The Periodic Table
5 marks · EasyThis question is about calculating and understanding how to fill electron shells.
State the maximum number of electrons that can fit in each of the first three shells for the first 20 elements.
Calcium has atomic number 20. Calculate how its electrons are distributed in the shells. Show your working.
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.
Show answer
(a) First shell: 2; Second shell: 8; Third shell: 8
(b) 2,8,8,2 (explanation: 2 electrons in 1st shell, 8 in 2nd, 8 in 3rd, 2 in 4th)
Common mistakes: Incorrect shell maxima (especially for third shell); Not filling lower shells first
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States maximum for first shell: 2 electrons.
B2 (a) · 1 mark · States maximum for second shell: 8 electrons.
B3 (a) · 1 mark · States maximum for third shell (first 20 elements): 8 electrons.
M1 (b) · 1 mark · Correct method shown (fills shells in order, does not exceed shell maxima).
A1 (b) · 1 mark · Final configuration given as 2,8,8,2.
Worked solution
a) First shell holds up to 2 electrons. Second shell up to 8. Third shell up to 8 (for first 20 elements).
b) Calcium (atomic number 20):
So, these shells take 18 electrons. 20 - 18 = 2 left for the next shell.
Electronic configuration: 2,8,8,2.