1^st period of the periodic table has only 2 elements and 2^nd period consists of only eight elements. The first element of the third period is sodium with atomic number 11. Now, there are nine elements in the third period. The last element of the third period is Argon which has atomic number of 18. So, the 17^th group element of third period is chlorine with atomic number = 17 and atomic mass = 35.453g. It belongs to p-block.