HOW TO COUNT ATOMS
1. The ​symbol​ of an element represents one atom of that element.
E.g.
Na
Type of Atom
Sodium
# of Atoms
1
Total
1
2. A ​subscript​ is a number written at the ​lower right​ corner ​beside the symbol​ of an
element. If there is more than one atom of the element, then a subscript is used to
indicate the number of atoms.
E.g.
H​2
Type of Atom
Hydrogen
# of Atoms
2
Total
2
3. If there is more than one atom, then each atom is counted individually and subscripts
are applied to each atom individually.
E.g.
H​2​SO​4
Type of Atom
Hydrogen
Sulphur
Oxygen
Total
# of Atoms
2
1
4
7
4. A ​subscript outside a bracket​ multiplies all the elements inside the brackets.
E.g.
Mg​3​(PO​4​)​2
Type of Atom
Magnesium
Phosphorus
Oxygen
Total
# of Atoms
3
2
8
13
5. a) A ​coefficient​ is a number written ​in front of​ a chemical ​symbol​ and indicates the
number of atoms of the element.
E.g.
3C
Type of Atom
Carbon
# of Atoms
3
Total
3
E.g.
3N​2​ =
Type of Atom
Nitrogen
# of Atoms
6
Total
6
5 b) Also, a ​coefficient​ is a number written ​in front​ of a chemical ​formula​ and
indicates the number of molecules of that compound.
**Note:​ A ​coefficient​ multiplies the number of atoms of each element in formula.
E.g.
2H​2​O
Type of Atom
Hydrogen
Oxygen
# of Atoms
4
2
Total
6
E.g. 3CuSO​4
Type of Atom
Copper
Sulphur
Oxygen
Total
# of Atoms
3
3
12
18
E.g. 4 Pb (NO​3​)​2
Type of Atom
Lead
Nitrogen
Oxygen
Total
# of Atoms
4
8
24
36
COUNTING ATOMS WORKSHEET
Complete the following charts.
Na​2​CO​3
Type of atom
Sodium
Carbon
Oxygen
Total
# of
Atoms
2
1
3
6
Ca​3​(PO​4​)​2
Type of atom
Phosphorous
Oxygen
Calcium
Total
# of
Atoms
2
8
3
13
Create charts and complete the following.
K​2​CrO​4
Type of atom
Potassium
Chromium
Oxygen
Total
# of
Atoms
2
1
4
7
​3BaCl​2
Type of atom
Barium
Chlorine
# of
Atoms
3
6
Total
9
Pb(NO​3​)​2
Type of atom
NH​4​C​2​H​3​O​2
Lead
Nitrogen
Oxygen
# of
Atoms
1
2
6
Total
9
2(NH​4​)​2​Cr​2​O​7​
Type of atom
Nitrogen
Helium
Chromanium
Oxygen
Total
Type of atom
Nitrogen
Hydrogen
Carbon
Oxygen
Total
# of
Atoms
1
7
2
2
12
4Al​2​(CO​3​)​3
# of
Atoms
4
16
4
14
38
Type of atom
Aluminum
Carbon
Oxygen
# of
Atoms
8
12
36
Total
56