Hydrogen bonds
A hydrogen bond is an attraction between a hydrogen atom and an electronegative atom, most common ones being nitrogen (N), oxygen (O) or fluorine (F). Hydrogen bonds appear frequently within biological molecules and it exists in polar compounds, a common example of this being water where the attractive nteraction exists between the oxygen and hydrogen. Hydrogen bonding occurs as intermolecular attractions, where the Hydrogen bond is found between different molecules, or intramolecular, where the bond exists between different parts of the same molecule [1].
A hydrogen bond is a non-covalent bond; they have much stronger attractions than Van der Waals forces and permanent dipole-permanent dipole interactions, but are weaker than ionic bonding or covalent bonding. Evidence for hydrogen bonding can be found when comparing the boiling points of hydrogen molecules across groups 5,6 and 7 of the periodic table. The compounds where hydrogen bonding is present produce a much higher boiling point as hydrogen bonds require more energy to be broken than Van der Waals forces [2].
The distance between two parts of the same molecule, or different molecule, can vary and this has an effect on the strength of the hydrogen bond. This why the hydrogen bonds are said to be "elastic," the greater the distance between the hydrogen atom and the electronegative atom the longer the hydrogen bond will be and this results in a weaker hydrogen bond.