Name _________________________
Naming Molecular Compounds
As you remember, molecular compounds are relatively small groups of covalently bonded
nonmetal atoms, that have weak to moderate intermolecular forces. There are two methods for
naming binary molecular compounds. The most commonly used method is to use prefixes
(Table 1) to indicate how many of each type of atom there is in the molecular formula. In
addition, the suffix –ide is added to the root of the final element’s name. However, if there is
only one atom for the first element in the formula we do not use the prefix mono-, but we do use
the prefix mono- if there is only one atom for the second element in the formula. As examples,
Cl2O7, CF4 and CO are named dichlorine heptaoxide, carbon tetrafluoride, and carbon monoxide,
respectively. Exceptions include the binary molecular compounds of hydrogen and the halogens,
HF, HCl, HBr and HI, which do not use the prefix mono-, e.g. hydrogen fluoride. There are a
number of binary compounds with traditional names that do not follow this system, with the
most important examples being water (H2O) and ammonia (NH3). In addition, organic
compounds, molecules containing carbon bonded to hydrogen, have their own unique naming
system. The International Union of Pure and Applied Scientists (IUPAC) have tried to
standardize the methods for naming compounds, but are faced with years and years of tradition
associated with the naming of chemical formulas.
Table 1. Prefixes used for naming binary molecular compounds.
Prefix
monoditritetrapentahexahepta-
Number
1
2
3
4
5
6
7
An alternate method1 is the “stock system,” whose approach is identical to that described for
ionic compounds. The utility of this system is that we do not need to remember separate
methods for ionic (metal and nonmetals) and molecular (all nonmetals) compounds. The
drawback is that we then need to be able to identify the oxidation numbers or the charges of
each individual atom in a formula. In the case of molecular compounds it is assumed that the
more electronegative element, which will usually be the second element in the formula, has a
negative oxidation number (charge) and the more electropositive element, which will be the first
element in the formula, has a positive oxidation number. It is also important to remember that
for molecular compounds, having not net charge, the sum of the positive and negative oxidation
numbers must equal zero. For example, carbon tetrafluoride, CF4, would be carbon (IV) fluoride
(reverse the criss-cross to give C4+ and 4 F-) using the stock system. For carbon dioxide, CO2,
which has two oxides (2 O2-), the oxidation number of carbon must be 4+, giving the stock name
of carbon (IV) oxide. In contrast, carbon monoxide, CO (C2+ and O2-), is carbon (II) oxide in the
stock system.
1
Primarily used on the New York State Chemistry Regents exam and nowhere else, to my knowledge.
© Van Der Sluys, 2004
Name _________________________
Naming Molecular Compounds
Fill in the blanks for following molecular compounds and their names.
Formula
Common Name
Stock Name
1. CO2
____________________
____________________
2. ________
carbon monoxide
____________________
3. ________
____________________
sulfur (VI) oxide
4. SO2
____________________
____________________
5. ________
dinitrogen monoxide
____________________
6. ________
____________________
nitrogen (II) oxide
7. NO2
____________________
____________________
8. ________
dinitrogen trioxide
____________________
9. N2O4
____________________
____________________
10. N2O5
____________________
____________________
11. ________
phosphorus trichloride
____________________
12. ________
____________________
phosphorus (V) chloride
13. NH3
____________________
____________________
14. H2O
____________________
____________________
15. ________
____________________
sulfur (VI) fluoride
16. ________
sulfur tetrafluoride
____________________
17. P2O5
____________________
____________________
18. ________
____________________
carbon (IV) chloride
19. CS2
____________________
____________________
20. ________
____________________
oxygen (II) fluoride
© Van Der Sluys, 2004
Name _________________________
Naming Molecular Compounds
Answers:
1. carbon dioxide, carbon (IV) oxide
2. CO, carbon (II) oxide
3. SO3, sulfur trioxide
4. sulfur dioxide, sulfur (IV) oxide
5. N2O, nitrogen (I) oxide
6. NO, nitrogen monoxide
7. nitrogen dioxide, nitrogen (IV) oxide
8. N2O3, nitrogen (III) oxide
9. dinitrogen tetroxide, nitrogen (IV) oxide
10. dinitrogen pentoxide, nitrogen (V) oxide
11. PCl3, phosphorus (III) chloride
12. PCl5, phosphorus pentachloride
13. ammonia, ammonia
14. water, water
15. SF6, sulfur hexafluoride
16. SF4, sulfur (IV) fluoride
17. diphosphorus pentoxide, phosphorus (V) oxide
18. CCl4, carbon tetrachloride
19. carbon disulfide, carbon (IV) sulfide
20. OF2, oxygen difluoride
© Van Der Sluys, 2004