Question

why are radicals unstable and what are flat and pyramidal radicals?

Answer 1

Radials are unstable, to as far as my knowledge and many chemists who aren't physicists, because a radical is an unpaired electron, and electrons are most stable when there are two of them, one in the spin up and spin down states.

Flat and pyramidal radicals are based on their shape. For instance, a flat radical looks like this:

The lines represent the flat plane, while the two orbitals represent where the electron can go. This means that the radical can react from either side, and if all 3 groups attached to it are different, then when it adds on in one way or the other it will look like its mirror image.