Atomic number and mass number

2 marks · Easy

An atom has an atomic number of 9 and a mass number of 19. Calculate the number of neutrons in this atom. Show your working.

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10 neutrons

Also accept: There are 10 neutrons in the atom.

Common mistakes: Subtracting mass number from atomic number instead of the other way round

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Marking points

M1 · 1 mark · Uses N = mass number − atomic number (19 − 9)

A1 · 1 mark · Correctly calculates number of neutrons: 10 (follow-through)

Worked solution

Number of neutrons = mass number − atomic number

There are 10 neutrons in the atom.