Ionic compounds are characterized by a huge separation in electronegativity...in other words, one participant in the bond is so eager to lose an elecron to attain noble gas config, while the other wants the electron to attain n.g. config, that the electron is essentially biased in one direction. This is why we see Group 1, 2 metals with relatively small ionic (and atomic, of course) radii when compared to their counterparts in Group 16 and 17.
Another way to think about this is delocalization...thre is almost none of this in ionic compounds (beign selfish doesnt allow for it) while there is a lot in covalent bonds. Yet another way to see this scenario is in terms of oxidation potentials - individual metals will go buck wild and lose their electrons thus showing an enormous gain in stability when participating in ionic compounds. You will almost NEVER find metals by themselves, but in ionic compounds.
Finally, metals DO form metallic bonds and sometimes participate in coordinate covalent bonds. The former is not really a molecule, but relaly just a network of the same metal atoms where the electron IS actually shared betweent he valence shells of all the atoms in the compound. Metal alloys can also be formed, with one metal being the solvent and the other being the solute, but this is not an ionic OR covalent bond.
