BELLRINGER
1. What is the chemical formula for
water?
2. How many hydrogen atoms are in
a water molecule?
3. How many oxygen atoms are in a
water molecule?
COUNTING
ATOMS
CHEMICAL FORMULAS
• Tell what elements make up the
compound and the number of
atoms of each element.
RULES FOR COUNTING
ATOMS
1. SUBSCRIPTS only refer to the
atom that they are BEHIND. For
example…
H2S
There are TWO atoms of
HYDROGEN and only ONE atom
of SULFUR.
COEFFICIENTS
2. COEFFICIENTS apply to the entire
compound. You MULTIPLY the
coefficients and SUBSCRIPTS.
2 H2S
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!
Polyatomic ions
• A group of atoms that act as a single atom.
• Put in parentheses
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
LET’S PRACTICE!
MgCl2
Atoms of Magnesium: 1
Atoms of Chlorine: 2
Al2S3
Atoms of Aluminum: 2
Atoms of Sulfur: 3
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
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
What about this BAD BOY???
2Ca3(PO4)2
Atoms of Calcium: 6
Atoms of Phosphorus: 4
Atoms of Oxygen: 16
COUNTING ATOMS PRACTICE
Name
Use
Formula
Atoms in Formula
Ex: Calcium Carbonate
Limestone
CaCO3
Ca= calcium- 1
C= carbon- 1
O= oxygen – 3
1. Aspirin
Pain reliever
C9H8O4
2. Paradichlorobenzene
Moth Crystals
C6H4Cl2
3. Acetic Acid
Vinegar
C2H4O2
4. TNT
AKA Trinitrotoluene
Explosive
C7H5(NO2)3
5. Calcium dihydrogen
phosphate
Fertilizer
Ca(H2PO4)2
6. Pyrite
Fool’s Gold
FeS2
7. Magnesium
hydroxide
In Milk of
Magnesia
Mg(OH)2
8. Sodium perborate
Solid Bleach
NaBO3
9. Sucrose
Sugar
C12H22O11
10. Isopropyl Alcohol
Rubbing Alcohol
(CH3)2CHOH