5 Steps to a 5 AP Chemistry

Writing Formulas

To write the formula from the name of a binary compound containing only nonmetals,

simply write the symbols for the separate atoms with the prefixes converted to subscripts.

In all compounds, the total charge must be zero. There are NO exceptions. Thus, to

determine the formula in those cases where no prefixes are given, it is necessary to have

some idea what the individual charges are. The species with the positive charge is listed and

named first; this is followed by the species with the negative charge. Subscripts may be

needed to make sure the sum of the charges (valances) will equal zero. Examples:

1. Magnesium oxide

This gives MgO.

2. Sodium oxide

Na^1 +O^2 −=+ 1 − 2 =−1 thus a subscript is needed

This gives Na 2 O.

3. Aluminum oxide

Al^3 +O^2 −=+ 3 − 2 =+1 thus a subscript is needed

Al 2 2(3+)O 3 3(−2)= 2(+3) +3(−2) = 0

This gives Al 2 O 3.

If a polyatomic ion must be increased to achieve zero charge, parentheses should be

used. An example of this is shown as:

This gives (NH 4 ) 2 SO 4.

One way of predicting the values of the subscripts is to crisscross the valences. This is

not a rule of nomenclature, but for practice purposes in this exercise it will be referred to

as the crisscross rule. It works most of the time and therefore is worth considering.

Example:

Al^3 +O^2 − crisscross the 2 from the oxygen charge to the aluminum and the 3 from the

aluminum charge to the oxygen

Al 23 +O 3 −^2

If the crisscross rule is applied, you should reduce the formula if possible. For example:

Mn^4 +O^2 crisscrosses to Mn 2 O 4 , which reduces to MnO 2

If a formula is given, the crisscross rule can be reversed to give the valences:

Al 2 O 3

Al 23 +O 32 −

As a first approximation, the valences of the representative elements can be predicted

from their position on the periodic table. Hydrogen and the metals have positive charges

NH SO

NH SO 2

4

+

4

2

44

2

−

(+) −

=+ − =−

( ) =+

12 1

2

21

(^11 )−=^20

Na 221 ()+−O^2 =+−=21 2 0()

Mg O^22 +−=+ − =22 0

56  STEP 4. Review the Knowledge You Need to Score High

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