Homolytic reaction
A reaction in which the electrons of a covalent bond, which is broken, are shared equally between the two products. Also a reaction in which species with an odd number of electrons, such as free radicals react to form a covalent bond.

Humic Pertaining to the soil or water brone substances resulting from the partial decay of leaves, plant material or any other organic waste.

Hydrobromic Acid (HBr)
A colourless pungent smelling gas obtained by the hydrolysis of phosphorus bromide, it is highly soluble in water. It is used as laboratory reagent in organic chemistry.

Hund’s rule
The electrons are distributed among the orbitals of a subshell in a way that gives the maximum number of unpaired electrons with parallel spins.

Hybridisation
The phenomenon of mixing of the atomic orbitals and formation of new orbitals of equal energy is known as hybridisation and the new orbitals formed are known as hybrid orbitals. The bonds formed by such orbitals are called hybrid bonds. The compounds which result in from these bonds are known as hybrids.

Hydrocarbon resins
Thermoplastic polymers of M wt. less than 2000 obtained as a by-product in petroleum cracking, e.g., petroleum resins, terpene resins, cyclopentadine resins, etc.

Hydrazoic acid (N₃H)
A colourless volatile liquid obtained by the action of nitrous acid on hydrazine or its alt, e.g., hydrazine chloride.

Hydrazone
Organic compounds formed by the reaction of hydrazine with aldehydes or ketones in the presence of an acid.

Hydrides
Hydrogen possesses a unique property by virtue of which it may combine with metals and non-metals by direct or indirect methods. These binary compounds of hydrogen are termed as hydrides.