Counting Atoms The formula for a compound indicates the elements that make up the compound and the number of atoms of each element present in the compound. These numbers of atoms are indicated by the use of small numbers called subscripts. Sometimes groups of atoms act as a single atom.. When a subscript appears outside the parentheses, it indicates that all the elements inside the parentheses should be multiplied by that subscript. For example, the formula Fe (OH)3 indicates the combination of one atom of iron. Fe= three atoms of oxygen (O) and three atoms of hydrogen (H).
In the following examples, list each element in the compound and the number of atoms of each element present. The first example has been done for you. You may already be familiar with some of the compounds.

Name
Use
Formula
Atoms in Formula

Calcium carbonate

Limestone

CaCO3
Ca= calcium 1
C= carbon 1
O= oxygen 3

Aspirin

Pain reliever

C9H8O4

Magnesium hydroxide
Found in milk of magnesia

Mg(OH)2

Paradichlorobenzene

Moth crystals

C6H4Cl2

Acetic acid

Found in vinegar

C2H4O2

TriNitroToluene (TNT)

Explosive

C7H5 (NO2)3

Calcium dihydrogen phosphate

Fertilizer

Ca(H2PO4)2

Pyrite

Fool’s gold

FeS2

Sucrose

Sugar

C12H22O11

Cellulose
Found in wood products such as your pencil and paper

C6H7O2(OH)3