Atomic number and mass number

3 marks · Normal

An atom of magnesium has atomic number 12 and mass number 25.

Calculate the number of neutrons in this atom. Show your working.

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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13 (±0)

Must show: mass number – atomic number = neutrons

Also accept: Neutrons = 25 – 12 = 13

Common mistakes: Wrong formula used; Arithmetic error

Show mark scheme and worked solution

Marking points

M1 · 1 mark · Writes correct formula: number of neutrons = mass number – atomic number.

M2 · 1 mark · Substitutes 25 and 12 into the formula.

A1 · 1 mark · Correct answer: neutrons = 13.

Worked solution

Number of neutrons = mass number – atomic number