Question

What is Aufbau principle and filling up of electrons with orbitals?

Answer 1

Hint : Electron configuration system is the arrangement of electrons of a molecule or an atom in their orbitals, shells, subshells. It gives us ideas about the number of valence electrons that particular atom or molecule has.

Complete Step By Step Answer:
To deal with this question we have to keep in mind three main and common rules that govern this system and they are – Aufbau principle, Pauli-exclusion principle and Hund’s rule.
So, Aufbau principle says that in an atom, electrons should fill the empty available orbitals such that the atom attains the maximum stability. This principle says that electrons should start filling from the orbital with lowest energy that means the electron first should enter the orbital with lower energy and then proceed to higher energy levels accordingly.
Now, to understand the filling of orbitals, one should know the criteria that determines the energy level of the orbital
So, the energy level of the orbital is determined by the sum of principal quantum number $(n)$ and azimuthal quantum number $(l)$ .
$Energy = n + l....(i)$
Principal quantum number is given to shells. $(n)$
And, Azimuthal quantum number $(l)$ is assigned to the subshells $(l)$ $=$ $(s,p,d,f)$
Now let’s have a look at the value of, Azimuthal quantum number $(l)$ for a particular, $(n)$ principal quantum number.

ORBITAL	principal quantum number $(n)$	Azimuthal quantum number $(l)$	$Energy = n + l$
$1s$	$1$	$0$	$1$
$2s$	$2$	$0$	$2$
$2p$	$2$	$1$	$3$
$3s$	$3$	$0$	$3$
$3p$	$3$	$1$	$4$
$4s$	$4$	$0$	$4$
$3d$	$3$	$2$	$5$
$3p$	$3$	$1$	$5$

Hence, from equation (i), and above table, we can say
This arrangement is in increasing order of their energies and the orbitals are filled in the increasing order of their energies only , such as the first electron will go in the s orbital then in p then d and so on.
$1s < 2s < 2p < 3s < 3p < 4s < 3d$

Note :
As we get the same $Energy = n + l$ value, then in such cases we look for the principal quantum number. The one having the higher value of $(n)$ , will have high energy and will be filled after filling the orbital with a lower principal quantum number having the same energy.