The outermost shell of an atom is called its Valence shell. It is the decisive shell during a chemical reaction. The electrons of only this outermost shell are involved during chemical combinations; electrons are either given out from the outermost shell, or accepted into the outermost shell, or shared with the electrons in the outermost shell of another element.
Valance shell electronic configuration is the representation of the participation of electrons in orbits of atom.
The electrons in an atom are filled by the three basic principles:
AufBau Principle
Pauli Exclusion Principle
Hund’s Rule
As per AufBau’s principle, the electrons will be filled first in orbital’s with lower energy like 1s will be filled first before 2s and 2s will be filled before 2p.
As per Pauli’s exclusion principle, no two electrons within a particular atom can have identical can have identical quantum numbers. In similar meaning it says that two electrons occupy the same orbital must have opposite spins.
As per Hund’s rule, when an electron come to an atom, and has to choose between two or more orbital’s of same energy, then the preference will be given to empty orbital rather than the occupied one. As more electrons will be occupied, they tend to follow half-filled and fully filled rule.
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