Rules for electronic configuration


            The method of distribution of electrons in sub shells and orbitals is called electronic configuration.  There are three rules for electronic configuration.  They are

Auf-Bau– Principle:-It states that electrons enter the orbital in the order of increasing energy.The energy of the electron (orbital) is given by the value (n + l).

˜        If two sub shells have the same value of (n + l) value then the sub shell with lower n value would have lower energy e.g.   l for S = 0, P = 1, d = 2, f = 3

Energy increasing order is 1S < 2S < 2P < 3S < 3P < 4S < 3d < 4P.

2.         Pauli Exclusion Principle:-     According to this principle

(1)        An orbital can hold a maximum of two electrons.

(2)        When there are two electrons in an orbital, they must have opposite spins.

OR       No two electrons in an orbital can have the same set of four Q.No.

OR       No two electrons in an orbital have the

same spins e.g. two electrons in an orbital

2p has quantum No.

For 1st  electron , n = 2, l = 1, ml = + 1 and ms = + ½

For 2nd  electron ,n = 2, l = 1, ml = + 1 but ms = – ½

Since three Q.No. are alike but spin Q. No. is different.

3.         Hund’s Rule:- According to this rule if orbitals of the same energy are available. Electrons will distribute in them in such away to give maximum number of unpaired electrons.  These electrons with the same energy (degenerate) have the same spins.

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