What is the bond polarity of h2s?

1 Answer
Dec 24, 2013

The short version: Sulfur is more electronegative than hydrogen, so the #"H-S"# bond is polar with electron density higher on the sulfur atom.


This leads to #"H"_2"S"# being a polar molecule.

However, the difference in polarity between #"H"# and #"S"# is very small, so neither the bond nor the molecule are very polar.

The longer version:

The difference in electronegativities of hydrogen (2.20) and sulfur(2.58) is almost exactly the same as that between hydrogen and carbon (2.55).

The #"C-H"# bond is viewed as non-polar and so, therefore, should the #"H-S"# bond.

However, there is some polar character to a #"C-H"# bond.

Its dipole moment is only 0.3 D.

We should therefore expect an #"H-S"# bond to have about the same dipole moment.

Yet, the measured molecular dipole moment of #"H"_2"S"# is 0.95 D.

If this were due entirely to the polar #"S-H"# bonds, the #"S-H"# bond dipole would be at most 0.6 D, with the negative end pointing to the #"S"# atom.

However, hydrogen sulfide has two lone pairs.

(from 2012 Book Archive)

Lone pairs contribute to the molecule's dipole moment even though they do not constitute a 'bond'.

The sulfur 'end' of the lone pair is positive, and the electron 'end' is negative, so one might think of a 'lone pair dipole' contributing to the polarity of the molecule in analogy to a bond dipole.

Thus, it may be the lone pairs that make the major contribution to the polarity of the molecule.