A hydrogen bond exists between two different groups of atoms that have a permanent dipole moment. So what that means is there's a permanent dipole moment right here and there's also permanent dipole moment right over, let's say right over here. And so these two permanent dipole moments interact with one another and they form a bond. Now, why is a hydrogen bond the most stable, the strongest type of inter molecular bond? Well, that's because hydrogen is the smallest nucleus that exists in nature and it's such a small atom that the H atom can get very close to this atom and this atom. And so what that means is because the distance between the two charges is so small that force is relatively large and that's exactly why hydrogen bonds are the strongest type of intermolecular bonds.