5 Steps to a 5 AP Chemistry 2019

(^58) › STEP 4. Review the Knowledge You Need to Score High
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
   Mg^22 +−O2=+−= 20

This gives MgO.

2. Sodium oxide
   Na^1 +O^2 - = + 1 - 2 = -1; thus a subscript is needed
   NaO 2 2(1)+−^2 =+2( 1)−= 20

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:

=+−=−

() =+−=

−

+ −

NH SO 12 1

(NH) SO 2( 1) 20

4

+

4

2

42

21

4

2

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 32 -

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
beginning with +1 on the left and increasing by one as you proceed to the right on the
periodic table (skipping the transition metals). Nonmetals begin with 0 in the rightmost

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