Ch. 5: Molecules and
Compounds
Dr. Namphol Sinkaset
Chem 152: Introduction to
General Chemistry
I. Chapter Outline
I. Introduction
II. Chemical Formulas
III. Views of Elements/Compounds
IV. Naming “Type I” Compounds
V. Naming “Type II” Compounds
VI. Polyatomic Ions
VII. Naming Acids
VIII. Naming “Type III” Compounds
IX. Molecular Masses
I. Sugar
• Sugar is composed
of carbon, oxygen,
and hydrogen
atoms.
• Properties of sugar
completely different
than elements from
which it’s made.
I. Sodium and Chlorine…
I. Versus Sodium Chloride
I. Elements in Compounds
• When an element forms a compound,
its properties change completely.
• Generally, properties of the compound
have no correlation to the original
elements.
• In this chapter, we see how elements
become compounds and cover
chemical nomenclature.
I. Molecules of a Compound
Are the Same
• Law of Constant
Composition: all
samples of a given
compound have the
same proportions of
their constituent
elements.
• Generally, this is
expressed as a
mass ratio.
I. Water’s Mass Ratio
• If 18.0 g of water is decomposed into it’s
elements O and H, there would be 16.0
g of O and 2.0 g of H.
• The O:H mass ratio is thus 8.0:1.0.
• Any sample of water would have this
exact same ratio.
II. Representing Compounds
• Chemical formulas are used to refer to
compounds.
• chemical formula: a way to show the
elements present in a compound and
the relative numbers of each elemental
atom.
• The most common is the molecular
formula.
II. Chemical Formulas
• There are three types of
formulas.
 molecular: gives the actual
number of atoms of each
element in a molecule of a
compound (e.g. H2O2)
 empirical: gives the relative
number of atoms of each
element in a compound (e.g.
HO)
 structural: uses lines to
represent covalent bonds and
shows interconnectivity
II. Writing Molecular Formulas
• The more metallic element is generally
listed first.
 Metallic character increases to the left and
down on the periodic table.
• Subscripts indicate the number of that
type of atom in the compound.
• If groups of atoms behave as an
independent entity, parentheses are
used.
II. Molecular Formulas
II. Chemical Models
• Formulas lead to models which give an idea
of the 3-D shape of a molecule.
II. From Macroscopic to
Symbolic
III. Pure Substances
III. Atomic Elements
• If element exists as
individual atoms, it
is named as
“atomic.”
• e.g. atomic mercury
III. Molecular Elements
• Some elements
occur naturally as
groups of two or
more atoms.
• These are named
“molecular” or
“diatomic” (for
two).
III. Molecular Compounds
• Compounds
formed from two
or more
nonmetals.
III. Ionic Compounds
• Comprised of
cations and anions.
• A formula unit is
the smallest
electrically-neutral
collection of ions.
IV. Chemical Nomenclature
• Like any specialized field, chemistry has
its own language.
• The ability to name and recognize
names of chemical entities is very
important.
• The naming system is LOGICAL!!
• The periodic table is indispensable
when you are first learning
nomenclature.
IV. Type I Compounds
• Type I compounds are ionics that have a
metal from Groups 1 or 2 and a nonmetal
from Groups 14-17.
• Examples:
 NaCl = sodium chloride
 MgBr2 = magnesium bromide
 K2S = potassium sulfide
IV. Type I Compounds
• To get a formula from a name,
remember that a compound must be
neutral.
• Ion charges can be found by locating
the element on the periodic table.
• “The charge on one becomes the
subscript of the other.”
IV. Type I Compounds
IV. Sample Problem
• Give the correct name or formula for the
compounds below.
a)
b)
c)
d)
sodium nitride
CaCl2
potassium sulfide
MgO
V. Transition Metals
• Transition metals are found in the
“Valley,” Groups 3-12, of the periodic
table.
• Transition metal cations often can carry
different charges, e.g. Fe2+ and Fe3+.
• Thus, a name like “iron chloride” is
ambiguous.
V. Type II Compounds
• Type II compounds are ionics that have a
transition metal (Groups 3-12) and a
nonmetal (Groups 14-17).
• Examples:
 FeCl2 = iron(II) chloride
 FeCl3 = iron(III) chloride
V. Sample Problem
•
e.g. Give the correct name or formula
for the compounds below.
a)
b)
c)
d)
e)
MnO2
copper(II) chloride
AuCl3
molybdenum(VI) fluoride
W2O3
V. Some Transition Metal Cations
VI. Additional Complications
• To make naming ionic compounds
harder, sometimes polyatomic ions are
involved.
• polyatomic ion: two or more atoms that
are bonded covalently and have a net
positive or negative charge
VI. Common Polyatomic Ions
VI. Oxyanion Families
• oxyanion: anion containing oxygen
• There are families of oxyanions, and
they have a systematic naming system.
• Have either two- or four-member
families.
 e.g. NO2- and NO3 e.g. ClO-, ClO2-, ClO3-, and ClO4-
VI. Two-Member Families
• For a two-member family, oxoanion with
fewer O atoms is given the “–ite” suffix
while the one with more O atoms is
given the “–ate” suffix.
 e.g. NO2- = nitrite
and NO3- = nitrate
VI. Four-Member Families
• For the four-member families, the
prefixes “hypo-” and “per-” are used to
indicate fewer or more oxygen atoms.
• e.g. the chlorine oxyanions




ClO- = hypochlorite
ClO2- = chlorite
ClO3- = chlorate
ClO4- = perchlorate
VI. Oxoanion Naming
Summary
VI. Sample Problem
•
e.g. Give names or formulas for the
following compounds.
a)
b)
c)
d)
e)
f)
Na2CO3
magnesium hydroxide
potassium nitrate
CoPO4
nickel(II) sulfate
NaClO2
VII. Acids
•
•
Acids are special ionic compounds
that have H+ as the cation.
There are two categories of acids that
have different naming rules.
1) Binary acids contain only hydrogen and a
nonmetal.
2) Oxyacids contain hydrogen, a nonmetal,
and oxygen.
VII. Naming Binary Acids
• Examples:
 HCl = hydrochloric acid
 HBr = hydrobromic acid
 H2Se = hydroselenic acid
VII. Naming Oxyacids
Examples of oxyacids:
• Set 1
• Set 2










HNO3 = nitric acid
H2SO4 = sulfuric acid
HClO3 = chloric acid
HClO4 = perchloric acid
H2CO3 = carbonic acid
H3PO4 = phosphoric acid
HNO2 = nitrous acid
HClO2 = chlorous acid
HClO = hypochlorous acid
H2SO3 = sulfurous acid
VII. Naming Oxyacids
-ate oxyanions become –ic acids.
-ite oxyanions become –ous acids.
VIII. Type III Compounds
•
•
Type III compounds are covalent (nonmetal
bonded to nonmetal).
Naming rules:
1) More metallic element is named 1st using the
normal element name EXCEPT when halogens
are bonded to oxygen.
2) Second element is named using its root and the
“-ide” suffix.
3) #’s of atoms indicated with Greek prefixes
EXCEPT when there is only one atom of the first
element.
VIII. Greek Prefixes
VIII. Type III Compounds
• Some examples:




ClO2 = chlorine dioxide
N2O5 = dinitrogen pentoxide
S2Cl2 = disulfur dichloride
SeF6 = selenium hexafluoride
VIII. Naming Practice
•
e.g. Indicate the “Type” and give the correct
formula or name of the compounds below.
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
l)
m)
CoCl3
dichlorine heptaoxide
SrO
magnesium hydroxide
carbon tetrachloride
HF(aq)
sodium hydride
V2O5
Ru(ClO4)3
hydrosulfuric acid
H2SO4
titanium(IV) oxide
N2F2
IX. Masses of Compounds
• Atomic masses are readily accessible via
the periodic table, e.g. H = 1.008 amu.
• Formula masses (a.k.a. molecular masses
or molecular weights) are calculated by
adding up the masses of each atom in the
compound.
IX. Molecular Mass of Water
• The formula for water is H2O, so it is
comprised of 2 H atoms and 1 O atom.
IX. Formula Mass
• e.g. What is the formula mass of barium
nitrate?