Tutorial – Counting Atoms in Molecules

 

The first skill required to be able to balance chemical equations is counting atoms in molecules.  The following 3 rules and the examples will help you connect on how this counting system works.

 

1.   Coefficients multiply each atom in the molecule

                  Examples:

3 KCl

3 K

3 Cl

2 MnO

2 Mn

2 O

7 KOH

7 K

7 O

7 H

 

                       

2.   If an atom has a subscript the coefficient multiplies every atom’s subscript

                  Examples:

3 H2O2       

6  H

6  O

3 K2CrO4

6   K

3   Cr

12 O

4 K2SO4

8   K

4   S

16 O

 

3.   If there is a subscript after a parenthesis, it multiplies everything inside the parentheses, but not anything in the molecule outside of the parenthesis

Examples:

5 Fe3(SO4)2

15 Fe

10 S

40 O

3 Mn(SO4)2

3   Mn

6   S

24 O

3 Cu(ClO3)2

3   Cu

6   Cl

18 O