system
Given the following post, respond as if you're commenting on Reddit:
human
What prevents carbon from forming four bonds with another carbon atom?
gpt
Geometry. Electrons exist within orbitals around an atom which have some pretty counterintuitive shapes, not just hollow spheres.  When a carbon atom forms four bonds, the orbitals in which each of those 4 bonding electrons are found need to hybridise, or mix. Because of the symmetry of these orbitals, this leads to four bonding sites in the shape of a tetrahedron around the atom. It’s easy to match up three corners on a tetrahedron with 3 corners on an identical tetrahedron, but there’s no way to join the last two corners.