|Class 11 Chemistry Structure of Atom||Rules Of Writing Electronic Configuration|
RULES OF WRITING ELECTRONIC CONFIGURATION:
“It is the number of electrons in different orbitals. “
There are certain set of rules that are followed:
Aufbau principle: It states that in ground state of atom, the orbitals are filled in order of their increasing energies. That is electrons first occupy the lower energy orbital then higher energy orbitals are filled.
The orbitals are filled in order:
To know the energy of orbital with lower energy the n+l rule is followed:
According to it:
For example: Out of 2s and 2p ,2s is filled first as n+l for 2s, is 2+0=2 and for 2p it is 2+1=3. Therefore, 2s is filled first.
For example: Out of 3p and 4s, n+l for both is 4 . Therefore, 3p is filled first as it possess lesser value of n.
For example: If orbital has s2, then the orbital has the arrangement as given below.
For example,if the d orbital has d6 ,then the filling is not:
Electronic configuration = 1s2,2s2,2p6,3s1
Electronic configuration = 1s2,2s2,2p6,3s2,3p6,4s1
For Aluminum ion
Atomic number of Aluminum =13
Aluminum ion (Al3+)=1s2,2s2,2p6
EXCEPTIONAL ELECTRONIC CONFIGURATION OF CHROMIUM AND COPPER
For Copper (Cu)= [Ar]18,4s23d9
But actually, it has : [Ar]18,4s13d10
Similarly, for Chromium it should be: [Ar]18,4s23d4
But in actual it is:=[Ar]18,4s13d5
The reason behind is:
If configuration is 4s2 3d9 , then the d orbital is not fully filled. If configuration is 4s1 3d10 , then the d orbital is completely filled. That means it becomes more stable.
If configuration was 4s23d4 the exchange energy is:
Total number of exchanges = 3 + 2 + 1 = 6
If configuration is 4s13d5the exchange energy is:
Total number of exchanges = 4 + 3 + 2 + 1 = 10
So, more is the exchange energy -more is the stability.