Define the principal quantum number $(n)$.

The principal quantum number $(n)$ is a positive integer $(n = 1, 2, 3, \dots)$ that describes the main energy level or shell of an electron in an atom.
Key characteristics of the principal quantum number $(n)$:
$1$. It determines the size of the orbital and,to a large extent,its energy.
$2$. As the value of $n$ increases,the electron is located further from the nucleus,and the energy of the orbital increases.
$3$. The total number of orbitals in a given shell is given by $n^{2}$.
Representation of shells:

Main shell $(n)$	$1$	$2$	$3$	$4$
Symbol (shell)	$K$	$L$	$M$	$N$
No. of orbitals $(n^{2})$	$1$	$4$	$9$	$16$