Filling of orbitals in atom

 Ans-:

The electronic configuration can be done by following 3 rules

The filling of electrons into the orbitals of an atom is governed by the Aufbau principle, Pauli's exclusion principle and Hund's rule of maximum multiplicity.

Aufbau principle:

"In the ground state of the atom, the orbitals are filled in the order of their increasing energies"

Pauli's exclusion principle:
This principle can also be stated as: "Only two electrons may exist in an orbital, which must have the opposite spin".

Ex: Two electrons present in 1s-orbitals have same values three quantum numbers, but differ in spin quantum number value.

•     Hund's rule of maximum multiplicity:
This rule helps in filling of electrons into the orbitals of same sub-shell. The orbitals belonging to the same sub-shell have similar energy and are called Degenerate orbitals.

•     Ex: There are three degenerate orbitals present in p sub-shell.

•     EXAMPLE:

•      electron configuration of Carbon will be 1s²2s²2p².

•     electron configuration of Neon will be 1s²2s²2p⁶

Some exceptions

The actual electronic configuration of some elements such as chromium and copper slightly differ from the expected electronic configuration in accordance with the Aufbau principle.

 

For chromium - 24

Expected configuration :

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ 4s²

Actual configuration :

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

 

For copper - 29

Expected configuration :

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁹ 4s²

Actual configuration :

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹