Class 11 | Stucture Of Atom | Hund's Rule For Maximum Multiplicity

3. Hund's rule of maximum.      Multiplicity 

Statement : When several orbitals of equal energy (degenerate orbitals) are available, the electrons first fill all the orbitals singly before pairing in any of these orbitals.

For example,

The Configuration of four electrons occupying p-orbitals is

Electronic configuration of nitrogen :

·Completely filled or half filled orbitals give extra stability.

Anomalous behaviour of  Cr :

· Atomic number Cr is 24.

· Expected configuration of 24Cr is 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d⁴ .

· The 3d orbital is not half filled. Hence it has less stability.

· Due to Interelectronic repulsion forces one 4s electron is transfer to empty 3d orbital to make 4s and 3d orbitals half filled.

· So Cr acquires extra stability and its configuration becomes, Is², 2s², 2p⁶, 3s², 3p⁶, 4s¹, 3d⁵ .

· In this configuration, both 4s and 3d subshells are half filled, This gives extra stability to Cr atom.

Anomalous behaviour of  Cu :

· Atomic number of Cu is 29.

^· The expected configuration of Cu is 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d⁹ .

· The 3d orbital is neither half filled nor completely filled. Hence it has less stability.

· Due to Interelectronic repulsion forces one 4s electron is transfer to one half filled 3d orbital to make it completely filled and 4s orbital half filled.

· So Cu acquires configuration 1s², 2s², 2p⁶, 3s², 3p⁶, 4s¹ 3d¹⁰

· In this configuration 3d sub shell is completely filled and 4s is half filled. This gives extra stability to Cu atom.