What is Hydrogen Acceptor and Hydrogen Donor?

Explanation:

Hydrogen bonds are formed between a hydrogen atom bound to a small, highly electronegative atom and another small, highly electronegative atom with an unshared electron pair. The elements that usually participate in hydrogen bonds are nitrogen, oxygen, and fluorine. A hydrogen bond is much weaker than a covalent bond (10-40 kJ/mol vs. hundreds of kJ/mol for a covalent bond), but when a lot of hydrogen bonds are added together, they can have a significant influence on the structure of molecules. An example of hydrogen bonding is the lattice formed by water molecules in ice. In biological molecules, the electronegative atoms the hydrogen atom is "shared" by are usually oxygen and nitrogen.

Scientists distinguish between the electronegative atoms in a hydrogen bond based on which atom the hydrogen atom is covalently bonded to. In the diagram at left below, the oxygen atom of the hydroxy group is called the hydrogen bond donor, because it is "donating" its hydrogen to the nitrogen. The nitrogen atom is called the hydrogen bond acceptor, because it is "accepting" the hydrogen from the oxygen. In the picture of two water molecules at lower right, the oxygen of the water molecule B is the hydrogen bond donor. The oxygen of water molecule A is the hydrogen bond acceptor.