Counting the number of electrons in an atom

4 marks · Normal

The following information is given about three atoms:

Data about three atoms
Atom	Atomic number
A	6
B	18
C	2

(a)

State the number of electrons in atom A.

[1 mark]

(b)

State the number of electrons in atom B.

[1 mark]

(c)

State the number of electrons in atom C.

[1 mark]

(d)

Explain why the number of protons is equal to the number of electrons in a neutral atom.

[1 mark]

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(a) 6

(b) 18

(d) Because the positive charge of protons cancels the negative charge of electrons, so there is no overall charge.

Common mistakes: Atomic mass confused with atomic number

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Marking points

A1 (a) · 1 mark · States atom A has 6 electrons.

A2 (b) · 1 mark · States atom B has 18 electrons.

A3 (c) · 1 mark · States atom C has 2 electrons.

B1 (d) · 1 mark · Gives correct explanation: charges cancel or balance in a neutral atom; number of protons equals number of electrons.

Worked solution

a) Number of electrons in atom A = atomic number = 6.

b) Number of electrons in atom B = atomic number = 18.

c) Number of electrons in atom C = atomic number = 2.

d) In a neutral atom, the number of positive charges from protons equals the number of negative charges from electrons, so there is no overall charge.