Relative atomic mass

Elements contain atoms that differ slightly in atomic mass. This difference, as we encountered earlier, is due to different numbers of neutrons in the nucleus of atoms of the same element. Although all atoms of the same element behave in an identical manner chemically, they differ in mass. For example oxygen is made up of three different isotopes, its isotopic composition is:
- Oxygen with mass 16 makes up 99.76% of the naturally occuring oxygen;
- Oxygen with mass 17 makes up 00.04% of the naturally occuring oxygen;

- Oxygen with mass 18 makes up 00.20% of the naturally occuring oxygen;

So how do we come up with the relative atomic mass of oxygen as being 16? The relative atomic mass of any element is the weighted average of the isotopic masses. The calculations are as follows.

relative atomic mass of oxygen = (16 X 99.76 + 17 X 0.04 + 18 X 0.2) / 100

= 16 atomic mass units.

So the formula is
rel. atomic mass= (% X isotopic mass1 + (100 - %) X isotopic mass2) / 100
We simply multiply all the isotopic masses by their respective relative abundance(%) and then divide by 100.

Try some exercises (click on the coloured cells for more information, double click to hide the information)

Element
Isotopic mass
Relative abundance
Relative atomic mass.
Lithium
7.02
8.0%
6.02
92.0%
Chlorine
34.97
75.80%
36.97
24.20%
Copper
62.93
69.10%
64.93
30.90%
Oxygen
16.00
99.76%
17.00
0.04%
18.00
0.20%

An element "X" has two isotopes 22X and 24X. If the average atomic mass of X is 23.4 find the percent abundance of each isotope.

Solution

Continue with mass spectrum