Why electronic configuration of some elements is 2 8 8 , some have 2 8 18 ,some have 2 8 18 18.Please explain.
Energy levels in an atom are shown in the following image. These shells are represented by the letters K, L, M, N…or the numbers, n=1,2,3,4…..
Electronic configuration is the arrangement of electrons in different shells. The maximum number of electrons which a shell can accommodate is 2n 2. Thus,
K-shell (n=1) can maximum have 2 [2x12=2] electrons.
L-shell (n=2) can maximum have 8 [2x22=8] electrons.
M-shell (n=3) can have 18[2x32=18] electrons.
K-shell (n=4) can maximum have 2[2x42=32] electrons.
For the following configurations,
Element Number of electrons K L M N
A 18 2 8 8
B 28 2 8 18
C 36 2 8 18 18
All the elements have different electronic configuration as they have different atomic numbers, hence, have different number of electrons. As a result, they have different outermost shell with different number of electrons present in it.
When electrons are filled in a particular shell then first that shell is completely filled with electrons then filling of next shell is started.
A ,B and C have these type of configurations as they have different number of electrons present in them.
A has 18 eletcrons. First K is filled with 2 electrons then L is filled with 8 electrons. Now 10 electrons are accommodated and 8 electrons are left. These electrons will go to M shell. That is why A has completely filled K, L shells and incompletely filled M shell.
Similarly,
B has completely filled K,L and M shells.
C has completely filled K,L,M shells and incompletely filled N shell.