Question

How many pairs of electrons are shared between two adjacent carbon atoms in a saturated hydrocarbon?

Answer 1

How many pairs? Just the one.

Explanation:

Consider the ethane molecule, `H_3C-CH_3`. Each carbon atom has 6 electrons, 2 of which are inner core and are along for the ride. Thus, there are 4 electrons in the valence shell.

When carbon makes music with the hydrogen atom (1 electron), 2 electrons are conceived to form the `H-C` bond. So far we have `H_3C*`, a neutral carbon radical.

So the free electron on one carbon nucleus is conceived to couple to form a `C-C` bond:

`H_3C*+*CH_3 rarr H_3C-CH_3`

Of course, all of this is a formalism. But you can use this approach to assign formal charge.