Chemistry Reference
In-Depth Information
Naming Simple Binary Compounds
Elements in Compound
Chemical Formula
Name of Compound
LiBr
Lithium and bromine
Lithium bromide
CaF
2
SrO
Calcium and fluorine
Calcium fluoride
Strontium and oxygen
Strontium oxide
BaI
2
MgS
Barium and iodine
Barium iodide
Magnesium and sulfur
Magnesium sulfide
Figure 5-3a
All of the examples used here involve elements with only one possible
oxidation state. When we are required to name compounds that contain
elements with more than one oxidation state, we use the stock system, which
uses Roman numerals to indicate the oxidation number of the less elec-
tronegative element. We can usually determine the oxidation number of
the less electronegative element by considering the rules for oxidation num-
bers presented in Lesson 5-1. It would probably be a good idea to review
those rules before you try to follow these next examples.
Suppose you were asked to name the compound with the formula FeO.
Following the method for naming simple binary compounds, we would get
iron oxide. The problem is that there is more than one form of iron oxide.
There is another compound, for example, with the formula Fe
2
O
3
, which
you might also call iron oxide. How do we differentiate between the names
of the two compounds?
What we need to do is make use of Roman numerals, in order to indi-
cate the oxidation number of the iron in each of the compounds. We can
figure out the oxidation number of the iron by paying attention to the ratio
by which it is combining with the oxygen in each compound. You will recall
from the Rules for Assigning Oxidation Numbers in Lesson 5-1 that the
oxidation number for oxygen (except in peroxides, and these are not per-
oxides) is -2. If the oxidation number of oxygen is -2, what does that make
the oxidation number of the iron in each compound, if the net charge on
each compound must be 0?
For the first compound, FeO, the iron and oxygen combine in a 1:1
ratio. If the oxidation number of the oxygen is -2, the oxidation number of
the iron must be +2, because +2 + (-2) = 0. We indicate the fact that iron
takes on an oxidation number of +2 by inserting a Roman numeral for
