Writing Names & Formulas for Ionic & Molecular Compounds
Textbook Pages: 64 – 68
Valence
Definition: the charge assigned to an ion or atom.
***Overall, when added the charges must equal
!!!
EX.
NaCl
Cl is 1−
EX.
HCl
EX.
N2
Na is 1+
Some elements have more than one valence.
EX.
What is the valence of Pb in:
1. PbO
O2−
Therefore, Pb must be 2+
2. PbO2
O2−
Therefore, Pb must be
Elements that have more than one valence are called
(***See back of Periodic Table)
Naming for Simple Binary Ionic Compounds
Binary means two (2). Therefore, simple binary compounds is the naming of compounds with
.
The first element only has one valence.
Steps for naming simple binary ionic compounds:
1. Name the first element regularly from the Periodic Table
2. Change the ending of the second element to “ide”
EX.
NaCl
EX.
Na2O
EX.
CaH2
Sodium Chloride
Writing Formulas for Simple Binary Ionic Compounds
Steps for writing simple binary ionic compounds:
1. Determine the valence for each element in the compound
2. Criss-cross the valence numbers (ignore the +/− charges) and write them as subscripts
3. If possible, reduce to the simplest ratio
EX.
Magnesium Oxide
1. Mg2+ O2−
2. Mg2 O2 forms Mg2O2
3. Mg2O2 gets reduced to MgO
EX.
Aluminum Chloride
.
Naming Compounds with Multiple Valences
Some elements can have more than one valence.
EX.
Pb can be Pb2+ or Pb4+
This means it can form
EX.
compounds that have different
properties.
Lead Oxide
-If lead is 2+, Pb2+ O2−  PbO
-If lead is 4+, Pb4+ O2− 
-Both these compounds are
and
from one another!
One of two methods can be used to distinguish between the compounds that have more than one valence:
Method 1: IUPAC Method
-For multiple valence compounds ONLY, we can indicate which one it is my putting its
in brackets after its
.
EX.
EX.
PbO Lead (II) Oxide
PbO2 Lead (IV) Oxide
-Note: Roman Numbers are used
I-1, II-2, III-3, IV-4, V-5, VI-6, VII-7
Method 2: Ous – Ic Method (Classical System)
-For multiple valence compounds ONLY, we can indicate the
valence with an “ic” ending and the
ending.
EX.
EX.
valence with an “ous”
PbO Plumbous Oxide
PbO2 Plumbic Oxide
-Note: Often the historical name is used. (***See back of Periodic Table)
Naming and Writing Molecular (Covalent) Compounds
***Do NOT worry about valences
Steps for naming molecular (covalent) compounds:
1. Name the first element regularly from the Periodic Table
2. Change the ending of the second element to “ide”
3. Use prefixes (below) to say how many
are in the compound.
Prefixes:
Mono – 1
Hexa – 6
Di – 2
Hepta – 7
Tri – 3
Octa – 8
Tetra – 4
Nona – 9
Penta – 5
Deca – 10
EX.
Carbon Tetrachloride
Note: “Mono” is never used on the
EX.
EX.
EX.
EX.
NO
Nitrogen Trihydride
Diphosphorus Pentoxide
PCl3
CF4
of each
EX.
H2O
element.
Dihydrogen Monoxide