Counting the number of electrons in an atom

2 marks · Hard

Cobalt has an atomic number of 27.

Give the number of electrons in a neutral atom of cobalt and explain your answer.

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27; because the number of electrons equals the atomic number in a neutral atom.

Must show: States 27; reason = number of electrons equals atomic number in a neutral atom

Also accept: 27; because number of electrons equals number of protons in a neutral atom; 27; in a neutral atom, electrons = protons = atomic number

Common mistakes: Giving number of protons or neutrons instead of electrons; Just stating 27 without explanation

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Marking points

B1 · 1 mark · States correct number of electrons as 27.

B2 · 1 mark · Gives correct reason: number of electrons equals atomic number in a neutral atom.

Worked solution

Number of electrons = atomic number = 27.

Reason: In a neutral atom, the number of electrons equals the atomic number because protons and electrons balance each other to give no overall charge.