COUNT ATOM
How to count atoms
2H2O
COEFFICIENT
SUBSCRIPT
2H2O
The 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!!!!
2H2O
The 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.
2H2O
How 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
Ca
C
O
4CaCO3
Write down the coefficient
next to each symbol
Ca
4
C
4
O
4
4CaCO3
Multiply by the subscript
Ca
4
C
O
4CaCO3
X
1
4
X
1
4
X
3
Multiply…
Ca
4
C
O
4CaCO3
X
1 =
4
4
X
1 =
4
4
X
3 =
12
Add totals together to get
the total amount of atoms
4CaCO3
Ca
4
X
1 =
4
C
4
X
1 =
4
O
4
X
3 =
12
20 atoms
3CO2
3CO2
2C6H12O6
GLUCOSE
2C6H12O6
6K2SO4
6K2SO4
CaMg(CO3)2
CaMg(CO3)2
3Pb(NO3)2
3Pb(NO3)2