Question

If n = 6, the correct sequence for filling of electrons will be:
A. $ns \to np \to (n - 1)d \to (n - 2)f$
B. $ns \to (n - 2)f \to (n - 1)d \to np$
C. $ns \to (n - 1)d \to (n - 2)f \to np$
D. $ns \to (n - 2)f \to np \to (n - 1)d$

Answer 1

This question can be solved by Aufbau’s principle. The Aufbau’s principle says that the atomic orbital with lower energy level is filled first and then the atomic orbital with higher energy level.

Complete step by step answer:
The filling of electrons is done by Aufbau’s Principle. The Aufbau’s principle tells about the way of filling the electrons in the atomic orbital of the atom. The Aufbau’s principle states that the electrons are filled into the atomic orbital in the increasing order of energy level. According to Aufbau’s principle, the atomic orbital with lowest energy level is filled first than the orbital with highest energy level.
When the (n + 1) rule is followed, that means when the sum of the principal quantum number and azimuthal quantum number is equal for two or more orbits, the higher value of n will have a higher energy level.
When the value of n is greater than the energy is high and when the value of n is low than the energy is low.
When n = 6, the filling of electrons is done as $ns \to (n - 2)f \to (n - 1)d \to np$
Substitute the value of n,
$\Rightarrow 6s \to (6 - 2)f \to (6 - 1)d \to 6p$
$\Rightarrow 6s \to 4f \to 5d \to 6p$
For 6s (n + l) = (6 + 0 = 6)
For 4f (n + l) = (4 + 3 = 7)
For 5d (n + l) = (5 + 2 = 7)
For 6p (n + l) = (6 + 1 =7)
Thus, the electrons are filled in order 6s, 4f, 5d, 6p.
Therefore, the correct option is B.

Note:
There is an exception for Aufbau’s principle for the chromium atom as its electronic configuration is $[Ar]3{d^5}4{s^1}$and not $[Ar]3{d^4}4{s^2}$. This is governed by several factors like increased stability because of the half-filled subshell and low energy gap between the 3d and 4sorbital.