-But thats not all. You also have to know in what percentage these Isotopes occur in nature. With
that information you can find out the average atomic mass of an element. The only way to find this out is to use a bit of
mathematics.

1)The first step is to write down the percentage in which the Isotopes occur.

75%
25%

Of Cl atoms
Of Cl atoms

are Cl-35 are Cl-37

2)The second step is to convert our percentages into decimals. A simple mathematical equation
does the trick. Take your percentage and divide it by 100.

75/100
25/100

.75
.25

3)Next take your new decimal and multiply it by its Atomic Mass.

.75x35
.25x37

26.25
9.25

4)After the multiplication you are left with two decimals. Add them together to find your average
atomic mass for the element you are dealing with.

26.25 + 9.25 = 35.5

5)Finally you are left with that atomic mass units (amu) of the element you were dealing with.

35.5 amu

-To help make sense of the work you did following a procedure can help in remembering steps and aid in organization.
Here is a final product following a structured procedure:

Cl

(Chlorine)
Atomic # Atomic Mass

Cl-35 - 75%
17
35

Cl-37 - 25%
17
37

75/100
25/100

.75
.25

.75x35
.25x37

26.25
9.25

26.25
+ 9.25

35.5amu

If
you need a simple way to remember the procedure of writing Isotopes memorize this:

%

% -->
decimal

Decimal
x Atomic Mass

Decimal
+ Decimal + ...