Barrons SAT Subject Test Chemistry, 13th Edition

To determine the formula for aluminum oxide, first write the ionic forms with
their associated charges.

Next move the numerical value of the Al’s superscript (without the charge)
to the subscript of the O symbol. Do the same with the 2 of the O. In other words,
crisscross the values. You now have the 2 as the subscript of the aluminum and the
3 as a subscript of the oxygen. You now have Al 2 O 3 as the final formula for

aluminum oxide.
This crisscross rule generally works very well. In one situation, though, you
have to be careful. Suppose you want to write the compound formed when
magnesium reacts with oxygen. Magnesium, an alkaline earth metal in group 2

forms a 2+ cation, and oxygen forms a 2− anion. You would predict its formula be
Mg 2 O 2 , but this is incorrect. After you do the crisscrossing (unless you know that

the compound actually exists, like H 2 O 2 ), you need to reduce all the subscripts by

a common factor. In this example, you can divide all the subscripts by 2 to get the
correct formula for magnesium oxide, MgO.

REMEMBER

Reduce all subscripts by a common factor unless you are sure the compound exists, like

H 2 O 2.

When you attempt to write a formula, you should know whether the substance
actually exists. For example, you could easily write the formula for carbon
nitrate, but no chemist has ever prepared this compound.

Examples of Category I Binary Ionic Compounds

Ions Present*

Formula

Name

K +, Cl−

KCl

Potassium chloride

Na +, I−

NaI

Sodium iodide

Ca 2+, S2−

CaS

Calcium sulfide

Al 3+, F−

AlF 3

Aluminum fluoride