Polarity reflects charge separation of electron density between the bound atoms....
Homonuclear diatomic molecules, such as H−H, and X−X, dihydrogen, and the halogens are necessarily NON-POLAR because the bound atoms have the same electronegativity and thus electron density is shared equally...Bond polarity is also not a feature of the wealth of carbon chemistry, due to the small difference in electronegativity of carbon and hydrogen....and the capacity of carbon to catenate....to form long C−C chains, along which substitution by heteroatoms can occur.
When two or more non-polar atoms are bound, say between H−F or H−O−H, bond polarity manifests due to the difference in bond polarity, and this is the origin of intermolecular hydrogen bonding, i.e. on the back of dipole-dipole interactions.... +δH−Fδ−, and +δH2Oδ−. Of course this represents the extreme end of polar bond.....
THE bond in say H2S, Br−Cl would display much less charge separation, and much less polarity....because the bound atoms have a SMALLER difference in electronegativity.
