Name: _________________________________
Date: _______
Period: ________
How to Count Atoms
Chemical formulas use the chemical element symbols found on the periodic table. Element symbols
are either one capital letter or one capital letter followed by a lower case letter. For example, using
H for hydrogen or He for helium. When counting atoms of each element in a chemical formula, first
we need to identify which atoms are present. Next, we look for subscripts. The subscript affects
only the atom that if follows. If no subscript is written, the atom is considered to have a subscript of
1. If a subscript follows parentheses, the subscript of each atom in the parentheses is multiplied by
the subscript outside of the parentheses. Rewrite the problem after these calculations. Finally, we
identify coefficients. Coefficients are distributed to all atoms in the chemical formula. The
coefficient is multiplied by the subscripts. If no coefficient is present, it is assumed to be 1. Let’s
work some examples.
Example 1: 3H2O
1.
Identify the atoms.
H and O
2.
Identify subscripts.
H subscript = 2
O subscript = 1
3.
Identify and distribute coefficients.
Coefficient = 3
3 x H subscript = 3 x 2 = 6 H
3 x O subscript = 3 x 1 = 3 O
Name of Atom
Hydrogen
Oxygen
3H2O
coefficient
Atom Symbol
H
O
Total of All Atoms
subscript
Total # of each atom
6
3
9
Example 2: 4Co(NO3)2
1.
Identify the atoms.
4Co(NO3)2
Co, N, O
2.
Identify subscripts.
Co subscript = 1
(NO3) subscript = 2 This subscript needs to be distributed because it is outside of
parentheses!
2 x N subscript = 2 x 1 = 2
2 x O subscript = 2 x 3 = 6
The problem can now be rewritten as CoN2O6
3.
Identify and distribute coefficients.
Coefficient = 4
4 x Co subscript = 4 x 1 = 4 Co
4 x N subscript = 4 x 2 = 8 N
4 x O subscript = 4 x 6 = 24 O
Name of Atom
Cobalt
Nitrogen
Oxygen
Type of Atom
Co
N
O
Total of All Atoms
Total # of each atom
4
8
24
36
Name: _________________________________
Date: _______
Period: ________
Counting the Number of Atoms in a Molecule Activity
Count each of the atoms, find the total number, and number for each element.
Subscript: The number telling you how many of each atom there are (H20)2
Coefficient: The large number in front tells you how many of that molecule (2NaOH)
Total No.
Substance
of Atoms
Element 1
Element 2
Element 3
1
CaCO3
Ca=
C=
O=
2
2CaSO4
Ca=
S=
O=
3
3Ca(NO3)2
Ca=
N=
O=
4
2Mg(OH)2
Mg=
O=
H=
5
2Na2CO3
Na=
C=
O=
6
Na3PO4
Na=
P=
O=
7
2K2Cr2O7
K=
Cr=
O=
8
C6H7O2
C=
H=
O=
9
2SiO2
Si=
O=
-
10
2CH4
C=
H=
-
11
2Al2(SO)3
Al=
S=
O=