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 ___________ cannot be ___________ to a __________ ______________ by normal means.
An element is easy to think of as a _______________. There are about ____ different elements.
A _____________ is a _____________________ composed of two or more _____________
types of atoms / elements
4 Rules for Counting Atoms
Rule #1: An element can have either ___ or ___ letters for its symbol. The first
letter is always ________, and the second one is ______ capitalized.
Elements with two letter symbols
Elements with one letter symbols
Mr. Chapman
Science 9
Norquay School
Atoms in Compounds
Rule #2: When looking at a ___________, you will sometimes see small ____
written beside some elements. These numbers are called _________, and they
represent _________ 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
_______________, and the other is _____________. In water there are _______________ and
__________________. 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 _______.
Identify each type of element and the number of atoms of each type in the following:
1. NaCl
2. SO2
3. LiHCO3
Mr. Chapman
Science 9
Norquay School
Atoms in Compounds
Rule #3: If some chemical formulas, there are _______. There is always a
_________ outside of the bracket. To count the number of atoms in these formulas,
_______ the number outside the brackets by everything _____ the brackets.
Examples: Identify each type of atom, and count the number of each in the formula.
1. Ba(OH)2
2. Mg(NO3)2
3. Al2(SiO3)3
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 ________________. 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
2. 5 MgBr2
Ultra Challenge:
5 Ge(SiO3)3