Electronic configurations and the periodic table

3 marks · Normal

Argon is a noble gas with atomic number 18. Calculate the number of electrons in its outer shell and state which group and period it belongs to.

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Argon has 8 electrons in its outer shell, belongs to Group 0 (or 8), and is in Period 3.

Also accept: Group 0 or Group 8 both accepted

Common mistakes: Counting group as Group 2; Stating only 2 shells

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Marking points

M1 · 1 mark · Works out correct electron configuration for argon (2,8,8)

A1 · 1 mark · States correct number of outer electrons (8)

A2 · 1 mark · Correctly states group and period (Group 0 or 8, Period 3)

Worked solution

The atomic number of argon is 18. Filling shells in order: 2 electrons in first shell, 8 in second, 8 in third = 2,8,8.

Number of outer electrons is 8 (third shell).

Argon is in Group 0 (or Group 8), Period 3.