The periodic table
2 marks · NormalA phosphorus atom has a mass number of 31 and an atomic number of 15. State the number of protons and electrons in this atom.
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Number of protons = 15; Number of electrons = 15
Also accept: Protons = 15, electrons = 15
Common mistakes: Using the mass number instead of atomic number
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Marking points
B1 · 1 mark · Correct number of protons given (15)
B2 · 1 mark · Correct number of electrons given (15)
Worked solution
The atomic number of phosphorus is 15, so the atom has 15 protons.
The number of electrons in a neutral atom = number of protons, so there are also 15 electrons.