Relative atomic mass
4 marks · NormalConsider two isotopes of carbon: and .
State the number of protons and neutrons in .
State the number of protons and neutrons in .
Explain why and are considered isotopes of the same element.
Describe one way in which the two isotopes differ.
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(a) 6 protons, 6 neutrons
(b) 6 protons, 8 neutrons
(c) They are considered isotopes because they have the same number of protons but different numbers of neutrons.
(d) They have different numbers of neutrons/mass numbers.
Common mistakes: Giving the wrong number of protons
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Marking points
B1 (a) · 1 mark · Correct number of protons and neutrons for : 6, 6
B2 (b) · 1 mark · Correct number of protons and neutrons for : 6, 8
B3 (c) · 1 mark · Explains both isotopes have the same number of protons but different neutrons.
B4 (d) · 1 mark · Describes one difference: number of neutrons or mass number.
Worked solution
a) : 6 protons, 6 neutrons (12 – 6).
b) : 6 protons, 8 neutrons (14 – 6).
c) Both have 6 protons (atomic number = 6) but different numbers of neutrons, so they are isotopes.
d) They differ in the number of neutrons (6 vs 8).