1. COUNT ATOM
How to count atoms

2. 2H2O
COEFFICIENT
SUBSCRIPT

3. The coefficient gets applied to the ENTIRE formula
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

4. 2H2O
2MOLECULES!!!!

5. The subscript only gets applied to the element it follows
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.

6. 2H2O 4 2 How many hydrogen atoms are present?
How many oxygen atoms are present? 2

7. 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

8. Let’s try an example:
4CaCO3

9. 4CaCO3
Ca -1 x 4 = 4
C – 1 x 4 = 4
O – 4 x 3 = 12
This molecule has 4-Ca, 4-C, and 12-O.
The total number of atoms present is = 20

10. 3CO2

11. 3CO2 C – 3 x 1 = 3 O – 3 x 2 = 6 There are 3 – C, 6 – O.
The total number of atoms is = 9

12. 2C6H12O6
GLUCOSE

13. 2C6H12O6
C – 2 x 6 = 12
H – 2 x 12 = 24
O – 2 x 6 = 12
The total number of atoms is 48

14. 6K2SO4

15. 6K2SO4
K – 6 x 2 = 12
S – 6 x 1 = 6
S – 6 x 1 – 6
O – 6 x 4 = 24
Total number of atoms 48

16. CaMg(CO3)2

17. CaMg(CO3)2 Ca – 1 Mg – 1 C – 1 x 2 = 2 O - 3 x 2 = 6
Total number of atoms is 10

18. 3Pb(NO3)2 Pb – 3 x 1 = 3 N – 3 x 1 x 2 = 6 O – 3 x 3 x 2 = 18
Total number of atoms 27