Question

The maximum number of electrons in any shell is given​by 2n². Then the 3rd shell can hold 18 electrons [ 2(3²) = 18]. But electronic configuration of element 20 is 2,8,8,2.
why it can't be 2,8,10? Explain clearly! :)

Answer 1

Hey there !

Lets take a look at the rules that are followed in the distribution of electrons in an atoms :-

1) The maximum number of electrons present in a shell is given by the formula 2n², where ‘n’ is the orbit number or energy level
K shell = 2n² = 2 × 1² = 2 electrons
L shell = 2n² = 2 × 2² = 8 electrons

etc......

2)The maximum number of electrons that can be accommodated in the outermost orbit is 8.

3)Electrons are not accommodated in a given shell, unless the inner shells are filled.
Which means that , the shells are filled in a step-wise manner.

Even if 3rd shell can hold a maximum of 18 electrons , it has to give some electrons to the next shell . this is because according to the second rule , there must be at least 8 electrons in the outermost orbit .

If the electronic configuration is 2,8 ,10 ,
then the number of electrons in the outermost orbit is more than 8.It is not possible for the outermost orbit to have more than 8 electrons

Hence , the correct electronic configuration is 2 , 8 , 8 , 2

Answer 2

Hi

Dhairya at ur service !

★Calcium is a S- block element . ( atomic number 20 )
elements which have their last electron filled in the S subshell are called Sblock element .

★What u r saying is that when the 3rd shell can accommodate 18 electrons then why Calcium has 2,8,8,2 configuration .
The Reason -
★Aufbau's Principle - Electrons dont fill continuously in a shell , they fill in subshells first , each shell has respective subshells .
★ 1 st Shell has 1 subshell named S .( s can accommodate 2 electrons at MAX.)
★2nd Shell has 2 subshell named S,P( P CAN accommodate 6 ELECTRON AT MAX)
★3rd Shell has 3 subshell S,P and D.(D CAN ACCOMMODATE 10 ELECTRON AT MAX )
and so on...
★ Now the subshells are filled in this order I have shown in the picture .
So , before the 3rd Shell is fully filled , 4th shell starts to fill after 3p subshell .HENCE AS only 3s and 3p are filled in Calcium from the 3 rd shell it has this configuration .

Overall : the formula gives the number of electrons in shell.
but electrons fill subshell wise in an elements electronic configuration.

Electrons are filled by the (n+l) rule , n is the shell number and l is the subshell .

For S subshell , l = 0
P subshell l = 1
D subshell l = 2

the lower value of n+l fills first.
if 2 values of n+l are same then the one with lower shell number is filled first .

Congratulations , what I just explained was a part of the Quantum Mechanical Model Of Atom ( given by Einstein )