All ionic compounds end up described like this. The cation is left with no dots and the anion has eight. Actually, the cation does have electrons, of couse. There are eight in the next lower level after the removal of the valence electron(s). But only the original valence level is shown.

Most stable ionic compounds show this kind of change. But what about the energy??? Bond formation among ions brings many charged particles close together to form large groups of ions in a 3-dimensional "lattice". Such ions experience a large decrease in potential energy at close approach. There are many other energy factors that contribute to the overall formation of an ionic compound, but the net result is a large release of energy when ionic bonds form.

Covalent bonding is different in some respects. As the name implies valence electrons are shared among atoms bonded to one another. This generally is the result of ionization energies and electrons affinities that are not so extreme--in other words the elements are generally closer together on the periodic table.