The document outlines rules for counting atoms in chemical formulas: 1) Subscripts refer only to the atom behind them, coefficients apply to the entire compound. 2) If elements are inside parentheses, the subscript behind applies to everything inside. 3) Examples are provided to demonstrate how to use the rules to count atoms in different chemical formulas.

1. COUNTING
ATOMS

2. RULES FOR COUNTING
ATOMS
1. SUBSCRIPTS only refer to the
atom that they are BEHIND. For
example…
H2 S
There are TWO atoms of
HYDROGEN and only ONE atom
of SULFUR.

3. COEFFICIENTS
2. COEFFICIENTS apply to the entire
compound. You MULTIPLY the
coefficients and SUBSCRIPTS.
2 H2 S
ATOMS OF HYDROGEN: 4
ATOMS OF SULFUR: 2
IF THERE ISN’T A SUBSCRIPT BEHIND AN
ELEMENT, ASSUME THERE IS ONLY ONE ATOM OF
THAT ELEMENT!

4. PARENTHESES
3. If elements or compounds are inside
of PARENTHESES, then the
SUBSCRIPT behind the parentheses
applies to everything inside.
Ba(OH)2
ATOMS OF BARIUM: 1
ATOMS OF OXYGEN: 2
ATOMS OF HYDROGEN: 2

5. LET’S PRACTICE!
MgCl2
Atoms of Magnesium: 1
Atoms of Chlorine: 2
Al2S3
Atoms of Aluminum: 2
Atoms of Sulfur: 3

6. PRACTICE
H2SO4
Atoms of Hydrogen: 2
Atoms of Sulfur: 1
Atoms of Oxygen: 4
CH3OH
Atoms of Carbon: 1
Atoms of Hydrogen: 4
Atoms of Oxygen: 1

7. THIS COULD BE A LITTLE
TRICKY…
Ca3(PO4)2
Atoms of Calcium: 3
Atoms of Phosphorus: 2
Atoms of Oxygen: 8
Al2(SO4)3
Atoms of Aluminum: 2
Atoms of Sulfur: 3
Atoms of Oxygen: 12

8. What about this BAD BOY???
2Ca3(PO4)2
Atoms of Calcium: 6
Atoms of Phosphorus: 4
Atoms of Oxygen: 16