Question

How many shells, orbitals and electrons are there in an atom having atomic mass 24 and atomic number 12 respectively?

Answer 1

We know,
Atomic number = No. of protons = No. of electrons = 12
So,
Electronic configuration = 2, 8, 2
Hence number of of shells $=3(K,L\text{and}M)$
Number of orbitals on which electrons are present
= (one $1s$ + one $2s$ + three $2p$ + one $3s$)
It can be expressed as $1s^{2}2s^{2}2p^{6}3s^2$