Name
Period
Date
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. Such a group of atoms is called a radical. If a radical is used in a
formula more than once, the radical is put in parentheses and the subscript appears outside the parentheses. 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
Calcium carbonate
Limestone
CaCO3
Aspirin
Pain reliever
C9H8O4
Magnesium hydroxide
Found in milk of
magnesia
Mg(OH)2
Paradichlorobenzene
Moth crystals
Acetic acid
Found in vinegar
Trinitrotoluene
(TNT)
Explosive
Calcium dihydrogen
phosphate
Fertilizer
Pyrite
Fool’s gold
C6H4Cl2
C2H4O2
C7H5 (NO2)3
Ca(H2PO4)2
FeS2
Sucrose
Sugar
Cellulose
Found in wood
products such as your
C6H7O2(OH)3
pencil and paper
C12H22O11
Atoms in Formula
Ca= calcium 1
C= carbon 1
O= oxygen 3
1. What does the atomic number represent in the atom?
2. What information about the atom does the atomic mass tell us?
Complete the following chart using your periodic table:
Element
Name
Element
Symbol
Atomic
Number
Atomic
Mass
# of
Protons
# of
Electrons
# of
Neutrons
Ex: Lithium
Li
3
7
3
3
7-3 = 4
3. Carbon
4. Hydrogen
5. Nitrogen
6. Oxygen
7. Phosphorus
8. Sulfur
9. Magnesium
10. Argon
11. Sodium
12. Potassium
13. Calcium
Color the letters based on the percentages.
# of e- in
energy levels