Counting Atoms
As we look towards the next unit, we need to be able to count atoms in a chemical
compound. You already of some experience with counting atoms from an earlier worksheet
and from writing chemical formulas. In this lesson, we will be a little more methodical with
how we do this.
It starts out pretty simple, the subscript tells the number of atoms. For example, in
MgCl2, there is 1 magnesium and 2 chlorines. Things get a little bit more interesting when
polyatomic ions are involved. There are 2 ways to handle these: one is to keep the polyatomic
ion together and the other is to separate it. You will need to do both. Later, you will learn how
to decide.
First, let’s look at keeping the polyatomic ion together. If there is no parenthesis, then
there is just one of that ion. If there is parenthesis around the ion with a subscript following it,
then the subscript tells the number of that polyatomic ion in the compound. For example, in
the compound Mg3(PO4)2, there are 3 Mg’s and 2 PO4’s.
Next, let’s consider when we want to separate the polyatomic ion into the atoms that
make it up. If we start with an example without parenthesis, count the atoms like you would in
a compound that does not have a polyatomic ion in it. In HCO3, CO3 is a polyatomic ion. There
is 1 H, 1 C and 3 O’s. Pretty simple. If parenthesis, it is a little more complicated. To count
these atoms individually, multiply the subscript by any subscripts inside the parenthesis. Using
the earlier example, Mg3(PO4)2, there are 3Mg’s, 2 P’s and 6 O’s.
We will be adding another number to the formula in the unit coming up. These
numbers are called coefficients. Coefficients are whole numbers written in front of the formula
that tell how many of that compound there are. These are added when chemical equations are
balanced. To count atoms in a compound with coefficients, multiply the coefficient by the
subscripts. With polyatomic ions in the compound, you need to be careful. It is probably
easiest to first count the atoms in the compound without including the coefficient. Then
multiply the number by the coefficient. Here are the examples to review with:
compound
element
MgCl2
2MgCl2
Mg3(PO4)2- keep ion together
2Mg3(PO4)2- keep ion together
Mg3(PO4)2- separate ion
2Mg3(PO4)2- separate ion
Mg
2 Mg’s
3 Mg’s
6 Mg’s
3 Mg’s
6 Mg’s
element or
ion
2 Cl’s
4 Cl’s
2 PO4’s
4 PO4’s
2 P’s
4 P’s
element
8 O’s
16 O’s
Counting Atom Worksheet
Count all the atoms present in each of the following compounds. Keep polyatomic ions
together:
Ca(OH)2 Ca=
OH=
CuCl2
Cu=
Cl=
K3PO4
(NH4)2SO4
Hg3(PO4)2
4Al2O3
PbCrO4
Mg(ClO3)2
Fe(OH)3
3Ca(OH)2
Adapted From:
http://homepage.mac.com/pattysue/egs/pdf_chemistry07.files.html/CountingAtoms207.pdf