Calculation of Ar based on Isotopic Abundance

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, A_r 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 A_r.

A_r =  ∑(Isotopic Mass x % Abundance) / 100

Putting the formulae into action, we will have:

A_r 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.

Related Articles:

• Calculating Relative Atomic Mass from Isotopic Abundance
• Calculation of Ar based on Isotopic Abundance – Challenging Question
• Isotopes & Isotopic Abundance
• Key Definitions & Formulae in Atoms, Molecules and Stoichiometry
• Sub-Atomic Particles of Atoms