COUNT ATOMHow to count atoms

2H2OCOEFFICIENT SUBSCRIPT

2H2OThe coefficient gets applied to the

ENTIRE formula

-this means that there are 2 molecules of water present!

-you MULTIPLY this number by every subscript in the formula

2H2O

2MOLECULES!!!!

2H2OThe subscript only gets applied to the

element it follows

-If there is no subscript present, it is assumed that there is only 1 atom of

that element in the molecule.

2H2OHow many hydrogen atoms are present?

4

How many oxygen atoms are present?

2

The easiest way to count atoms:

1. Write down all of the symbols for all of the elements present in the formula

2. If there is a coefficient, write this number next to each of the symbols

3. Multiply the coefficient by the subscripts of each of the elements present in the formula

Let’s try an example:

4CaCO3

4CaCO3

1. Write down symbols

4CaCO3

Ca

C

O

Write down the coefficient next to each symbol

4CaCO3

Ca

C

O

4

4

4

Multiply by the subscript

4CaCO3

Ca

C

O

4

4

4

X

X

X

1

1

3

Multiply…

4CaCO3

Ca

C

O

4

4

4

X

X

X

1

1

3

=

=

=

4

4

12

Add totals together to get the total amount of atoms

4CaCO3

Ca

C

O

4

4

4

X

X

X

1

1

3

=

=

=

4

4

12

20 atoms

3CO2

3CO2

2C6H12O6

GLUCOSE

2C6H12O6

6K2SO4

6K2SO4

CaMg(CO3)2

CaMg(CO3)2

3Pb(NO3)2

3Pb(NO3)2