Drawing diagrams of electronic configurations

3 marks · Normal

An element has atomic number 8. Calculate how its electrons fill the shells. Show the number of electrons in each shell. State the rules used for filling shells.

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First shell: 2 electrons; Second shell: 6 electrons; Rules: first shell max 2, second shell max 8, third shell max 8; Fill inner shells before outer.

Must show: 2 electrons in first shell, 6 electrons in second shell, States rule about maximum per shell and order

Also accept: States correct max per shell (first 2, second 8, third 8) in any order

Common mistakes: Distributes electrons incorrectly (e.g. 4+4); Misstates shell maximums

Show mark scheme and worked solution

Marking points

M1 · 1 mark · States or applies rule: first shell up to 2 electrons, then second shell up to 8.

A1 · 1 mark · Correctly distributes electrons: 2 in first shell, 6 in second shell (adds to 8).

M2 · 1 mark · States electrons fill inner shells before filling the next shell.

Worked solution

Atomic number 8 means 8 electrons to place.

First shell can hold maximum 2 electrons. Place 2 there (8 - 2 = 6 left).

Second shell can hold up to 8 electrons. Place remaining 6 in second shell.

So, arrangement: 2 electrons (1st shell), 6 electrons (2nd shell).

Rules: Fill inner shells first. Maximum electrons: 1st shell = 2, 2nd shell = 8, 3rd shell = 8.