has four outer electrons, and thus can complete its eight-electron
"neon" shell by sharing electrons in four covalent bonds. The simplest
carbon compound is methane, CH4.
The methane molecule is diagramed at the right, first with Lewis electron
pairs, then with stick bonds, and finally in a representation of the
actual tetrahedral shape of the molecule.
The four hydrogen atoms in methane are at the four corners of a tetrahedron
with the carbon atom at the center. All of the atoms are as far from
one another as they can be, given a fixed C-H distance. This is a
general principle: The four electron pairs around a central atom
such as a carbon atom repel one another as all negative charges do,
and the lowest energy state is that with the four bonds directed tetrahedrally
away from the central atom. Any other arrangement of C-H bonds
in methane would bring two bonding pairs closer together, and electrostatic
repulsion would push them apart again.