I'm not a physicist just a curious mind, so please go easy!
I was just watching a BBC Horizon Documentary that featured a piece on the recently discovered material Graphene. One of the facts mentioned on the documentary was that a sheet of graphene, despite being a lattice of carbon atoms just one atom thick, could hold the weight of a household cat without breaking.
They showed an illustration of graphene's hexagonal structure and how a crude form of graphene could be created using Sellotape and piece of graphite.
OK so here's my question.. From what I can remember from Chemistry lessons in school, carbon has a valency of 4, so it can bind to 4 more carbon atoms. Surely (in theory at least) a sheet of carbon could exist that adopts a square grid lattice, as oppose to graphene's hexagonal structure. If so wouldn't such a material be stronger than graphene? Also would such a lattice naturally form under any circumstance or does carbon always assume a hexagonal structure when reduced to layers?
Best Answer
Although it's not strictly what happens, you can think of the bonds around a carbon atom as repelling each other because the electrons localised into those bonds want to get as far away from each oither as possible. That's why when a carbon atom forms three bonds you get the bonds separated by 120º. When you have four bonds they arrange themselves into a tetrahedral shape with an angle of about 109º between each pair of bonds.
It is possible to force the bonds closer together, and molecules, like cyclobutane, exist that have a four carbon ring. However these tend to be more easily attacked by reagents than five or six carbon rings, so they tend to be unstable. These molecules have only two bonds separated by a 90º angle, with the other two bonds at more like the usual 109º and in a plane at right angles to the two 90º bonds. I don't know of any molecule with four bonds at 90º in the same plane. My guess is that the energy of this arrangement would be so high it would spontaneously reorganise to something more stable.