When combining with nonmetallic atoms, metallic atoms generally will..
1.lose electrons and form negative ions
2.lose electrons and form positive ions
3.gain electrons and form negative ions
4.gain electrons and form positive ions

1. metals lose electrons and become positive ions

This is because metals usually have 1 or more outer shell electrons. Due to the other interior electron shells, the nucleus doesn't have the same force of attraction on the outer shell electrons as it does on the ones closer to it. This enables metals to lose their outer electrons relatively easily. The atom become a +ve ion because the metal loses a couple of electrons so the proton number exceeds the number of electrons.

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.