Thompson/Candelario
IPC
(A309)
Name _______________________________________ Date _______________________Per _____
Number of Atoms in a Formula
Determine the number of atoms of each element in each of the following chemical formulas.
Chemical Formula
NaCl
H2SO4
2 KNO3
CaCl2
C2H6
NH4Br
KMnO4
Fe2O3
3 H2O2
Hg2Cl2
K2SO3
H3PO4
3 NaC2H3O2
(NH4)3PO4
Mg(C2H3O2)2
Ba(OH)2
Ca3(PO4)2
Al2(SO4)3
Mg(NO3)2
Cu(NO3)2
2 Na3(PO4)2
Number of Atoms of each Element
Na = 1
Cl = 1
H=2
S=1
O=4
Total atoms in the
formula
2
7
Cross / Kothari
IPC
(J118)
Name ________________________________________ Date __________________________ Per _____
Determining the Number of Atoms in a Formula Notes
Counting the number of atoms of each element is essential when learning to balance chemical
equations. Use the following rules to help you practice counting atoms in formulas.
Rule 1- Each symbol represents a separate element in a formula and if there is no subscript
will count as one atom of that element
Ex: NaCl
1 atom of Sodium & 1 atom of Chlorine
Rule 2- The subscript follows the symbol and represents the number of atoms of the element
it follows
Ex : H2O
the subscript 2 represents 2 atoms of Hydrogen
Rule 3- If a subscript comes directly after a set of ( ) parenthesis, then it gets DISTRIBUTED TO
ALL ELEMENTS INSIDE THE ( ) and is multiplied with any subscript inside the ( ).
Ex: (SO4)2 the subscript 2 will distribute across the entire polyatomic ion, so there are 2
Sulfurs and 8 Oxygens.
Rule 4- a Coefficient is the number that comes in front of the formula and will also distribute
to ALL ELEMENTS in the formula and also multiply with any subscript.
Ex: 3 Na2PO4 in this formula there are 6 Sodium, 3 Potassium, and 12 Oxygen.
Complete the chart that follows:
Chemical Formula
MgO
SO2
2 H2CO3
C12H22O11
NaOH
3 MgSO4
Sr(OH)2
Number of atoms of each element
Mg =
S=
H=
H=
Na =
Mg =
Sr =
C=
C=
O=
S=
O=
Total number of
atoms
O=
O=
O=
O=
H=
O=
H=
Cross / Kothari
IPC
(J118)
Name ________________________________________ Date __________________________ Per _____
Determining the Number of Atoms in a Formula Notes
Counting the number of atoms of each element is essential when learning to ____________________
__________________. Use the following rules to help you practice counting atoms in formulas.
Rule 1- Each symbol represents a ____________________________ in a formula and if there is no
subscript will count as one atom of that element.
Ex: NaCl
1 atom of Sodium & 1 atom of Chlorine
Rule 2- The subscript _________________the symbol and represents the number of atoms of the
element before it.
Ex : H2O
the subscript 2 represents 2 atoms of ______________
Rule 3- If a subscript comes directly __________ a set of ( ) parenthesis, then it gets
_______________________________________________________ and is multiplied with any subscript
inside the ( ).
Ex: (SO4)2 the subscript 2 will ________________________________ polyatomic ion, so there
are ______ Sulfurs and _______ Oxygens.
Rule 4- a ____________________ is the number that comes ____________________________
_________________ and will also distribute to _____________________ in the formula and also
multiply with any subscript.
Ex: 3 Na2PO4 in this formula there are 6 Sodium, 3 Potassium, and 12 Oxygen.
Complete the chart that follows:
Chemical Formula
MgO
SO2
2 H2CO3
C12H22O11
NaOH
3 MgSO4
Sr(OH)2
Number of atoms of each element
Mg =
S=
H=
H=
Na =
Mg =
Sr =
C=
C=
O=
S=
O=
Total number of
atoms
O=
O=
O=
O=
H=
O=
H=