Relative atomic mass - Higher tier

The relative atomic mass of an element (A_r) is an average value for the isotopesof the element. For example, the A_r for chlorine is 35.5 because it contains two different isotopes.

Chlorine

Chlorine has two isotopes:

 

Isotope	Protons	Electrons	Neutrons
	17	17	35 - 17 = 18
	17	17	37 - 17 = 20

75 per cent of chlorine atoms are ³⁵Cl and 25 per cent of chlorine atoms are ³⁷Cl.

This means that in 100 chlorine atoms, 75 will be ³⁵Cl and 25 will be ³⁷Cl.

The total A_r for these chlorine atoms will be (75 × 35) + (25 × 37) = 2625 + 925 = 3550.

So the average A_r for chlorine is 3550 ÷ 100 = 35.5.

A standard atom

The mass of the ¹²C isotope is the 'standard atom' that the masses of other atoms are compared to. The A_r of ¹²C is defined as 12. This means, for example, that one ²⁴Mg atom has twice the mass of a ¹²C atom.