Question

Complete the below table:

Symbol	Atomic number	Electronic configuration
$H$	\[1\]
$Cl$	$17$
$Ne$	$10$
$Na$	$11$

Answer 1

Hint: While assigning electrons to orbitals we must follow a set of three rules: the Aufbau Principle, the Pauli-Exclusion principle and Hund’s rule. According to the rule electrons are always filled in the lower energy orbitals before filling higher energy orbitals.

Complete step by step answer:
The filling of electrons into the orbitals of different atoms takes place according to the Aufbau principle which is based on Pauli's exclusion principle, the Hund’s rule of maximum multiplicity and the relative energies of the various orbitals.
Aufbau principle states that in the ground state of the atoms, the orbitals are filled in the order of increasing energies. We can say that electrons first occupy the lower energy orbitals and then the higher energy orbitals are filled.
According to Pauli’s exclusion principle each orbital can have a maximum of two electrons so $1s,2s\& 3s$ orbital have two electrons each and $2p\& 3p$ orbital can have a maximum of 6 electrons each.
Hence the following order of energies of the orbitals is useful:
$1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,4f,5d,6p,7s$
So, the filling of electrons takes place according to this order. The above series is a useful guide to the building up of electronic structure of atoms. Therefore the table given in the question can be filled according to Aufbau principle.

Symbol	Atomic number	Electronic configuration
$H$	\[1\]	$1{s^1}$
$Cl$	$17$	$1s{}^22{s^2}2{p^6}3{s^2}3{p^5}$
$Ne$	$10$	$1{s^2}2{s^2}2{p^6}$
$Na$	$11$	$1{s^2}2s{}^22{p^6}3{s^1}$

Note: So, the filling of electrons takes place according to Aufbau principle but there are some exceptions to the filing of electrons in some atoms like chromium and copper. In these cases a completely full or half full d subshell is more stable than partially filled d subshell therefore an electron is excited from \[4s\] to $3d$ orbital.