Question

the third period contains only 8 elements even through the electron capacity of the 3rd shell is 18

Answer 1

As per the bhor theory maximum number of electrons in the outer most shell should be 8
if any element have it's outermost shell filled with 8 electrons it can shift the the shell to the next one

Answer 2

Heya .

Sorry , dear you are mixing periods and electonic configuration of last shell formulae of 2n^2 .


Thanks