COUNTING
ATOMS
RULES FOR COUNTING
ATOMS
1. SUBSCRIPTS (baby numbers)=
(# of atoms) only refer to the
element that they are BEHIND.
For example…
H2S
There are TWO atoms of
HYDROGEN and only ONE atom
of SULFUR.**No number = 1**
“We Do”
1. Ca2O
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
2. HgCO3
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
“You Do”
1. PbSO3
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
2. NH4OH
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
COEFFICIENTS
2. COEFFICIENTS (BIG # in front)
apply to the entire compound. You
MULTIPLY the coefficients and
SUBSCRIPTS. 2 H2S
ATOMS OF HYDROGEN: 2 x 2 = 4
ATOMS OF SULFUR: 1 x 2 = 2
IF THERE ISN’T A SUBSCRIPT BEHIND AN
ELEMENT, ASSUME THERE IS ONLY ONE ATOM OF
THAT ELEMENT!
“We Do”
1. 4Ca2O
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
2. 2HgCO3
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
“You Do”
1. 3PbSO3
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
2. 4NH4OH
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
PARENTHESES
3. If elements or compounds are inside of
PARENTHESES, then the SUBSCRIPT
behind the parentheses applies to
everything inside the parentheses ONLY.
Ba(OH)2
ATOMS OF BARIUM: 1
ATOMS OF OXYGEN: 2
ATOMS OF HYDROGEN: 2
“We Do”
Ca3(PO4)2
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
“You Do”
Al2(SO4)3
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
• Element: _______ # of Atoms: ______
What about this BAD BOY???
2Ca3(PO4)2