Chapter 2
Atoms,
Molecules,
and Ions
2|1
Contents and Concepts
Atomic Theory and Atomic Structure
The key concept in chemistry is that all matter is
composed of very small particles called atoms. We
look at atomic theory, discuss atomic structure,
and finally describe the periodic table, which
organizes the elements.
1.
2.
3.
4.
5.
Atomic Theory of Matter
The Structure of the Atom
Nuclear Structure; Isotopes
Atomic Masses
Periodic Table of the Elements
2|2
Chemical Substances: Formulas and Names
We explore how atoms combine in various ways to
yield the millions of known substances.
6.
7.
8.
Chemical Formulas; Molecules and Ionic
Substances
Organic Compounds
Naming Simple compounds
Chemical Reactions: Equations
9. Writing Chemical Equations
10. Balancing Chemical Equations
2|3
Learning Objectives
Atomic Theory and Atomic Structure
1. Atomic Theory of Matter
a. List the postulates of the atomic theory.
b. Define element, compound, and chemical
reaction in the context of these postulates.
c. Recognize the atomic symbols of the
elements.
d. Explain the significance of the law of
multiple proportions.
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2. The Structure of the Atom
a. Describe Thomson’s experiment in which
he discovered the electron.
b. Describe Rutherford’s experiment that led
to the nuclear model of the atom.
2|5
3. Nuclear Structure; Isotopes
a. Name and describe the nuclear particles
making up the nucleus of the atom.
b. Define atomic number, mass number, and
nuclide.
c. Write the nuclide symbol for a given
nucleus.
d. Define and provide examples of isotopes of
an element.
e. Write the nuclide symbol of an element.
2|6
4. Atomic Masses
a. Define atomic mass unit and atomic mass.
b. Describe how a mass spectrometer can be
used to determine the fractional abundance
of the isotopes of an element.
c. Determine the atomic mass of an element
from the isotopic masses and fractional
abundance.
2|7
5. Periodic Table of the Elements
a. Identify periods and groups on the periodic
table.
b. Find the main-group and transition
elements on the periodic table.
c. Locate the alkali metal and halogen groups
on the periodic table.
d. Recognize the portions of the periodic table
that contain the metals, nonmetals, and
metalloids (semimetals).
2|8
Chemical Substances: Formulas and Names
6. Chemical Formulas; Molecular and Ionic
Substances
a. Determine when the chemical formula of a
compound represents a molecule.
b. Determine whether a chemical formula is
also a molecular formula.
c. Define ion, cation, and anion.
d. Classify a compound as ionic or molecular.
e. Define and provide examples for the term
formula unit.
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f.
Specify the charge on all substances,
whether ionic and molecular.
g. Write an ionic formula, given the ions.
7. Organic Compounds
a. List the attributes of molecular substances
that make them organic compounds.
b. Explain what makes a molecule a
hydrocarbon.
c. Recognize some functional groups of
organic molecules.
2 | 10
8. Naming Simple Compounds
a. Recognize ionic compounds.
b. Learn the rules for predicting the charges of
monatomic ions in ionic compounds.
c. Apply the rules for naming monatomic ions.
d. Learn the names and charges of common
polyatomic ions.
e. Name an ionic compound from its formula.
f. Write the formula of binary compound from
its name.
2 | 11
g. Determine the order of elements in a binary
(molecular) compound.
h. Learn the rules for naming binary molecular
compounds.
i. Name a binary compound from its formula.
j. Write the formula of a binary compound
from its name.
k. Name a binary molecular compound from
its molecular model.
l. Recognize molecular compounds that are
acids.
2 | 12
m. Determine whether an acid is an oxoacid.
n. Learn the approach for naming binary acids
and oxoacids.
o. Write the name and formula of an anion
from the acid.
p. Recognize compounds that are hydrates.
q. Learn the rules for naming hydrates.
r. Name a hydrate from its formula.
s. Write the formula of a hydrate from its
name.
2 | 13
Chemical Reactions: Equations
9. Writing Chemical Equations
a. Identify reactants and products in a
chemical equation.
b. Write chemical equations using appropriate
phase labels, symbols of reaction
conditions, and the presence of a catalyst.
2 | 14
10. Balancing Chemical Equations
a. Determine if a chemical reaction is
balanced.
b. Master the technique for balancing
chemical equations.
2 | 15
Postulates of Dalton’s Atomic Theory
All matter is composed of indivisible atoms. An
atom is an extremely small particle of matter that
retains its identity during chemical reactions.
An element is a type of matter composed of only
one kind of atom.
2 | 16
A compound is a type of matter composed of
atoms of two or more elements chemically
combined in fixed proportions.
A chemical reaction involves the rearrangement
of the atoms present in the reacting substances to
give new chemical combinations present in the
substances formed by the reaction.
2 | 17
Atomic Symbol
A one- or two-letter notation used to represent an
atom corresponding to a particular element.
The first letter must be UPPERCASE.
The second letter, when present, must be
lowercase.
Symbols come from the element name;
several names come from Latin.
2 | 18
Law of Multiple Proportions
When two elements form more than one
compound, the masses of one element in these
compounds for a fixed mass of the other element
are in small, whole-number ratios.
2 | 19
As an example, compare the mass of oxygen in
the two oxygen compounds of carbon:
Carbon monoxide
12 g C
16 g O
Carbon dioxide
12 g C
32 g O
The ratio of oxygen in CO2 to CO:
oxygen in CO 2
32 g

2
oxygen in CO
16 g
2 | 20
?
A
Like Dalton, today’s chemists model
atoms using spheres. Modern models
are often drawn using computer
programs and use different colors to
represent atoms of different elements.
Which of these models depicts CO2?
B
C
E is CO2
D
E
2 | 21
Structure of the Atom
The atom is composed of two parts:
Nucleus
Central core
Positively charged
Contains most of the atom’s mass
Electrons
Very light
Negatively charged
Exist in the region around the nucleus
2 | 22
Discovery of the Electron
J. J. Thomson used an apparatus similar to the
one shown on the next slide, a cathode ray tube.
He discovered that the particles that make up the
cathode ray are negative and are part of all matter.
This finding is illustrated on the following slides.
As a result, Thomson concluded that atoms are
not indivisible, as Dalton had postulated.
2 | 23
Cathode Ray Tube
2 | 24
Cathode rays are attracted to
the positive end of the
magnet and repelled by the
negative end.
2 | 25
Thomson’s experiments allowed him to calculate
the ratio of the electron’s mass to its charge.
Robert Millikan calculated the charge on the
electron. He determined the charge on various
drops of oil and found the smallest increment in
charge—that is, the charge on the electron. This is
illustrated on the next slide.
2 | 26
2 | 27
Nuclear Model of the Atom
Experiments done in Ernest Rutherford’s
laboratory used positively charged alpha particles
to bombard very thin gold (and other metals) foil.
Most alpha particles passed through the foil, but a
few were scattered at large angles, sometimes
almost straight backward.
Based on these findings, Rutherford proposed that
99.95% of the atom’s mass is in the positively
charged nucleus.
2 | 28
2 | 29
2 | 30
Proton
A nuclear particle having a positive charge equal
to that of the electron and a mass more than 1800
times that of the electron.
The number of protons in an atom is called the
atomic number, Z.
An element is a substance whose atoms have the
same number of protons and thus the same atomic
number, Z.
2 | 31
Neutron
A nuclear particle having a mass almost equal to
that of the proton but no electrical charge
The mass number, A, is the total number of
protons and neutrons in the nucleus.
Isotopes are atoms whose nuclei have the same
atomic number (number of protons) but different
numbers of neutrons (mass number).
2 | 32
Nuclide
An atom characterized by a certain atomic
number, Z, and mass number, A.
Nuclide symbol
Examples:
A
Z Chemical symbol
23
11 Na
7
3 Li
13
6C
2 | 33
?
Write the nuclide symbol for the
atom that has 19 protons and 20
neutrons.
Atomic number: Z = 19
The element is potassium, K.
Mass number: A = 19 + 20 = 39
The nuclide symbol is
39
19 K
2 | 34
Atomic Mass
The average atomic mass for the naturally
occurring element expressed in atomic mass units
(amu).
Atomic Mass Unit
Equal to exactly one-twelfth the mass of a carbon12 atom
2 | 35
?
An element has four naturally occurring
isotopes. The mass and percentage of
each isotope are as follows:
Percentage Abundance
Mass (amu)
1.48
203.973
23.6
205.9745
22.6
206.9759
52.3
207.9766
What is the atomic weight and name of
the element?
2 | 36
To find the portion of the atomic weight due to
each isotope, multiply the fraction by the mass of
the isotope. The atomic weight is the sum of these
products.
Fractional
Abundance
0.0148
0.236
0.226
0.523
Mass (amu)
203.973
205.9745
206.9759
207.9766
Mass From
Isotope
3.01880040
48.6099820
46.7765534
108.771762
207.177098
The atomic weight is 207 amu; the element is lead.
2 | 37
Periodic Table of the Elements
A tabular arrangement of elements in rows and
columns, highlighting the regular repetition of
properties of the elements.
2 | 38
A period consists of the elements in one
horizontal row.
A group consists of the elements in one vertical
column.
Groups are numbered using two systems:
IUPAC
Numbers 1 through 18
Older system Roman numerals I–VIII and
the letters A and B
2 | 39
Metal
A substance or mixture that has a characteristic
luster or shine and is generally a good conductor
of heat and electricity.
Nonmetal
An element that does not exhibit the
characteristics of a metal
Metalloid
An element having characteristics of both metals
and nonmetals
2 | 40
Chemical Formula
A notation that uses atomic symbols with
numerical subscripts to convey the relative
proportions of atoms of the different elements in
the substance.
2 | 41
Molecule
A definite group of atoms that are chemically
bonded together (tightly connected by attractive
forces).
Molecular Formula
A formula giving the exact number of different
atoms of an element in a molecule.
2 | 42
Polymer
A very large molecule that is made up of a number
of smaller molecules repeatedly linked together.
Monomers
The small molecules that are linked together to
form a polymer.
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
2 | 43
An ion is an electrically charged particle obtained from an
atom or chemically bonded group of atoms by adding or
removing one or more electrons.
A cation is a positively charged ion formed by losing one or
more electrons. A cation is named by its element name
followed by the word ion. For example, Na+ is the sodium
ion, etc. Under the definition of anion add the following: An
anion is named by changing the last part of the element
name to –ide followed by the word ion. For example, F- is
the fluoride ion, O2- is the oxide ion, N3- is the nitride ion,
etc.
An anion is a negatively charged ion formed by gaining
one or more electrons.
2 | 44
An ionic compound is composed of cations and
anions.
Ions are arranged in a repeating three-dimensional
pattern, forming a crystal.
The formula of an ionic compound gives the
smallest possible integer number of ions in the
substance (without writing charges) so that the
combination is electrically neutral.
The formula gives the formula unit of the
compounds. A formula unit is not a molecule!
2 | 45
?
What is formula of the ionic
compound of Mg2+ and N3-?
The common multiple of the charges is 6, so we
need three Mg2+ and two N3-. The resulting formula
is
Mg3N2
2 | 46
?
What is the formula of the ionic
compound of Ca2+ and PO43-?
The common multiple of the charges is 6, so we
need three Ca2+ and two PO43-. The resulting
formula is
Ca3(PO4)2
2 | 47
Organic Compounds
An important class of molecular substances; they
contain carbon combined with other elements –
notably hydrogen, oxygen, and nitrogen.
Hydrocarbons contain only carbon and hydrogen.
2 | 48
A functional group is a reactive portion of a
molecule that undergoes predictable reactions.
Functional
Group
OH
O
O
C
Name of
Group
Examples
Alcohol
Methyl alcohol
Ether
Dimethyl ether
Carboxylic
acid
Acetic acid
OH
2 | 49
Chemical nomenclature is the systematic naming
of chemical compounds.
Compounds that are not organic are called
inorganic compounds.
Carbon monoxide, carbon dioxide,
carbonates, and cyanides are also classified
as inorganic compounds.
2 | 50
Naming Inorganic Compounds
1. The cation is always named first with the
name of the cation without the word ion.
2. The anion is named second with the name
of the anion without the word ion.
3. Examples:
Cation
Na+
Mg2Al3+
Anion
BrO2Cl-
Name
Sodium bromide
Magnesium oxide
Aluminum chloride
2 | 51
Monatomic ions
Ions formed from a single atom.
Common monatomic ions are listed on the next
slide by group and by period.
2 | 52
Common Monatomic Ions of the
Main-Group Elements
Period
IA
IIA
IIIA
IVA
VA
VIA
VIIA
H-
1
2
Li+
Be2+
N3-
3
Na+ Mg2+ Al3+
4
K+
Ca2+ Ga3+
5
Rb+
Sr2+
6
Cs+
Ba2+ Tl3+, Pb2+
Tl+
In3+
Sn2+
O2-
F-
S2-
Cl-
Se2-
Br-
Te2-
I-
Bi3+
2 | 53
Rules for Predicting the Charge on a
Monatomic Ion
1. Most main-group metals have one monatomic
ions with a charge equal to the group number.
Al, in Group IIIA(3), has one ion, Al3+.
Sr, in Group IIA(2), has one ion, Sr2+.
K, in Group IA(1), has one ion, K+.
2 | 54
2. Some main-group metals with high atomic
number have more than one cation. One cation
will have the charge of the group number minus
2; the second cation will have a charge equal to
the group number
Pb in Group IVA(14) has two ions:
Pb2+ and Pb4+
Tl in Group IIIA(13) has two ions:
Tl+ and Tl3+
2 | 55
3. Most transition metals form more than one
cation, of which one is +2.
Zn and Cd form only the +2 ion.
Ag forms only the +1 ion.
4. Nonmetal main-group elements form one
monatomic anion with a charge equal to the
group number minus 8.
F in Group VIIA(17) forms the F- ion.
S in Group VIA(16) forms the S2- ion.
N in Group VA(15) forms the N3- ion.
2 | 56
Naming Monatomic Ions
Monatomic cations are named after the element if
the element forms only one cation.
2 | 57
If more than one cation forms:
a. In the Stock system, the charge is written
using a Roman numeral and is enclosed in
parentheses.
Cu2+ is copper(II).
Cu+ is copper(I).
b. In an older system, the suffic –ic (for the
higher-charged cation) or –ous for the
lower-charged cation) is added to the
element’s stem.
Cu2+ is cupric.
Cu+ is cuprous.
2 | 58
Fe3+ is iron(III) or ferric ion.
Fe2+ is iron(II) or ferrous ion.
Hg2+ is mercury(II) or mercuric ion.
The second ion mercury forms is diatomic:
Hg22+ is mercury(I) or mercurous ion.
2 | 59
Cr3+ is chromium(III) or chromic ion.
Cr2+ is chromium(II) or chromous ion.
Mn2+ is manganese(II) or manganous ion.
Co2+ is cobalt(II) or cobaltous ion.
Zinc forms only Zn2+, so it is called zinc ion.
Cadmium forms only Cd2+, so it is called cadmium
ion.
Silver forms only Ag+, so it is called silver ion.
2 | 60
Polyatomic Ion
An ion consisting of two or more atoms chemically
bonded together and carrying an electrical charge.
Table 2.5 lists common polyatomic ions.
2 | 61
Cations
mercury(I) or mercurous
ammonium
Hg22+
NH4+
Anions
peroxide
hydroxide
cyanide
O2OHCN2 | 62
phosphate
monohydrogen phosphate
dihydrogen phosphate
PO43HPO42H2PO4-
carbonate
hydrogen carbonate (bicarbonate)
CO32HCO3-
sulfate
hydrogen sulfate (bisulfate)
SO42HSO4-
sulfite
hydrogen sulfite (bisulfite)
SO32HSO32 | 63
acetate
C2H3O2-
oxalate
C2O42-
chromate
dichromate
CrO42Cr2O72-
permanganate
MnO4-
nitrate
nitrite
NO3NO22 | 64
hypochlorite
chlorite
chlorate
perchlorate
ClOClO2ClO3ClO4-
2 | 65
?
What are the names of the following
ionic compounds?
BaO
Cr2(SO4)3
BaO is barium oxide.
Cr2(SO4)3 is chromium(III) sulfate or chromic
sulfate.
2 | 66
?
What are the chemical formulas for
the following ionic compounds?
potassium carbonate
manganese(II) sulfate
The ions K+ and CO32- form K2CO3
The ions Mn2+ and SO42- form MnSO4
2 | 67
Binary Molecular Compounds
A compound composed of only two elements.
Binary compound of a metal and a nonmetal are
generally named using ionic rules.
2 | 68
Naming Binary Molecular Compounds
We usually name the elements in the order given
in the formula.
Name the first element using the element name.
Name the second element using the element root
+ -ide suffix.
2 | 69
Add a prefix to each name to indicate the number
of atoms of that element. The prefix mono- is used
only when needed to distinguish two compounds
of the same two elements.
The final vowel of the prefix is often dropped when
followed by an element name that begins with a
vowel. Oxygen is the most common example.
N2O4 dinitrogen tetroxide
(“a” is dropped)
NO
nitrogen monoxide
(only one “o”)
(also called nitric oxide)
2 | 70
Prefixes (Greek)
One (1)
monoTwo (2)
diThree (3)
triFour (4)
tetraFive (5)
pentaSix (6)
hexaSeven (7)
heptaEight (8)
octaNine (9)
nonaTen (10)
deca2 | 71
Some compounds have common names that differ
from their systematic names:
H2S
H2O
NH3
hydrogen sulfide (the “di” is omitted)
water
ammonia
Common names need to be memorized.
2 | 72
Acids and Corresponding Anions
Oxoacids contain hydrogen, oxygen, and a third
central atom.
To name an acid from its anion name:
1. Change an –ate suffix to –ic.
2. Change an –ite suffix to –ous.
3. Add the word “acid.””
For example:
HNO3
H2SO4
nitric acid
sulfuric acid
2 | 73
Oxoanion
CO32-
Oxoacid
NO2-
carbonate
ion
nitrite ion
NO3-
nitrate ion
PO43-
phosphate ion H3PO4 phosphoric
acid
sulfite ion
H2SO3 sulfurous
acid
sulfate ion
H2SO4 sulfuric
acid
SO32SO42-
H2CO3 carbonic
acid
HNO2 nitrous
acid
HNO3 nitric acid
2 | 74
Oxoacid
Oxoanion
ClOClO2ClO3ClO4-
hypochlorite
ion
chlorite
ion
chlorate
ion
perchlorate
ion
HClO
HClO2
HClO3
HClO4
hypochlorous
acid
chlorous
acid
chloric
acid
perchloric
acid
2 | 75
?
What are the names of the
following compounds?
OF2
S4N4
BCl3
OF2 is oxygen difluoride
S4N4 is tetrasulfur tetranitride
BCl3 is boron trichloride
2 | 76
?
What are the formulas for the
following binary molecular
compounds?
carbon disulfide
nitrogen tribromide
dinitrogen tetrafluoride
The formula for carbon disulfide is CS2.
The formula for nitrogen tribromide is NBr3.
The formula for dinitrogen tetrafluoride is N2F4.
2 | 77
?
Bromine has an oxoacid, HBrO2,
bromous acid (compare to HClO2,
chlorous acid). What are the name
and formula of the corresponding
anion?
The anion corresponding to HBrO2 is
bromite, BrO2-.
2 | 78
Hydrate
A compound that contains water molecules weakly
bound in the crystals.
The formula of a hydrate is written with a dot
before the water molecule(s) included.
For example:
CuSO45H2O
2 | 79
Hydrates are named using the anhydrous (without
water) compound name followed by the prefix for
the number of water molecules included and the
word “hydrate.”
For example:
CuSO45H2O is named
copper(II) sulfate pentahydrate.
2 | 80
?
A compound whose common name
is green vitriol has the chemical
formula FeSO47H2O. What is the
chemical name of this compound?
FeSO47H2O is iron(II) sulfate heptahydrate.
2 | 81
?
Calcium chloride hexahydrate is
used to melt snow on roads. What
is the chemical formula of the
compound?
The chemical formula for calcium chloride
hexahydrate is CaCl26H2O.
2 | 82
A chemical equation is the symbolic
representation of a chemical reaction in terms of
chemical formulas.
For example:
2Na + Cl2  2NaCl
Reactants are the starting materials; they are
written on the left of the equation.
Products are the materials at the end of the
reaction; they are written on the right of the
equation.
2 | 83
Because a reaction must accurately describe the
chemical reaction, it must be consistent with the
law of conservation of mass.
When this is not the case, after correct formulas
are written for each reactant and product, the
coefficients are adjusted so that the same number
of each atom is present in both the reactants and
the products.
This is called balancing the equation.
2 | 84
For example, the reaction of sodium with chlorine
produced sodium chloride.
First, we determine the correct formula for each
compound.
Sodium is Na.
Chlorine is Cl2.
Sodium chloride is NaCl.
2 | 85
Second, we write the reaction.
Na + Cl2  NaCl
Third, we check the number of each atom on each
side of the equation.
This equation shows two Cl atoms on the reactant
side and only one Cl atom on the product side. To
balance the Cl atoms, we insert a coefficient of “2”
before NaCl on the product side.
Na + Cl2  2NaCl
2 | 86
Na + Cl2  2NaCl
Now the Na are not balanced: there is one on the
reactant side and there are two on the product
side. To balance Na, we insert the coefficient “2”
before Na on the reactant side.
2Na + Cl2  2NaCl
The reaction is now balanced!
2 | 87
?
Balance the following equation:
CS2 + O2  CO2 + SO2
Tally the number of each atom on each side:
C 1 on reactant side; 1 on product side
S 2 on reactant side; 1 on product side
O 2 on reactant side; 4 on product side
Begin by inserting the coefficient “2” before
SO2 on the product side. We leave O2 until
later because it is an element.
2 | 88
CS2 + O2  CO2 + 2SO2
Tally the atoms again:
C
1 on reactant side; 1 on product side
S
2 on reactant side; 2 on product side
O
2 on reactant side; 6 on product side
Insert a “3” before O2:
CS2 + 3O2  CO2 + 2SO2
2 | 89
CS2 + 3O2  CO2 + 2SO2
Tally the atoms again:
C
1 on reactant side; 1 on product side
S
2 on reactant side; 2 on product side
O
6 on reactant side; 6 on product side
The reaction is now balanced!
2 | 90
?
Balance the following equation:
NH3 + O2  NO + H2O
Tally the number of each atom on each side:
N 1 on reactant side; 1 on product side
H 3 on reactant side; 2 on product side
O 2 on reactant side; 2 on product side
Begin by inserting the coefficient “2” before
NH3 on the reactant side and the coefficient “3”
before H2O on the product side. We leave O2
until later because it is an element.
2 | 91
2NH3 + O2  NO + 3H2O
Tally the atoms again:
N
2 on reactant side; 1 on product side
H
6 on reactant side; 6 on product side
O
2 on reactant side; 4 on product side
To balance N, insert a “2” before NO:
2NH3 + O2  2NO + 3H2O
2 | 92
2NH3 + O2  2NO + 3H2O
Tally the atoms again:
N
2 on reactant side; 2 on product side
H
6 on reactant side; 6 on product side
O
2 on reactant side; 5 on product side
Since this gives us an odd number oxygens, we
double the coefficients on NH3, NO, and H2O
and to balance O, insert a “5” before O2.
2 | 93
Tally the atoms again to double check:
4NH3 + 5O2  4NO + 6H2O
N
H
O
4 on reactant side; 4 on product side
12 on reactant side; 12 on product side
10 on reactant side; 10 on product side
The reaction is now balanced!
2 | 94
?
Balance the following equation:
C2H5OH + O2  CO2 + H2O
Tally the number of each atom on each side:
C 2 on reactant side; 1 on product side
H 6 on reactant side; 2 on product side
O 3 on reactant side; 3 on product side
Begin by balancing H. Insert the coefficient “3”
before H2O on the product side. We leave O2
until later because it is an element.
2 | 95
C2H5OH + O2  CO2 + 3H2O
Tally the number of each atom on each side:
C
2 on reactant side; 1 on product side
H
6 on reactant side; 6 on product side
O
3 on reactant side; 5 on product side
To balance C, insert a “2” before CO2.
2 | 96
C2H5OH + O2  2CO2 + 3H2O
Tally the number of each atom on each side:
C
2 on reactant side; 2 on product side
H
6 on reactant side; 6 on product side
O
3 on reactant side; 7 on product side
To balance O, insert a “3” before O2.
2 | 97
C2H5OH + 3O2  2CO2 + 3H2O
Tally the number of each atom on each side:
C
2 on reactant side; 2 on product side
H
6 on reactant side; 6 on product side
O
7 on reactant side; 7 on product side
The reaction is now balanced!
2 | 98