Period:
Name:
Date:
Now 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 1:3H20
1. Identify the atoms.
H and 0
Identify subscripts.
2.
H subscript = 2
0 subscript = 1
3.
Identify and distribute coefficients.
Coefficient = 3
3xHsubscript= 3x2=6H
3xOsubscript= 3x1=30
coefficient
H
0
Hydrogen
Oxygen
Total
subscript
6
3
9
Example 2:4Co(N03)2
1. Identify the atoms.
4Co(NO3)2
Co, N, 0
Identify subscripts.
Co subscript = 1
(NO3) subscript = 2 This subscript needs to be distributed because it is outside of
parentheses!
2xNsubscript=2xl= 2
2xOsubscript=2x3 = 6
The problem can now be rewritten as CoN206
Identify and distribute coefficients.
Coefficient = 4
4xCosubscript = 4x 1=4Co
4xNsubscript =4x2 = 8N
4xOsubscript =4x6 = 240
Name of Atom
Cobalt
Nitrogen
Oxygen
Type of Atom
Co
N
0
Total of All Atoms
Total # of each atom
4
8
24
36
Name:
Date:
Period:
Count each of the atoms, find the total number, and number for each element.
The number telling you how many of each atom there are (HaO)~
Coefficient: The large number in front tells you how many of that molecule (2.NaOH)
3
4
5
7
8
O= ~
O=
O=
O= _~
O=
~3
O=
O=
H=
O=
O=
18
o=
o=