Relative Atomic Mass

IGCSE Edexcel Chemistry
1.16-1.17 Isotopes and relative atomic mass
Key Concept: Relative atomic mass is the weighted average of isotopes based on their abundances.

Section A: Isotopes

1. Define an isotope. [2]
2. Calculate the relative atomic mass of chlorine if 75% is Cl-35 and 25% is Cl-37. [2]

Section B: Calculations

3. An element X has isotopes 10 (40%) and 11 (60%). Calculate Ar. [2]
4. Explain why Ar is not a whole number for most elements. [2]

Total marks: 8

Mark Scheme

1. Atoms of the same element with same number of protons but different numbers of neutrons [2]
2. (35 x 0.75) + (37 x 0.25) = 35.5 [2]
3. (10 x 0.40) + (11 x 0.60) = 10.6 [2]
4. It is a weighted average of isotopes with different masses and abundances [2]