RULES FOR FILLING OF ELECTRONS IN VARIOUS ORBITALS

The atom is built up by filling electrons in various orbitals according to the following rules.

Aufbau Principle: This principle states that the electrons are added one by one to the various orbitals in order of their increasing energy starting with the orbital of lowest energy. The increasing order of energy of various orbital is 1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p,5f,6d,7p……………………

How to remember such a big sequence? To make it simple we are giving you the method to write the increasing order of the orbitals. Starting from the top, the direction of the arrow gives the order of filling of orbitals.

 

256_Aufbau Principle.JPG

 
Alternatively, the order of increasing energies of the various orbitals can be calculated on the basis of (n + l ) rule.

The energy of an orbital depends upon the sum of values of the principal quantum number (n) and the azimuthal quantum number ( l ). This is called (n +l ) rule. According to this rule,

 

“In neutral isolated atom, the lower the value of (n + l ) for an orbital, lower is its energy. However, if the two different types of orbitals have the same value of (n +l ), the orbitals with lower value of n has lower energy’’.

Illustration of (n +l ) Rule:
 
Type of orbitals
Value of n

Values of l

Values of (n+l)

Relative energy
1s
1
0
1 + 0 = 1
Lowest energy
2s
2
0
2 + 0 = 2

Higher energy than 1s orbital

2p
2
1
2 + 1 = 3

2p orbital (n=2) have lower energy than 3s orbital (n=3)

3s
3
1
3 + 1 = 4

 

Illustration:

Write the electronic configuration of nitrogen (atomic number = 7)

Solution:

 1524_Solution.JPG

Illustration:

Write the electronic configuration of following:

(i)  S2- ( Z = 16 ) (ii) Fe2+ = ( Z = 26 )  
 

Solution:

(i) 1s2 2s2 2p6 3s2 3p6   

(ii) 1s2 2s2 2p6 3s2 3p6 3d6 

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