RULES FOR WRITING ELECTRONIC CONFIGURATION
Pauli's exclusion principle:
No two electrons in the same atom can have the same set of values for all the four quantum numbers.
Two electrons in a given orbital have the same values of n, I and m but differ in spin quantum numbers.
Aufbau principle:
The orbitals are successively filled in the order of their increasing energy.
Among the available orbitals, the orbitals of lowest energy are filled first.
The relative energy of orbital can be known by (n+ℓ) formula.
If two orbitals have the same value of (n + ℓ), the orbital having lower n value is first filled.
As atomic number increases, (n + ℓ) formula is not useful to predict the relative energies of orbitals
for example, up to z = 20, 3d > 4s
Beyond z = 20, energy difference narrows up. Beyond z = 57, 3d < 4s.
upto to z = 57,4f > 5p
beyond z = 57, 4f > 5p ; At z = 90, 4f < 5s.
The order of filling of orbitals can be known from Moellar's diagram.
Hund's rule of maximum multiplicity:
Orbitals having the same values for n and are called degenerate orbitals.
Pairing of orbitals will begin after the available degenerate orbitals are half filled.
Orbitals with highest resultant spin value are more stable.
The degenerate orbtials are filled to have like spins as far as possible.
As per Hund's rule, the number of unpaired electrons in the ground state of C, N, O are 2, 3, 2 respectively.
In the absence of Hund's rule, the number of unpaired electrons in C, N, O are 0, 1, 0 respectively.
The filling of orbital is governed by Pauli's principle.
The filling of sub-orbit is governed by Hund's rule.
The filling of orbitals of various suborbits is governed by Aufbau principle.
The maximum number of electrons that are present in the outer most shell of any atom = 8
The maximum number of electrons that are present in the (n–1) most shell of any atom = 18
The maximum number of electrons that are present in the (n–2) most shell of any atom = 32
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