Counting Atoms in
Compounds or Molecules
Big Idea:
Given a chemical formula be able to count
the number of atoms for each element in
the compound or molecule.
Parts of a Chemical Formula
Element Symbol
2 H2O
Coefficient
Subscript
4 Pb(OH)4
3 Parts of a Chemical Formula
1. Chemical symbols
- Written in the order they appear on the
periodic table, from left to right.
2. The subscript
- Gives the number of atoms
- Must be written after the symbol, half size
3. The coefficient
- Gives the number of compounds or molecules
- Must be written regular size before the formula
Counting Atoms
Rules for counting atoms in a molecule or compound.
1. A subscript shows how many atoms of that element
are bonded in the molecule.
2. If there is no subscript then one atom of the element
is present.
3. If the subscript is outside the brackets, all the atoms
inside the brackets are multiplied by the subscript
outside the brackets.
4. Coefficients are multiplied last. The total number of
atoms in the formula is multiplied by the coefficient.
Count the Atoms
Oxygen – 1 atom
Hydrogen – 2 atoms
Formula - H2O
Count the Atoms
Ca(OH)2
Calcium – 1 atom
Oxygen – 2 atoms
Hydrogen – 2 atoms
Coefficients
The number of water
molecules is shown
with a coefficient.
Coefficients are written
in front of the
chemical formula.
3 H2O
Count all the atoms
3 H2O
The coefficient
multiplies all the
atoms after it.
Hydrogen 3 x 2 = 6
Oxygen 3 x 1 = 3
Test Yourself
Formula
Pb(OH)4
Mg3(PO3)2
As(IO4)5
2Cu(ClO2)2
Atom 1
present
How
Many?
Pb
1
Atom 2
present
O
How
Many?
Atom 3
present
How
Many?
4
H
4
Counting atoms
Formula
Atom 1
present
How
Many?
Pb
1
Mg3(PO3)2
Mg
As(IO4)5
Pb(OH)4
2Cu(ClO2)2
Atom 2
present
How
Many?
Atom 3
present
How
Many?
O
4
H
4
3
P
2
O
6
As
1
I
5
O
20
Cu
2
Cl
4
O
8