Relative atomic mass

2 marks · Normal

State the number of protons and the number of neutrons in an atom of magnesium-25 ().

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Also accept: 12 protons and 13 neutrons; Number of protons = 12, number of neutrons = 13

Common mistakes: Mixing up protons and neutrons; Giving atomic number as neutron count

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Marking points

B1 · 1 mark · Correct number of protons (12)

B2 · 1 mark · Correct number of neutrons (13, calculated by 25 - 12)

Worked solution

The atomic number of magnesium is 12, so it has 12 protons.

So, a magnesium-25 atom has 12 protons and 13 neutrons.