 4.0026 g
There are 6.022 x 1023 atoms in 1 mol of He and 1
mol of Xe – but they have different masses.
… 1 dozen eggs is much heavier than 1 dozen peas!
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Molar Mass and Problem Solving
Example
How many moles of copper are in a 320.0 g sample?
Cu-atom mass = 63.546 g/mol (periodic table)
Conversion factor: 1 mol Cu = 1
63.546 g
1 mol Cu
nCu = 320.0 g x
= 5.036 mol Cu
63.546 g
n = number of moles
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Molar Mass and Problem Solving
Calculate the number of atoms in a 1.000 g sample
of boron.
nB = (1.000 g) 1 mol B
10.81 g
= 0.092507 mol B
B atoms = (0.092507 mol B)(6.022  1023 atoms/mol)
= 5.571  1022 B atoms
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The Periodic Table
Summarizes
•
•
•
•
Atomic numbers.
Atomic weights.
Physical state (solid/liquid/gas).
Type (metal/non-metal/metalloid).
Periodicity
• Elements with similar properties are arranged in
vertical groups.
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The Periodic Table
In the USA, “A” denotes a
main group element…
International system
uses 1 … 18.
…”B” indicates a
transition element.
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The Periodic Table
Main group metal
Transition metal
Metalloid
Nonmetal
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The Periodic Table
Period
number
A period is a horizontal row
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The Periodic Table
Group 1A
Alkali metals (not H)
A group is a vertical column
Group 7A
Halogens
Group 8A
Noble gases
Group 2A
Alkaline
earth metals
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The Alkali Metals and Alkaline Earth Metals
Alkali metals (group 1A; 1)
Alkaline earth metals (group 2A; 2)
• Grey … silvery white
colored.
• Highly reactive.
• Never found as native
metals.
• Form alkaline solutions.
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Transition Elements, Lanthanides & Actinides
Transition Elements (groups 1B – 8B)
• Also called transition metals.
• Middle of table, periods 4 – 7.
• Includes the lanthanides & actinides.
Lanthanides and Actinides
• Listed separately at the bottom.
• Chemically very similar.
• Relatively rare on earth.
 (old name: rare earth elements)
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Groups 3A to 8A
Groups 3A to 6A
• Most abundant elements in the Earth’s crust
and atmosphere.
• Most important elements for living organisms.
Halogens (group 7A; 17)
• Very reactive non metals.
• Form salts with metals.
• Colored elements.
Noble gases (8A; 18)
• Very low reactivity.
• Colorless, odorless gases.
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