Pauli Exclusion Principle : No two electrons in an atom can have the same set of four quantum numbers . Only two electrons may exist in the same orbital and these electrons must have opposite spin .
Hund ’ s rule of maximum multiplicity : 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 .
Electronic configuration of atoms : Arrangement of electrons in different orbitals of an atom . The electronic configuration of different atoms can be represented in two ways . a . s a p b d c ...... notation . b . Orbital diagram :, each orbital of the subshell is represented by a box and the electron is represented by an arrow ( ↑ ) a positive spin or an arrow ( ↓ ) a negative spin .
Stability of completely filled and half filled subshells :
a . Symmetrical distribution of electrons- the completely filled or half filled sub-shells have symmetrical distribution of electrons in them and are more stable .
b . Exchange energy-The two or more electrons with the same spin present in the degenerate orbitals of a sub-shell can exchange their position and the energy released due to this exchange is called exchange energy . The number of exchanges is maximum when the subshell is either half filled or completely filled . As a result the exchange energy is maximum and so is the stability .