Question

What is the first example in the periodic table where the Aufbau principle is not followed?

Answer 1

Answer:

the 4s subshell is lower in energy than the 3d subshell

Explanation:

In the element number 24 which is chromium

the electronic configuration should be according to Aufbau principle

will be  = $1s^22s^22p^63s^23p^64s^23d^4$

but in reality it is = $1s^22s^22p^63s^23p^64s^13d^5$

The 6 electrons in the outermost shell will be divided as 5 in 3d subshell and 1 in 4s shell, because half filled orbital $4s^13d^5$ will be more stable than $4s^23d^4$.