what really happens when an electron pair is shared in a bond?

Question

I've never been able to understand how bonding between two atoms works.

1. When two atoms share an electron pair, does it mean the valence electrons of both atoms stop revolving in their shells and stop somewhere between the two atoms?
2. I have also never understood what an ionic bond is, because if an atom donates one of its electrons to another atom, how could there be a bond holding them together? since both the atoms have done so to attain stability, they wouldn't be charged or unstable, to still be attracted to each other.

Answer 1

1. Electrons do not actually revolve, though they do have angular momentum. That is an oddity of quantum mechanics.
2. Electrons don't exist at a point, though when observed they appear at a point. Think of them as a wave, with the amplitude of the wave being the likelihood of finding (observing) the electron in a place. Imagine a cotton ball, fluffed out at the edges and hollow at the center, with greatest density about 0.1 nm (~1 Å) from the center. Yep, another quantum quirk.

In the diagram above, adapted from from R. Bader, Atoms in Molecules,

• On the average electrons in the first diagram, covalent chlorine, $\ce{Cl2}$, can be found with equal probability around around either nucleus.

• On the average, an outer electron in the second diagram, ionic sodium chloride, $\ce{NaCl}$, is more likely to be found around the chlorine atom (well, ion), effectively giving it partial negative charge. It is less likely to be found around the sodium atom (yes, ion, if you prefer), giving it a partial positive charge.

  Now, like a balloon that has an electrostatic charge, they cling to each other.

So in a crystal lattice, a cube of table salt, static electricity holds things together... and not all that tightly; it's easy to cleave a salt crystal with a sharp blade. However, unless solvated, those electrons and ions don't roam. Dissolved in water, though, "Things fall apart; the centre cannot hold; Mere anarchy is loosed upon the world," as Yeats put it.