Ions are atoms that have as they have lost or gained electrons
whats in the gap?

Positively or negatively charged (im not 100% sure but the question says "as they have lost or gained electrons") because atoms that gain electrons are called anions and atoms that lose electrons are called cations. atoms losing electrons are positively charged and atoms gaining electrons are negatively charged.

I'm not sure if this helps a lot but now you know a few things about ions :)

beta decay is the answer