Relative Atomic Mass — Atomic Structure | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable

Medium

Stable9%

Subatomic particles and electronic configuration appear in every exam series.

Relative atomic mass (A_r): a weighted-average mass for an element, calculated from the masses and percentage abundances of all of its naturally occurring isotopes, expressed relative to 1/12 the mass of a carbon-12 atom

Multiply each isotope's percentage abundance by its mass number, sum the products, then divide by 100:

$A_r = \frac{(\%_A \times \text{mass}_A) + (\%_B \times \text{mass}_B) + \ldots}{100}$

Add one term to the top of the fraction for every isotope present
Example. Natural chlorine consists of 75% ³⁵Cl (mass 35) and 25% ³⁷Cl (mass 37); applying the formula:

$A_r = \frac{(75 \times 35) + (25 \times 37)}{100} = \frac{2625 + 925}{100} = \mathbf{35.5}$

This is why chlorine appears on the periodic table with A_r = 35.5, not a whole number