What are the rules that must be followed to write the electronic configuration of elements?

Electronic configuration:

• “The distribution of electrons into various orbitals of an atom is called its electronic configuration”.

Rules for writing electronic configuration:

There are three rules for writing electronic configuration:

• Aufbau principle
• Pauli’s exclusion principle
• Hund’s Rule

1. Aufbau principle:

• “It states that in the ground state of the atoms, the orbitals are filled in order of their increasing energies.”
• The order of filling of the electrons goes this way:1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,4f,5d,6p,7s…
• Each atomic orbital can just accommodate only 2 electrons that are in opposite spin only.
• So, the 1st shell can have 2 electrons in just 1s. The 2nd shell can have 8 electrons, 2 in 2s and 6 in 2p. The 3rd subshell can have 18 electrons, 2 in 3s, 6 in 3p and 10 in 3d.

2. Pauli’s exclusion principle:

• "No two electrons in an atom can have the same set of four quantum numbers. Pauli exclusion principle can also be stated as: “Only two electrons may exist in the same orbital and these electrons must have opposite spin.

3. Hund’s Rule:

• "It states: the pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each i.e., it is singly occupied."