Counting Atoms:
How to Count Atoms Review
1. The symbol of an element represents one atom of that element.
e.g. Ca =
2. A subscript is a number written at the lower right corner behind the symbol of an
element. If there is more than one atom of the element in the molecule, then a
subscript is used to indicate the number of atoms.
e.g. N2 =
3. A subscript outside a bracket multiplies all the elements inside the brackets.
e.g. Ba3(PO4)2 =
4. (a) A coefficient is a number written in front of a chemical symbol and indicates
the number of atoms of that element.
e.g. 3C =
or
(b) A coefficient is a number written in front of a chemical formula and indicates the
number of molecules of that compound.
Note: a coefficient multiples the number of atoms of each element in the formula.
e.g. 2H2O
3FeSO4
4Cu(NO3)2
Counting Atoms Worksheet
Fill in each table by counting atoms in each formula.
Na2CO3
Type of Atom
# of Atoms
Ca3(PO4)2
Type of Atom
Total
Total
K2CrO4
Type of Atom
3BaCl2
Type of Atom
# of Atoms
# of Atoms
# of Atoms
Total
Total
NH4C2H3O2
Type of Atom
4Al2(CO3)3
Type of Atom
# of Atoms
# of Atoms
Total
Total
Pb(NO3)2
Type of Atom
Total
2(NH4)2Cr2O7
Type of Atom
# of Atoms
# of Atoms
Total
Fill in the following chart. An example is given for you.
Formula
Name of
Compound
MgO
SO2
NH3
ammonia
H3CO3
Carbonic
acid (soda
water)
Sucrose
(Table
sugar)
C12H22O12
MgSO4
(Epsom
salts)
NaOH
H2O2
(lye)
Hydrogen
peroxide
Fe2O3
Na2HCO3
(rust)
Sodium
bicarbonate
(baking
soda)
Number of
Elements
Names of
Number of
the Elements Atoms of
Each
Element
2
Magnesium
Oxygen
1
1
Total
Number of
Atoms in
One
Molecule
2
Writing Word and Skeleton Equations:
Word equations represent chemical reactions using words and symbols.
E.g zinc + sulfur  zinc sulfide
Skeleton equations represent chemical reactions using formulae and symbols.
E.g. Zn + S  ZnS
The items on the left of the arrow are the items that enter the reaction. They are
called reactants. The items on the right of the arrow are the items that exit the
reaction. They are called products.
For the following descriptions, write word and skeleton equations to represent that is
happening in the reactions:
1.
2.
3.
4.
5.
6.
7.
8.
9.
copper and chlorine form copper (II) chloride.
Aluminum metal rusts in oxygen gas. The product is aluminum oxide.
magnesium and iodine form magnesium iodide.
sodium and bromine form sodium bromide.
hydrogen gas and chlorine gas form hydrogen chloride (aka hydrochloric acid)
hydrogen gas and sulfur form dihydrogen sulfide
calcium and chlorine gas form calcium chloride
potassium and iodine gas form potassium iodide
zinc rusts in oxygen gas to form zinc oxide.
Complete the word equations by filling in the blanks. Then write skeleton equations
for each.
1.
+ oxygen  zinc oxide
 potassium iodide
2. potassium +
3.
+ oxygen  carbon dioxide
 sulfur dioxide
4. sulfur +
5.
+ iodine  calcium iodide
6.
+
 magnesium bromide
7. magnesium +
8.
 sodium chloride
+ sulfur  carbon disulphide
9. zinc + sulfur 
10.
+
 water