Compare the 3 types of bonds based on what happens to the valence electrons of the atoms

In ionic bonds, one atom gives one or more electrons to another atom so both can get closer to 8 valence electrons.  Example:  In potassium chloride (KCl), Potassium gives up one valence electron to chlorine, so that the outer shell of potassium has 8 valence electrons.  This happens only between metals and nonmetals.

In covalent bonds, atoms share their electrons to reach 8 valence electrons.  Example: In water (H2O), Oxygen shares one valence electron with one atom of hydrogen, and another valence electron with another atom of hydrogen.  Oxygen now has 8 (4 unshared + 2 of its own + 1 from hydrogen + 1 from hydrogen), and each hydrogen has 2 valence electrons: one of its own and one from oxygen [ note that hydrogen only needs 2 valence electrons to be complete instead of 8].

In metallic bonds between metals, the valence electrons move much more freely than in other bonds.  This free characteristic makes metals how they are: ductile, malleable, sectile, conductive, etc.