1. How to count atoms

2. 2H2O
COEFFICIENT
SUBSCRIPT

3. 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
2 MOLECULES!!!!

5. 2H2O
The subscript only gets applied to the element it follows -In the case of a Water Molecule (H2O) there are 2 Hydrogen atoms in the molecule - If there is no subscript present, it is assumed that there is only 1 atom of that element in the molecule

6. 2H2O
How many hydrogen atoms are present? 2 (coefficient) x 2 (subscript) = 4 How many oxygen atoms are present? 2 (coefficient only) = 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. 1. Write down symbols
4CaCO3 Ca C O

10. 2. Write down the coefficient next to each symbol
4CaCO3 Ca C O 4

11. 3. Multiply by the subscript
4CaCO3 Ca C O 4 X 1 3

12. Multiply…
4CaCO3 Ca C O 4 X 1 3 = 4 12

13. Add totals together to get the total amount of atoms
4CaCO3 Ca C O 4 X 1 3 = 4 12
20 atoms

14. 3CO2

15. 3CO2

16. 2C6H12O6
GLUCOSE

17. 2C6H12O6

18. 6K2SO4

19. 6K2SO4

20. CaMg(CO3)2

21. CaMg(CO3)2