How are non polar covalent bonds formed?

Polarity in covalent bonds can be gauged by comparing electronegativity of each atom. The greater the difference, the more polar the bond. This is because more electronegative atoms tend to pull electron charge density towards themselves, creating a partial negative charge (and therefore a partial positive charge on the other atom).

Therefore, for binary compounds in order to have a truly non-polar covalent bond the two atoms should be identical. Therefore, the bonds in such things as #O_2# or #Cl_2# are non polar bonds. The bond between hydrogen and chlorine in HCl, however, is polar, as hydrogen is less electronegative than chlorine.

For more complex molecules, the degree of polarity in a bond will be influenced by the nature of the groups bonded to each atom. Therefore in methanol, #CH_3OH# the C-O bond is polar as oxygen is more electronegative than carbon. But in propanol #CH_3-CH_2-CH_2-OH# the C-O bond is still polar, but less so, due to the ethyl group having an inductive effect which pushes charge density back towards the oxygen atom.

To complicate things further, symmetry plays a part too. Therefore, whilst the bond between carbon and oxygen in #CO_2# is polar, the fact that the molecule is linear means that the polarity of the two C=O bonds cancel each other out. So overall the molecule is not polar (although the two bonds, separately, are).

All clear now ? LOL