Question

The electronic configuration of calcium is 2,8,8,2. So why we can't write it as 2,8,9,1?

Answer 1

• The maximum number of electrons present in the shell on the 2n² rule.

Name of Shell	The value of ‘n’	Maximum electrons present (2n2)
K	1	2
L	2	8
M	3	18
N	4	32

• These shells are further divided into subshells/orbitals s, p, d, and f.
• The maximum electrons present in a subshell = 2(2l +1).

Subshell	Maximum electrons	l
s	2	0
p	6	1
d	10	2
f	14	3

• The electrons are arranged in the shell in the following manner:

Name of the shell	Name of the subshell	Maximum no. of electrons present in the shell	Distribute the electron in the subshell
K	1s	2	1s2
L	2s, 2p	8	2s2, 2p6
M	3s, 3p, 3d	18	3s2, 3p6, 3d10
N	4s, 4p, 4d, 4f	32	4s2, 4p6, 4d10, 4f14

• The filling up of electrons in the shells is followed according to the Aufbau principle.
• The Aufbau principle states that an electron occupies orbitals in order from lowest energy to highest.
  
  

Calcium(20) has 20 electrons.

• Its electronic configuration is 1s²2s²2p⁶3s²3p⁶4s² instead of 1s²2s²2p⁶3s²3p⁶3d¹4s¹.
• As the energy of the 3d orbital is higher than 4s, the last two electrons in calcium get accommodated in the 4s energy level instead of 3d.
• It is because of this order in which the atomic orbitals are filled as stated in Afbau’s principle that the electronic configuration of calcium is 2, 8, 8, 2 neither 2, 8, 9, 1 nor 2, 8, 10.