[thedoc (OP)]

Aman Sharma  asked the Naked Scientists:
   
Hello Chris!

How do electrons revolve in covalent bonds?

Aman Sharma

What do you think?

[damocles]

Covalent bonds are a convenient way of describing the way that a few negatively charged electrons can fix their average location so as to hold two positively charged nuclei together. Chemists know about covalent bonds, electrons do not. All electrons are constantly in very fast motion, they are all indistinguishable from one another, and we have to think in terms not of the position of each electron specifically, but of the average distribution of electrons in a molecule. "Bonds" and "localized electrons" are a convenient fictional calculus and accounting system that allows chemists to work out the properties of molecules and the mechanisms of chemical reactions.

Think of a baseball team made up of nine identical siblings, who constantly and rapidly change fielding positions. You can still describe the team as having a short stop or a second base man, even though you cannot identify an individual person with any role.

(All right! Like any analogy that one has its weaknesses, but it might help provide an insight.)

[evan_au]

In the covalent bond analogy, each electron is viewed like a little bar magnet (electrons spin on their axis, and an electric current spinning around creates a magnetic field).

Magnets tend to stick to another magnet with the opposite orientation, and so electrons tend to stick to another electron with the opposite spin.

If one atom has a lone electron spinning in one direction, and a nearby atom has a lone electron spinning in the opposite orientation, they will tend to stick the atoms together, with a covalent bond. (If one atom attracts the electron much more strongly than the other, they will form an ionic bond, like common salt NaCl, rather than a covalent bond).

Depending on how many electrons each atom has, and what their energy is, the electron will reside (or orbit) in a different electron orbital, which has a particular shape. This shape influences the final shape of the molecule.
There is a table of orbital shapes at: http://en.wikipedia.org/wiki/Atomic_orbital#Orbitals_table