Question

What is the maximum number of electrons that can be present in an atom in which the highest principal quantum number is $4$.

Answer 1

Keeping in view Aufbau principle, the electronic configuration of atom having highest value of $n=4$ will be $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^6$ (After $4p$, filling of $5s$ stars)

Hence maximum number of electrons present $=36$ (Kr).