Naming Covalent Compounds
Now that we have some idea about the naming of inorganic compounds, its time
to tackle the compounds composed solely of non-metallic elements. While
much of the naming is very similar to what we already know. The major
difference in the naming is that we have to give the number of each element
found in the compound. In any compound formed between metals and nonmetals that we have already encountered we did not have to do this.
Similarities
We must name all elements found in
the compound
The most metallic element is always
named (and written) first.
The second element’s name is
shortened and the “ide” suffix used.
If only a single element appears in the
formula, we simply write the name of
the element concerned.
Differences
We must name the number of each
element found in the compound. We
use a series of suffix to denote the
number of these atoms.
The most metallic element will be the one closest to the left hand side of the
table or closest to the “stairs” separating the metals from the non-metals.
Hydrogen presents a slight problem here as it fits between carbon and nitrogen
in the scheme. We use a prefix to denote the number of each atom present. This
is shown in the table below.
Number of
atoms
Prefix
Number of
atoms
Prefix
1
2
3
4
5
mono-
di-
tri-
tetra-
penta-
6
7
8
9
10
nona-
deca-
hexa-
hepta-
octa-
Note that the term mono is used given but we tend to not use it for the first
named element. It seems a silly convention if we have to use the mono (or
“mon” prefix) on the first element. It is simply implied by having the symbol
being present.
Example 1 – A very common compound that is found burning material has the
formula CO2. This means that it will have the name carbon dioxide – note that
the mono is not used for the carbon.
Example 2 – Another gas that will be produced if there is insufficient oxygen in
the air when burning occurs has the formula CO. This means it will have the
name carbon monoxide.
Example 3 – One of the many oxides of nitrogen has the formula N2O4. In this
case we must add a prefix to help distinguish between the other possible oxides
of nitrogen. So the name becomes dinitrogen tetroxide.
When writing the correct formula for the compound, we simply follow the name.
We do not have to worry about the combining powers of the elements because it
is covered within the name.
Example 4 – Write the correct formula for sulfur dioxide. The name suggests
we have one sulfur atom combined with two oxygen atoms per molecule of
compound. So the formula would be written as
SOO or usually more correctly as SO2
Example 5 – Write the correct formula for phosphorus trichloride. The name
suggests we have one phosphorus atom combined with three chlorine atoms per
molecule of compound. So the formula would be written as
PClClCl or usually more correctly as PCl3
Example 6 – Write the correct formula for dinitrogen tetroxide. The name
suggests we have two nitrogen atoms combined with five oxygen atoms per
molecule of compound. So the formula would be written as
NNOOOOO or usually more correctly as N2O5
What to do
1.
2.
Use the rules for writing formula to correctly give the formula for:
a. Sulfur trioxide
b. Phosphorus pentabromide
c. Carbon tetrachloride
d. Nitrogen trihydride
e. Sulfur dichloride
Deduce the correct IUPAC name for:
a. H2O
b. SO3
c. P2O10
d. NF3
e. C3N4
f. CH4
g. CS2