# Aufbau Principle, Pauli's Exclusion Principle And Hund's Rule

In different atoms the filling of electrons into the orbitals takes place according to the three main principles naming the Aufbau Principle, Pauli exclusion principle and Hundâ€™s rule of maximum multiplicity:

## Aufbau principle

• The Aufbau Principle is basically a German word “Aufbauen” which means building up.
• According to this principle, the electrons will first occupy the orbitals that have the lowest energy. This means that the electrons enter first orbital and then enter the orbitals which have higher energy but only when the lower- energy orbitals are completely filled.
• The order in which the electrons should be filled is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7pâ€¦..
• The order of the increasing energy of the orbitals can also be calculated using (n+l) rule.
• The sum of the values of the principal quantum numbers (n) and azimuthal quantum number (l) is used to decide the energy level of an orbital.
• The lower value of the sum of (n+l) implies that the energy of the orbital is low. If the value of n+l for two orbitals is equal then the orbital with a lower value of n will have a lower energy level.

## Pauli exclusion principle

• According to this principle, an orbital can have maximum two electrons and these must have opposite spins.
• In other words, no two electrons in an atom can have all the four quantum numbers exactly the same.
• As we know that fourth quantum number is m, 1/2 is spin up and -1/2 is spin down and hence if the first three quantum numbers are same for two electrons then the electrons must have opposite spins in an orbital.

## Hundâ€™s rule

• According to this rule, theÂ electrons are filled in the degenerate orbitals of the same subshell.
• Electron pairing in p, d and f orbitals cannot be done until each sub-shell is occupied singly.
• This is because electrons are the same in charge and they repel each other. This repulsion is minimized if two electrons move away from each other by acquiring different degenerate orbitals.
• All the orbitals which are singly occupied have parallel spins which can be either clockwise or anticlockwise.
• If the electrons have parallel spin then there will be less inter-electronic repulsions and high spin multiplicity.
• So in order to maximize spin multiplicity, the pairing of electrons in a sub-shell does not take place until each sub-shell is singly occupied.

Filling Electron Diagram by Hund’s Rule

This was a brief introduction to the rules that govern the electronic configuration of elements. Learn how to write the electronic configurationÂ of an element and the related concepts, register with BYJU’S.

#### Practise This Question

"An orbital can have 10 electrons”.