The Distribution of electrons in the available orbits is governed by certain rules or principles such as (i) Paul’s exclusion principle, (ii) Aufbau principle (iii) (n+l) rule (iv) Hund’s rule etc.
The Aufbau principle:
It states that for any given atom, the electron are filled to the orbitals of lowest energy in sequence, two electrons to each orbital. in other words the electrons are fed in order of increasing orbital energy starting with Aufbau principle (German Aufbau Building up). Hypothetically we can build up the electrons in the lowest avaiable orbitals until the total number of electrons added is equal to the atomic number “Z” for example the electronic configuration of the first five elements is given in table.