Reactivity Trends of Periodic Table

Question

It would be much appreciated if anyone can assist me in these two questions:

a) How do you best explain the reactivity of the elements of group 17 as you move down the group?

b) How do you best explain the reactivity of the elements of group 1 as you move down the group?

Thanks, 

Answer 1

a) Group 17 are the halogens, and they desire to gain an electron to complete the octet.  As you move down the group, the reactivity decreases because the outer electron shell is further from the nucleus, thus the nuclear attraction is weaker, and hence it is harder to accept an additional electron.

 

b) Group 1 are the alkali metals and they desire or tend to lose an electron to become stable.  As you go down the group, the reactivity increases, and the reasons are analogous to those for group 17 gaining electrons.  As you go down the group, the outer electrons get further from the nucleus, the nuclear attraction is weaker, and so it is easier to lose that electron.