Answer 1

X belongs to Group 1. The mass number of element X is 40 and the number of neutrons in its atom is 21. In order to find out the group to which the element belongs, we need to know the atomic number of the element. Once we know the atomic number of the element, the electronic configuration and valency can be determined.
Atomic number = Mass number - Number of Neutrons.
Atomic number = 40-21 =19
The electronic configuration of X is 2,8,8,1. The number of valence electrons in X is 1. Therefore, the valency of element X is 1. We know that group 1 elements have a valency of 1.