1.8 Isotopes and relative atomic mass
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What are isotopes?
Mark scheme
- atoms of the same element / same number of protons (1)
- with different numbers of neutrons (1)
Model answer
Isotopes are atoms of the same element, with the same number of protons but different numbers of neutrons.
Carbon-12 and carbon-14 are isotopes of carbon (atomic number 6). Give the number of neutrons in each.
Mark scheme
- carbon-12: 6 (1)
- carbon-14: 8 (1)
Model answer
Carbon-12 has 6 neutrons and carbon-14 has 8.
Boron is 20% boron-10 and 80% boron-11.
Calculate the relative atomic mass of boron.
Mark scheme
- (10 × 20 + 11 × 80) ÷ 100 (1)
- = 10.8 (1)
Model answer
A_\mathrm{r} = (10 × 20 + 11 × 80) ÷ 100 = (200 + 880) ÷ 100 = 10.8.
Explain why isotopes of an element have the same chemical properties.
Mark scheme
- same number of electrons / same electron arrangement (1)
- chemical reactions involve electrons (1)
Model answer
They have the same number of electrons arranged in the same way, and it is electrons that take part in chemical reactions.
Explain why the relative atomic mass of chlorine is 35.5 and not a whole number.
Mark scheme
- chlorine is a mixture of isotopes (35 and 37) (1)
- A_\mathrm{r} is an average weighted by abundance (1)
Model answer
Chlorine is a mixture of chlorine-35 and chlorine-37, so its relative atomic mass is a weighted average of their masses.
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