How do atoms gain or loose electrons to form ions?

Question

Also, what are the relative sizes of the atoms and their ions?

Answer 1

The sizes are basically identical, except for a few cases:
- group I metal (not H): ion is slightly smaller since the outermost electron shell is no longer occupied.
- H+ ion is MUCH smaller than the H atom since it is only a nucleus (indeed, normally only a single proton) now.

As to "how" it happens, electrons move around based on electromagnetic forces. If one atom is much more electrongative than the other, the electron experiences an electric force which moves it over to the first atom.

Answer 2

God made physics. Physics begat Chemistry. Then, after a flood of experimental data came the Biologists...and at the very very bottom were the religious enthusiasts.