Drawing diagrams of electronic configurations
5 marks · HardAnswer the following about the rules for filling electron shells up to the third shell.
State the maximum number of electrons that can occupy:
the first shell
the second shell
the third shell, up to atomic number 10
Describe the order in which electrons fill these shells in an atom.
Explain why the first shell is always filled before electrons occupy the second shell.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a.i) 2
(a.ii) 8
(a.iii) 8
(b) Electrons fill the first shell completely before starting to fill the second shell, and the second before the third shell begins to fill.
(c) The first shell is filled before the second because it is the lowest energy level and electrons occupy the lowest available energy level first.
Also accept: Maximum electrons for shell 3 accepted as 8 for elements up to Ne; clear description of order and correct explanation phrased differently.
Common mistakes: Stating the third shell holds 18 electrons before atomic number 10; Mixing up order of filling
Show mark scheme and worked solution
Marking points
A1 (a(i)) · 1 mark · First shell: maximum 2 electrons.
A2 (a(ii)) · 1 mark · Second shell: maximum 8 electrons.
A3 (a(iii)) · 1 mark · Third shell: maximum 8 electrons up to atomic number 10.
B1 (b) · 1 mark · Describes correct sequential filling (each shell is filled before the next starts).
C1 (c) · 1 mark · Explains first shell fills first due to lowest energy.
Worked solution
a.i) The first shell holds a maximum of 2 electrons.
a.ii) The second shell holds a maximum of 8 electrons.
a.iii) The third shell holds a maximum of 8 electrons for elements up to atomic number 10.
b) Electrons fill the first shell completely before any are placed in the second shell, and fill the second shell before starting the third.
c) The first shell is filled before the second because it is closest to the nucleus and is the lowest available energy level, so electrons occupy it first for stability.