How can valence electrons be used to predict the type of compounds formed?

Valence electrons are the electrons in the outermost shell. Those are t he electrons on an atom that can be gained or lost in a chemical reaction.

Elements that are left on the periodic table have relatively few valence electrons, and can form ions more easily by losing their valence electrons to form positively charged cations.

Nonmetals are further to the right on the periodic table, so they gain electrons relatively easily and lose them with difficulty.