Potassium
also could either gain 7 electrons (and become like the atom of
Krypton) or lose 1 electron (and become like the atom of Argon).
What
does happen with the atoms of the first group elements when they react
with another substance is that they lose 1 electron instead of gaining
7.
They
lose 1 electron for the simple reason that it is easier than gaining 7.
Now,
what about the explanation for the increasing trend going down the group?