Mr. Chapman
Science 9
Norquay School
Atoms in Compounds
Counting Atoms in Compounds
Now and in the future, it is going to be very important for you to
be able to look at a chemical formula and determine how many
of each type of atom it has. Remember our previous definitions
of elements and compounds:
An element cannot be reduced to a simpler substance by normal means. An element is easy to
think of as a type of atom. There are about 114 different elements.
A compound is a pure substance composed of two or more different types of atoms / elements
4 Rules for Counting Atoms
Rule #1: An element can have either one or two letters for its symbol. The first letter
is always capitalized, and the second one is never capitalized.
Elements with two letter symbols
Elements with one letter symbols
Na – Sodium
K - Potassium
Fe – Iron
O - Oxygen
Ne – Neon
F - Fluorine
Ba – Barium
H – Hydrogen
Mr. Chapman
Science 9
Norquay School
Atoms in Compounds
Rule #2: When looking at a chemical formula, you will sometimes see small numbers
written beside some elements. These numbers are called subscripts, and they represent
how many of that atom is present in the chemical formula.
Example: The chemical formula for water is H2O. There are two elements in water, one is
Hydrogen (H), and the other is Oxygen (O). In water there are 2 hydrogen atoms and
1 oxygen atom. You can tell this by the subscripts that are on the right of each atom. If there is
no subscript, the number is assumed to be a one.
Identify each type of element and the number of atoms of each type in the following:
1. NaCl Sodium (1) Chlorine (1)
2. SO2 Sulfur (1) Oxygen (2)
3. LiHCO3 Lithium (1) Hydrogen (1) Carbon (1) Oxygen (3)
Mr. Chapman
Science 9
Norquay School
Atoms in Compounds
Rule #3: If some chemical formulas, there are brackets. There is always a subscript
outside of the bracket. To count the number of atoms in these formulas, multiply the
number outside the brackets by everything inside the brackets.
Examples: Identify each type of atom, and count the number of each in the formula.
1. Ba(OH)2 Barium (1) Hydrogen (2) Oxygen (2)
2. Mg(NO3)2 Magnesium (1) Nitrogen (2) Oxygen (6)
3. Al2(SiO3)3 Aluminum (2) Silicon (3) Oxygen (9)
Super Tough Challenge: Pb(CH3COOH)3
Mr. Chapman
Science 9
Norquay School
Atoms in Compounds
Rule #4: If some reactions, you will find a number in front of the formula. This number is
called a numerical coefficient. Multiply each atom in the formula by the number in front
of the chemical formula.
Examples: Identify each type of atom, and count the number of each in the formula.
1. 2 NaNO3 Sodium (2) Nitrogen (2) Oxygen (6)
2. 5 MgBr2 Magnesium (5) Bromine (5)
Ultra Challenge:
5 Ge(SiO3)3