What is meant by cleavage of covalent bonds?

Cleavage of covalent bonds refers to the process where the bonds between atoms are broken, resulting in the formation of different substances. This breaking of bonds can lead to the rearrangement of atoms and the creation of new chemical species, such as when a molecule is split into two or more smaller molecules or fragments.

Understanding this process is crucial in the context of chemical reactions, as it often involves the release or absorption of energy and can result in significant changes in the properties and behaviors of the substances involved. This concept is foundational in both organic and inorganic chemistry, where bond breaking is a key step in many reaction mechanisms.

In contrast, the other choices relate to different processes: combining atoms pertains to bond formation, transforming a liquid into a gas involves phase changes rather than bond cleavage, and filling outer shells of atoms relates to atomic stability and electron configuration, not to the breaking of bonds.