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5.ATOMIC STRUCTURE eLearn.Punjab

Aufbau Principle Table (5.4) Arrangement of orbitals
according to (n+1) rule

The electrons should be filled in energy 1s n  n+ 
subshells in order of increasing energy values. 2s
The electrons are first placed in Is, 2s, 2p and 2p 1 0 1+0=1
soon. 3s 2 0 2+0=2
3p 2 1 2+1=3
Pauli’s Exclusion Principle 3d 3 0 3+0=3
4s 3 1 3+1=4
This principle can be stated as follows: 4p 3 2 3+2=5
It is impossible for two electrons residing 4d 4 0 4+0=4
in the same orbital of a poly-electron atom to 4f 4 1 4+1=5
have the same values of four quantum num- 5s 4 2 4+2=6
bers, or Two electrons in the same orbital 5p 4 3 4+3=7
should have opposite spins ( ↓↑ ). 5d 5 0 5+0=5
5f 5 1 5+1=6
Hund’s Rule 6s 5 2 5+2=7
6p 5 3 5+3=8
6d 6 0 6+0=6
6f 6 1 6+1=7
7s 6 2 6+2=8
6 3 6+3=9
7 0 7+0=7

If, degenerate orbitals are available and more than one electrons are to be placed in them,
they should be placed in separate orbitals with the same spin rather than putting them in the same
orbital with opposite spins.
According to the rule, the two electrons in 2p subshell of carbon will be distributed as follows.

↓↑ ↓↑ ↑ ↓ o

6C = 1s 2s 2px 2py 2pz
The three orbitals of 2p subshell are degenerate.

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