The periodic table
3 marks · NormalAn atom of calcium has a mass number of 40 and an atomic number of 20.
Calculate the number of:
protons
neutrons
electrons
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(a) 20
(b) 20
(c) 20
Common mistakes: Using wrong arithmetic for neutrons; Using mass number for electrons
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Marking points
B1 (a) · 1 mark · Correct number of protons (20)
M1 (b) · 1 mark · Correctly calculates number of neutrons: 40 - 20 = 20 (follow-through)
B2 (c) · 1 mark · Correct number of electrons (20)
Worked solution
(a) The atomic number of calcium is 20, so number of protons = 20
(b) Number of neutrons = mass number - atomic number = 40 - 20 = 20
(c) Number of electrons = atomic number (in a neutral atom) = 20