Counting the number of electrons in an atom
2 marks · NormalState the number of electrons in:
(a)[1 mark]
an atom of magnesium (atomic number = 12)
(b)[1 mark]
an atom of phosphorus (atomic number = 15)
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(a) 12
(b) 15
Common mistakes: Confusing atomic mass with atomic number
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Marking points
M1 (a) · 1 mark · States that magnesium atom has 12 electrons.
M2 (b) · 1 mark · States that phosphorus atom has 15 electrons.
Worked solution
a) The atomic number of magnesium is 12. Number of electrons = number of protons = 12.
b) The atomic number of phosphorus is 15. Number of electrons = number of protons = 15.