Atomic number and mass number

2 marks · Hard

Helium-3 is an isotope of helium with a mass number of 3. It has 2 protons. State the number of neutrons and electrons in a neutral atom of helium-3.

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1 neutron, 2 electrons

Also accept: neutrons: 1, electrons: 2; number of neutrons: 1, number of electrons: 2; 1 neutron and 2 electrons

Common mistakes: Confusing number of neutrons with protons; Giving the mass number as number of neutrons

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Marking points

B1 · 1 mark · States correct number of neutrons (1)

B2 · 1 mark · States correct number of electrons (2)

Worked solution

Number of neutrons = mass number - number of protons = 3 - 2 = 1

Number of electrons in a neutral atom = number of protons = 2

Final answer: 1 neutron, 2 electrons