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Atomic Orbitals: Filling Of Electrons

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Atomic Orbitals: Filling Of Electrons - Lesson Summary

The filling of electrons into the orbitals of an atom is governed by the Aufbau principle, Pauli's exclusion principle and Hund's rule of maximum multiplicity.

Aufbau principle:
"In the ground state of the atom, the orbitals are filled in the order of their increasing energies".

So the electrons first occupy the lowest energy orbitals available to them and then enter into the higher energy orbitals successively. The energy of an orbital is determined by using the (n+l) rule.

      Type of Orbital      Value of n      Value of l     Value of n + l           1s           1           0           1+0 = 1           2s           2           0           2+0 = 2           2p           2           1           2+1 = 3           3s           3           0           3+0 = 3           3p           3           1           3+1 = 3           4s           4           0           4+0 = 4           3d           3           2           3+2 = 5           4p           4           1           4+1 = 5

According to this rule, the lower the value of (n+l) for an orbital, the lower is its energy. If two orbitals have the same (n+l) value, the orbital with lower value of n has lower energy and hence is filled first.

Ex: For 4s orbital, the (n+l) value is (4+0) is 4. For 3d orbital, the (n+l) value is (3+2) is 5. So, 4s orbital will be filled first as it has lower value of (n+l).

Pauli's exclusion principle:
It states that, no two electrons in an atom can have the same set of four quantum numbers. This principle can also be stated as: "Only two electrons may exist in an orbital, which must have the opposite spin".

Ex: Two electrons present in 1s-orbitals have same values three quantum numbers, but differ in spin quantum number value.


Hund's rule of maximum multiplicity:
This rule helps in filling of electrons into the orbitals of same sub-shell. The orbitals belonging to the same sub-shell have similar energy and are called Degenerate orbitals.

Ex: There are three degenerate orbitals present in p sub-shell.

The pairing of electrons in the orbitals belonging to the same sub-shell doesn't take place until each orbital belonging to the same sub-shell gets one electron each.

Therefore, electron pairing in p, d, or f sub-shells cannot take place until each orbital of a given sub-shell contains one electron each.

All the singly occupied orbitals will have spins in the same direction.

And it is observed that half filled and completely filled sub-shells impart greater stability to atoms due to the symmetrical distribution of electrons.

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