Q.2. How does the valency of elements vary in going down a group of the periodic table?

The valency does not change going down a group since the bonding behavior is not affected by the core electrons.

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.