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.