Question

Will Fan & Medal !

Which of the following explains why elements in column 17 (the halogens) are very reactive?

Their atoms contain three energy levels.
Their atoms need only one electron to complete their outer energy level.
Their atoms have large masses.
Their atoms contain the same number of protons as electrons.

Answer 1

It is all about the electrons, the electrons, electrons. The elements that have one excess electron or are short one electron tend to be the most reactive. For Halogens they are one electron short of being Nobel Gases which are very non-reactive so Halogens will do anything to complete their collection of electrons, knock over children, elderly people and fellow shoppers in order to get that one electron as if it were the last hottest selling toy in the store on Christmas eve.

So the answer is
Their atoms need only one electron to complete their outer energy level.

Answer 2

When I said "short of being Nobel Gases" I mean having the same electron structure as Nobel gases not actually becoming a nobel gas.