an element X has a mass number 40 and contains 21 neutrons in its atom .To which group of periodic table does it belong?

We are given that the mass number of the element is 40. Mass number of an element is the total number of protons and neutrons present in the nucleus of an atom. We are also given that the number of neutrons in the element is 21. So the number of protons will be

= mass number - number of neutrons 

= 40 - 21

= 19

As in an atom the number of protons are equal to the number of electrons, therefore the number of electrons in the atom are also 19. Thus the electronic configuration of the element will be 2, 8, 8, 1. This element belongs to 1^st group and 4^th period of the periodic table and is potassium.