Learn how to count atoms.

1. COUNT ATOM
How to count atoms

2. How many elements?
• Remember the symbol for an
element is made up of one or
two letters.
• It is easy to figure out because
they are capitalized.
• HF – (Hydrofluoric acid) is made up of H (hydrogen)
and F (fluor)
• NaCl (sodium chloride) is made up of Na (sodium)
and Cl (chlorine)

3. What about these?
• H2Se
H – hydrogen and Se - selenium
Na – sodium O - oxygen
P - phosphorus
• Na4O7P2
• HCl
H – hydrogen Cl - Chlorine
• K2Te
K – Potassium Te - Tellurium
• C232 N92O139P22
C – carbon N – nitrogen
O – oxygen P - phosphorus

4. Subscripts
• Subscripts tell us how many
atoms of the element BEFORE
them there are.
H2O
SUBSCRIPT

5. No subscript
• If there are no subscripts it
means there is only one atom of
the element.
• HF – Hydrogen = 1 Fluorine = 1
• CaCl2 – Calcium = 1 Chlorine = 2
• H3PO4 –
Hydrogen = 3
Phosphorus = 1
Oxygen = 4

6. What about these?
• C3H4O3
C=3
H=4
O=3
• H2O
H=2
O=1
• C232 N92O139P22
C = 232
H = 92
O = 139
P = 22

7. But it has a parenthesis
• Parenthesis group atoms into
polyatomic ions.
• Don’t worry, the only thing you have
to do is multiply the number of
atoms inside the parenthesis by the
subscript outside the parenthesis.

8. Say what?
• Think distributive property:
Zn (MnO4)2
Zn = 1 atom
Mn = 1 x 2 = 2 atoms
O = 4 x 2 = 8 atoms

9. What about these:
• Fe(OH)3
Fe = 1
O=1x3=3
H=1x3=3
• Sr3(PO4)2
Sr = 3
P=1x2=2
O=4x2=8

10. Finally the Coefficient
2H2O
COEFFICIENT
SUBSCRIPT

11. 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

12. 2H2O
2 MOLECULES!!!!

13. 2H2O
How many hydrogen atoms are present?
4
How many oxygen atoms are present?
2

14. Let’s try an example:
4CaCO3

15. 1. Write down symbols
4CaCO3
•
Ca
C
O

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

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

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

19. 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

20. Now with your table
• 3CO2
• 2C6H12O6
• 6K2SO4
• CaMg(CO3)2
• 3Pb(NO3)2