Why there is a break in the thirdperiod elements of the long form of the periodic table? 

Explanation

In the third period, 3s  and 3p orbitals are only filled successively. Four orbitals together can hold eight electrons. Hence, eight elements are present. No elements to come in between 3s and 3p, because of the absence of `(n-1)d and (n-2)f` sub-shells. 7