How to Count Atoms
Worksheet
1. The symbol of an element represents one atom of that element.
e.g., Ba = 1 atom of Barium
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, then a subscript is used to indicate the number of
atoms.
e.g., Cl2 = 2 atoms of Chlorine
3. A subscript outside a bracket multiples all the elements inside the brackets.
e.g., Ca3(PO4)2 =
Ca
P
O
3 atoms
= ________
2 atoms
= ________
8 atoms
= ________
3. A coefficient is a number written in front of a chemical symbol and indicates the number of
atoms of that element or number of molecules
3 carbon atoms
e.g., 3C = ____
sodium atoms
2 sulfur_______
atoms 8 oxygen atoms
2NaSO4 = 2 _______
_______
A subscript is a number written after an atom in a formula and indicates the number of atoms of
the kind in the molecule.
4
e.g H2SO4 The subscript of H = 2 and the subscript of O = ____
Note: a coefficient multiples the number of atoms of each element in the formula
e.g.,
2 H 2O
_____
2 molecules of H2O
_____
4 H (hydrogen)
_____
O (oxygen)
2
3 Na2SO4
_____
3 molecules of Na2SO4
6 Na (copper)
_____
3 S (sulphur)
_____
12
_____ O (oxygen)
4 Pb(NO3)2
4
_____
molecules of Pb(NO3)2
4 Pb (Lead)
_____
8 N (nitrogen)
_____
24 O (oxygen)
_____
Counting Atoms
Worksheet
Count the atoms present in the different compounds by using the coefficients and subscripts.
K2CO3
Ba3(PO4)2
Type of Atom
# of Atoms
K
Potassium
2
______________
______
C Carbon
1
______________
______
O Oxygen
3
______________
______
6
Total
______
Type of Atom
Ba Barium
_______________
P Phosphorous
_______________
O Oxygen
_______________
Total
Na2CrO4
Type of Atom
# of Atoms
2
Na
Sodium
______________
______
Cr Chromium
1
______________
______
4
O Oxygen
______________
______
7
Total
______
NH4C2H3O2
Type of Atom
# of Atoms
N
Nitrogen
1
______________
______
______________
______
7
H Hydrogen
2
______________
______
C Carbon
2
O Oxygen
______________
______
12
Total
______
Pb(NO3)2
Type of Atoms
# of Atoms
1
______________
______
Pb Lead
N Nitrogen
______________
______
2
O Oxygen
______________
______
6
Total
______
9
# of Atoms
3
______
2
______
8
______
13
______
3 CaCl2
Type of Atom
# of Atoms
3
Ca
Calcium
_______________
______
Cl Chlorine
6
_______________
______
9
Total
______
4 Al2(CO3)3
Type of Atom
# of Atoms
8
Al Aluminum
________________
______
12
C Carbon
________________
______
O Oxygen
________________
______
36
Total
______
56
2 (NH4)2Cr2O7
Type of Atom
Nitrogen
________________
Hydrogen
________________
Cr
Chromium
________________
O Oxygen
________________
Total
# of Atoms
4
______
______
16
2
______
7
______
______
29