What is a hydrogen bond?

Answer 1

It is an atrtraction of a slightly positively charged hydrogen atom, covalently bonded to a second atom, to yet a third atom that is slightly negatively charged. The simplest example is water, in which one H of H-O-H is attracted to the oxygen atom of another water molecule. H-O-H***O

Answer 2

A hydrogen bond occurs between a hydrogen atom covalently bonded to a small electronegative atom (such as N,O,F - let's call it X from now on) and a second small electronegative atom (Y), usually in a neighbouring molecule.

e.g. X-H ........Y

They come about because the electronegative X pulls electron density away from H leaving it slightly positively charged. This in turn pulls electron density away from Y leading to a build-up of electron density between H and Y (which attracts both the H and Y nuclei giving rise to the 'hydrogen bond'). H-bonds are generally much weaker than normal covalent bonds (like the X-H one above) but stronger than other intermolecular forces such as dipole-dipole interactions and van der Waals' forces. This is why water (which does have hydrogen bonding) is a liquid but H2S (which doesn't) is a gas at room temperature.

Answer 3

As far as I know, hydrogen bond is happened when Hydrogen bonds another atom with high electronegativity like Oxygen, Nitrogen, or Fluorin. We need a large amount of energy to break it because this kind of bond is very strong.