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Atoms, Molecules and Ions
Chapter 2
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Atomic Theory
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•
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Democritus – teacher, 4th Century BC
Atomist school of thought
Matter is composed of tiny particles called
atoms
These atoms are invisible, indestructible
fundamental units of matter
John Dalton
• English school teacher
• Studied the ratios in which
elements combine in
chemical reactions
• formulated Dalton’s Atomic
Theory
Dalton’s Atomic Theory (1808)
1. Elements are composed of extremely small particles
called atoms.
2. All atoms of a given element are identical, having the
same size, mass and chemical properties. The atoms
of one element are different from the atoms of all other
elements.
3. Compounds are composed of atoms of more than one
element. In any compound, the ratio of the numbers of
atoms of any two of the elements present is either an
integer or a simple fraction.
4. A chemical reaction involves only the separation,
combination, or rearrangement of atoms; it does not
result in their creation or destruction.
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Dalton’s Atomic Theory
3. Compounds are composed of atoms of more than one element. In any
compound, the ratio of the numbers of atoms of any two of the elements
present is either an integer or a simple fraction.
Mass Ratio 16:12
Mole Ratio 1:1
Law of Definite Proportions (Proust-1799)
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Dalton’s Atomic Theory
Mass Ratio 16:12
Mole Ratio 1:1
Mass Ratio 32:12
Mole Ratio 2:1
Law of Multiple Proportions
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4. A chemical reaction involves only the separation,
combination, or rearrangement of atoms; it does not result in
their creation or destruction.
16 X
+
8Y
8 X2Y
Law of Conservation of Mass
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John Joseph Thomson
(1856-1940)
• English Physicist
• 1897 – discovered electrons:
negatively charged subatomic
particles
• Experiments using flow of electric
current through gases
Cathode Ray Tube
J.J. Thomson, measured mass/charge of e(1906 Nobel Prize in Physics)
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Cathode Ray Tube
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Thomson’s Model
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Robert Andrews Millikan
(1868-1953)
• American Physicist
• 1923 – Nobel prize in physics for
determining the charge of the
electron.
• Experiments using a single drop
of oil with a static charge.
Millikan’s Experiment
Measured mass of e(1923 Nobel Prize in Physics)
e- charge = -1.60 x 10-19 C
Thomson’s charge/mass of e- = -1.76 x 108 C/g
e- mass = 9.10 x 10-28 g
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Wilhelm Konrad Röntgen
(1845-1923)
• German Physicist
• 1901 – Nobel prize in physics for discovery
of x-rays.
• Observed that the cathode ray caused
glass and metals to emit unusual rays.
Becquerel and Curie continued experiments
with radioactivity
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Thomson’s Model
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Ernest Rutherford
(1871-1937)
• University of Manchester, England
• Tested theory of atomic structure
• Bombarded thin gold foil with a beam of
‘alpha’ particles.
• If the positive charge was evenly spread
out, the beam should have easily
passed through.
Rutherford’s Experiment
(1908 Nobel Prize in Chemistry)
 particle velocity ~ 1.4 x 107 m/s
(~5% speed of light)
1. atoms positive charge is concentrated in the nucleus
2. proton (p) has opposite (+) charge of electron (-)
3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)
“It was as incredible as if you had fired a 15-inch shell at a piece of tissue paper
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and it came back and hit you.”
Chadwick’s Experiment (1932)
(1935 Noble Prize in Physics)
H atoms - 1 p; He atoms - 2 p
mass He/mass H should = 2
measured mass He/mass H = 4
 + 9Be
1n
+ 12C + energy
neutron (n) is neutral (charge = 0)
n mass ~ p mass = 1.67 x 10-24 g
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Rutherford’s Model of
the Atom
atomic radius ~ 100 pm = 1 x 10-10 m
nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
“If the atom is the Houston
Astrodome, then the nucleus is a
marble on the 50-yard line.” 20
mass p ≈ mass n ≈ 1840 x mass e21
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Atomic number, Mass number and Isotopes
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Isotopes are atoms of the same element (X) with different
numbers of neutrons in their nuclei
Mass Number
A
ZX
Atomic Number
1
1H
235
92
2
1H
U
Element Symbol
(D)
238
92
3
1H
U
(T)
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Isotopes
Elements can also be represented
using name and mass number
Carbon – 12
How many protons?
How many neutrons?
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Problem 2.14
Calculate the number of neutrons in an atom
239
of 94 Pu .
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The Isotopes of Hydrogen
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How many protons, neutrons, and electrons are
14
in 6 C ?
How many protons, neutrons, and electrons are
11
in 6 C ?
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Problem 2.16
Indicate the number of protons, neutrons, and
electrons in each of the following species:
15
7
N
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16
S
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Cu
___ protons ____neutrons ____electrons
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Problem 2.16
Indicate the number of protons, neutrons, and
electrons in each of the following species:
84
38
Sr
130
56
Ba
186
74
W
202
80
Hg
___ protons ____neutrons ____electrons
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Rubidium has two common isotopes, 85Rb and
87Rb. If the abundance of 85Rb is 72.2% and the
abundance of 87Rb is 27.8%, what is the average
atomic mass of rubidium?
Uranium has three common isotopes. If the
abundance of 234U is 0.01%, the abundance of
235U is 0.71%, and the abundance of 238U is
99.28%, what is the average atomic mass of
uranium?
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Titanium has five common isotopes:
46Ti (8.0%), 47Ti (7.8%), 48Ti (73.4%),
49Ti (5.5%), 50Ti (5.3%)
What is the average atomic mass of titanium?
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The Periodic Table
• Dmitri Mendeleev (1834-1907)
– Russian chemist
– Listed elements in columns in order of
increasing atomic mass.
– Arranged columns so that elements
with similar properties were side by
side.
The Periodic Table
• Medeleev left blank spaces
where there were no known
elements with the appropriate
properties or mass.
• Predicted the properties of the
missing elements.
The Periodic Table
• Henry Mosely (1887-1915)
– British Physicist
– Determined the atomic number of the
atoms of the elements.
– Arranged elements in table by atomic
number instead of mass.
The Modern Periodic Table
• Each horizontal row is a period
– Seven periods
– From 2 to 32 elements in a period
– Properties of the elements change as
you move across a period.
– This pattern repeats from period to
period
The Periodic Law
The Modern Periodic Table
• Each column is a group or
family
– Elements in a group have similar
physical and chemical properties.
– Groups are identified by a number
and the letter A or B
– Group A are the representative
elements
– Group A can be divided into three
broad classes
The Modern Periodic Table
• Metals
– High electrical conductivity
– High luster when clean
– Ductile
– malleable
The Modern Periodic Table
• Metals
– Alkali metals
– Alkaline earth metals
– Transition metals
– Inner transition metals
The Modern Periodic Table
• Non-Metals
– poor electrical conductivity
– Non-lustrous
The Modern Periodic Table
• Non-Metals
– halogens
– Noble gases
The Modern Periodic Table
• Metalloids
– Elements with properties that are
intermediate between those of metals
and non-metals.
The Modern Periodic Table
Transition Metals
Inner Transition Metals
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Noble Gas
Halogen
Group
Alkali Earth Metal
Alkali Metal
Period
An ion is an atom, or group of atoms, that has a net
positive or negative charge due to a loss or gain of
electrons
Where are the electrons in the atom?
Definitions
Orbital – Area around the nucleus where electrons
reside
Valence electrons – electrons in the outermost orbital
Octet Rule – to be “happy” atoms want 8 valence
electrons – LOW ENERGY
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An ion is an atom, or group of atoms, that has a net
positive or negative charge.
Na
11 protons
11 electrons
Na+
11 protons
___
___
10 electrons
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
P
___ protons
___ electrons
3-
P
15 protons
___
18 electrons
___
anion – ion with a negative charge
If a neutral atom gains one or more electrons
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it becomes an anion.
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Ar
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Common Ions Shown on the Periodic Table
H+
Ga3+
In3+
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Common Ions Shown on the Periodic Table
H+
1+
12+
2-
Ga3+
In3+
3-
3+
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Common Ions Shown on the Periodic Table
1+
2+
3+
3- 2- 1-
H+
Ga3+
In3+
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A monatomic ion contains only one atom
Na+, Cl-, Ca2+, O2-, Al3+, N3-
A polyatomic ion contains more than one atom
OH-, CN-, NH4+, NO3-
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ionic compounds consist of a combination of cations
and anions (metal and a nonmetal)
•The sum of the charges on the cation(s) and anion(s) in each
formula unit must equal zero
The ionic compound NaCl
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Na Cl
Will magnesium and chlorine form the octet together?
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The most reactive metals (green) and the most reactive
nonmetals (blue) combine to form ionic compounds.
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Formula of Ionic Compounds
2 x +3 = +6
3 x -2 = -6
Al2O3
Al3+
1 x +2 = +2
Ca2+
1 x +2 = +2
Na+
O22 x -1 = -2
CaBr2
Br1 x -2 = -2
Na2CO3
CO3258
Write the formula for the ionic compound
chromium(III) sulfate.
Write the formula for the ionic compound
titanium(IV) oxide.
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A molecule is an aggregate of two or more atoms in a
definite arrangement held together by covalent bonds
H2
H2O
NH3
CH4
A diatomic molecule contains only two atoms
H2, N2, O2, Br2, HCl, CO
diatomic elements
A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4
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Formulas and Models
61
Write the molecular formula for methanol based on
the figure below.
1 - carbon
1 – oxygen
4 - hydrogen
CH4O or
CH3OH
Write the molecular formula for chloroform
based on the figure below.
1 - carbon
3 – chlorine
1 - hydrogen
CHCl3
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A molecular formula shows the exact number of
atoms of each element in the smallest unit of a
substance
An empirical formula shows the simplest
whole-number ratio of the atoms in a substance
molecular
Covalent
Compounds
Nonmetal +
nonmetal
H2O
C6H12O6
empirical
H2O
CH2O
O3
O
N2H4
NH2
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Write the empirical formula for each of the
following molecules:
molecular
empirical
Acetylene
C2H2
CH
Nicotine
C8H10N4O2
C4H5N2O
Nitrous oxide
N2O
N2O
Ethane
C2H6
CH3
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Problem 2.46
What are the empirical formulas of the
following compounds?
a) Al2Br6
b) Na2S2O4
c) N2O5
d) K2Cr2O7
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Ask Yourself…
• Binary or tertiary?
– Binary – composed of 2 different elements
• MgBr2, H2O, CrO
– Tertiary – composed of 3 or more different elements
• BaSO4, AgNO3
• Ionic, Covalent, or Acid?
– Ionic = metal + nonmetal - MgBr2
– Covalent = two nonmetals - SiO2
– Acid (first element is hydrogen)
• Binary Acids = acid that contains 2 elements (hydrogen and a
nonmetal)
• Oxyacids = hydrogen, oxygen, and a third element (nonmetal)
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Chemical Nomenclature
• Ionic Compounds
– Often a metal + nonmetal
– Anion (nonmetal), add “ide” to element name
BaCl2
barium chloride
K2O
potassium oxide
Mg(OH)2
magnesium hydroxide
KNO3
potassium nitrate
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• Some transition metals can have more than 1
charge
– indicate charge on metal with Roman numerals
FeCl2
2 Cl- -2 so Fe is +2
iron(II) chloride
FeCl3
3 Cl- -3 so Fe is +3
iron(III) chloride
Cr2S3
3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide
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Name the following compounds:
Cu(NO3)2
copper (II) nitrate
or cupric Nitrate
KH2PO4
potassium dihydrogen phosphate
NH4ClO3
ammonium chlorate
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• Molecular compounds
− Nonmetals or nonmetals + metalloids
− Common names
− H2O, NH3, CH4,
− Element furthest to the left in a period
and closest to the bottom of a group on
periodic table is placed first in formula
− If more than one compound can be
formed from the same elements, use
prefixes to indicate number of each kind
of atom
− Last element name ends in ide
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Molecular (covalent) Compounds
HI
hydrogen iodide
NF3
nitrogen trifluoride
SO2
sulfur dioxide
N2Cl4
dinitrogen tetrachloride
NO2
nitrogen dioxide
N2O
dinitrogen monoxide
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Exceptions – compounds containing Hydrogen
B2H6
diborane
CH4
methane
SiH4
silane
NH3
ammonia
H2O
water
H2S
hydrogen sulfide
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An acid can be defined as a substance that yields
hydrogen ions (H+) when dissolved in water.
For example: HCl gas and HCl in water
•Pure substance, hydrogen chloride
•Dissolved in water (H3O+ and Cl−),
hydrochloric acid
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An oxoacid is an acid that contains hydrogen,
oxygen, and another element.
HNO3
nitric acid
H2CO3
carbonic acid
H3PO4
phosphoric acid
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Naming Oxoacids and Oxoanions
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The rules for naming oxoanions, anions of
oxoacids, are as follows:
1. When all the H ions are removed from the
“-ic” acid, the anion’s name ends with “-ate.”
2. When all the H ions are removed from the
“-ous” acid, the anion’s name ends with “-ite.”
3. The names of anions in which one or more
but not all the hydrogen ions have been
removed must indicate the number of H ions
present.
For example:
– H2PO4- dihydrogen phosphate
– HPO4 2- hydrogen phosphate
– PO43- phosphate
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A base can be defined as a substance that yields
hydroxide ions (OH-) when dissolved in water.
NaOH
sodium hydroxide
KOH
potassium hydroxide
Ba(OH)2
barium hydroxide
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Hydrates are compounds that have a specific
number of water molecules attached to them.
BaCl2•2H2O
barium chloride dihydrate
LiCl•H2O
lithium chloride monohydrate
MgSO4•7H2O
magnesium sulfate heptahydrate
Sr(NO3)2 •4H2O
strontium nitrate tetrahydrate
CuSO4•5H2O
CuSO4
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