Science 10
MOLECULAR COMPOUNDS
What is a molecular compound?
 A substance made up of molecules that is only composed
of nonmetals
 They are different from ionic compounds because those
are made up of a metal and nonmetal
 Electrons are shared between nonmetals to form a
covalent bond
 The simplest molecules are diatomic (composed of two
nonmetals)

Ex. Cl2
 Other molecules are composed of different nonmetals

Ex. C6H12O6, H2O
How to Write the Formula of a Molecular
Compound
 The number of electrons that a nonmetal can share is
called its combining capacity
 When predicting the formula of a molecule, use the
combining capacity (group number) of the nonmetal
 Ex. C = group 14 (loss of 4 e-),
O = group 16 (gain of 2e-)
C4O2
C2O4
 These formulas are not reduced
Combining Capacities of Nonmetals
4
3
2
1
H
C
N
O
F
Si
P
S
Cl
As
Se
Br
I
Naming Molecular Compounds
 Instead of using the ionic compound naming rules,
we use the prefix system to name molecules
Prefix
Number
Example
mono-
1
carbon monoxide (CO)
di-
2
carbon dioxide (CO2)
tri-
3
carbon trioxide (CO3)
tetra-
4
carbon tetrafluoride (CF4)
penta-
5
phosphorous pentafluoride (PF5)
Some Rules to Remember
 If there is not a subscript on the first nonmetal, the
prefix mono does not have to be used
 If there are subscripts on both nonmetals, use
prefixes for both
 Use the common name for the compound when
applicable

Ex. H2O is dihydrogen monoxide but you can call it water