In GCE A-Levels, many question on Atomic Structure are based on Isotopes, more specifically it will be testing you on Isotopic Abundance.
We all know that Chlorine has a relative atomic mass, Ar of 35.5. How did we get it?
It is based on the Isotopic Abundance Calculation on its 2 isotopes, namely 35Cl (75% abundance) and 37Cl (25% abundance).
Let’s see how we get the value of 35.5 for its Ar.
Ar = ∑(Isotopic Mass x % Abundance) / 100
Putting the formulae into action, we will have:
Ar of Cl = [(75 x 35) + (25 x 37)] / 100 = 35.5
Got it?
How about trying out a question on your own?
Question:
Given that Lead has four isotopes: 204Pb, 206Pb, 207Pb and 208Pb. Relative abundance is 2%, 24%, 22% and 52% respectively. Calculate the relative atomic mass of Lead.
PS: Try it out and leave your answer in the comments section below.