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:
1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p,7s,5f,6d,7p…
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 s^{2}, then the orbital has the arrangement as given below.
Correct incorrect
For example,if the d orbital has d^{6} ,then the filling is not:
Atomic number=11
Electronic configuration = 1s^{2},2s^{2},2p^{6},3s^{1}
Electronic configuration = 1s^{2},2s^{2},2p^{6},3s^{2},3p^{6},4s^{1}
For Aluminum ion
Atomic number of Aluminum =13
Aluminum ion (Al^{3+})=1s^{2},2s^{2},2p^{6 }
EXCEPTIONAL ELECTRONIC CONFIGURATION OF CHROMIUM AND COPPER
For Copper (Cu)= [Ar]^{18},4s^{2}3d^{9}
But actually, it has : [Ar]^{18},4s^{1}3d^{10}
Similarly, for Chromium it should be: [Ar]^{18},4s^{2}3d^{4}
But in actual it is:=[Ar]^{18},4s^{1}3d^{5}
The reason behind is:
If configuration is 4s^{2 }3d^{9 }, then the d orbital is not fully filled. If configuration is 4s^{1 }3d^{10} , then the d orbital is completely filled. That means it becomes more stable.
If configuration was 4s^{2}3d^{4} the exchange energy is:
Total number of exchanges = 3 + 2 + 1 = 6
If configuration is 4s^{1}3d^{5}the exchange energy is:
Total number of exchanges = 4 + 3 + 2 + 1 = 10
So, more is the exchange energy -more is the stability.
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