Question

Why is the third shell of an atom not full completely with 18 electrons?

Answer 1

a shell can only have 8 electron to attain noble gas or for its stable electronic configuration it is according to lewis structure
but if we see correctly the shell has sub shells 
s , p , d , f orbitals
s can hold 2 electrons
p can hold 6 electrons 
d can hold 10 and f 14 electron these are their maximum capacity 
and when shell 3 is fully filled 
it is of the form  1s^2 2s^2 2p^6 3s^2 3p^6 3d^10
first one is 2 and adding next 2 is 8 and
if you add the last three you will get 18 
E.C = 18