Relative atomic mass

3 marks · Normal

Chlorine exists mainly as two isotopes, and . The Periodic Table gives chlorine a relative atomic mass of 35.5.

Explain why this value is not a whole number. Your answer should refer to isotopes and how the value is calculated.

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Because chlorine is a mixture of isotopes with different masses, and the relative atomic mass is the weighted average of their masses based on abundance, which is not a whole number.

Also accept: It is an average of the masses of its isotopes, weighted by their abundances; Because chlorine has two isotopes with different masses; the value is an average

Common mistakes: Saying there are half neutrons; Misunderstanding the weighting process

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Marking points

B1 · 1 mark · States that chlorine is a mixture of isotopes (not just one isotope).

B2 · 1 mark · Mentions that isotopes have different masses (mass numbers).

B3 · 1 mark · Explains that 35.5 is a weighted average based on abundance, not a whole number.

Worked solution

Chlorine's relative atomic mass is not a whole number because it is a mixture of two isotopes, and .

These isotopes have different masses (35 and 37), and the value 35.5 is the weighted average mass of all chlorine atoms, taking into account the proportion (abundance) of each isotope.