This very simple but useful way of predicting molecular shapes has
been given the grandiose name of the valence-shell electron-pair
repulsion theory, or VSEPR theory, but essentially it is
nothing more than common sense. We shall develop simple VSEPR theory
as it is needed, and will find that this theory accounts for almost
all of the observed geometries of molecules.
The electronegativities of carbon and hydrogen are almost the same,
2.5 and 2.1, respectively. Electrons in the C-H bonds are shared almost
equally by the two atoms, with little tendency to shift toward either
C or H. The bonds are said to be nonpolar, because there is no accumulation
of positive charge at one end and negative charge at the other caused
by the movement of the bonding electron pair toward one atom. In contrast,
the bond in H-F is quite polar because the high electronegativity
of F pulls the bonding electron pair toward F, leaving the molecule
with a positive charge on the H atom and a negative charge on the
F atom.
Most of the forces between molecules are electrostatic, caused by
attractions between the positive and negative charges on different
parts of the molecule.