RULES IN WRITING ELECTRON 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
a) for example, up to z = 20, 3d > 4s
Beyond z = 20, energy difference narrows up. Beyond z = 57, 3d < 4s.
b) 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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