Chem 155 - Eric Bullock
Santa Barbara City College
Naming Molecular Compounds
Molecular compounds make up the vast majority of the millions of known compounds. They consist
of nonmetal elements only. Many molecular compounds are very large and complicated such as
proteins and DNA. However, we will limit our naming to simple binary molecular compounds that
consist of only two different nonmetal elements. The names of these molecules consist of just two
words (for example: carbon dioxide).
All atoms in all molecules are electrically neutral. When you see an oxygen atom in the chemical
formula for a molecule, for example, it is not the O2- ion and does not have an electrical charge of -2.
It has an electrical charge of 0. All molecules as a whole are electrically neutral as well. They all have
an electrical charge of 0.
1) Naming molecular compounds from their chemical formulas
a) The first word in the name of a binary molecular compound is the name of the first element in its
chemical formula preceded by a Greek prefix. This Greek prefix is the number of atoms of that
element in the molecule. The exception is when there is only one atom of the first element in the
formula. In this case no prefix is used. Some examples are given below:
The first word in N2O is dinitrogen
The first word in NO2 is nitrogen
The first word in P3S4 is triphosphorus
The first word in SF6 is sulfur (or sulphur)
The Greek prefixes are:
Number of atoms
1
2
3
4
5
6
7
8
9
10
Greek prefix
mono
di
tri
tetra
penta
hexa
hepta
octa
nona
deca
b) The second word of a binary molecular compound also uses a Greek prefix to indicate the number
of atoms of the second element in the molecule’s chemical formula. After the prefix, the stem of
the name of the element is written, followed by the suffix -ide. Completing the names of the
examples above:
N2O is dinitrogen monoxide
NO2 is nitrogen dioxide
P3S4 is triphosphorus tetrasulfide
SF6 is sulfur hexafluoride
from: mono - ox - ide
here one ‘o’ is dropped
from: di - ox - ide
from: tetra - sulf - ide
from: hexa - fluor - ide
1
Chem 155 - Eric Bullock
Santa Barbara City College
2) Binary molecular compounds that begin with hydrogen
Group 6A and 7A in the Periodic Table form the following binary molecular compounds with H:
Group 6A
H2 O
H2 S
H2Se
H2Te
Name
water
hydrogen sulfide
hydrogen selenide
hydrogen telluride
Group 7A
HF
HCl
HBr
HI
Name
hydrogen fluoride
hydrogen chloride
hydrogen bromide
hydrogen iodide
The naming of these compounds is very similar to the naming of other binary molecular compounds
except that Greek prefixes are not used.
3) Common names
Some molecules are so common that we do not use their chemical names. Instead, we use their
common names. We never use the chemical names for these molecules. In this course, there are two
molecules for which we use common names:
H2 O
NH3
water
ammonia
4) Elemental molecules
Some elements exist in their pure forms as molecules. Because they consist of pure elements, they are
not compounds. Here is a table of all the elemental molecules.
Chemical Formula
Name
H2
hydrogen
N2
nitrogen
O2
oxygen
F2
fluorine
Cl2
chlorine
Br2
bromine
I2
iodine
P4
phosphorus
S8
sulfur
Notice that when we use the name of one of these elements such as ‘fluorine’, this can mean either:
1) F, an atom of fluorine
2) F2, a molecule of the element fluorine
2