Did you know that, back in the 1900s, it was believed that a hook and a loop type of closure was the covalent bond responsible for holding atoms in a molecule? Interesting, right?
Later, this hook-and-loop idea was discarded when G.N. Lewis, in 1916, proposed that two atoms are held in a covalent bond by two electrons. This theory is still relevant today.
So, any chemical reactions and molecular transformations must happen by breaking or making this two-electron bond.
The two-electron covalent bond can break in two ways, equal and unequal. Equal bond breaking, also known as the homolytic bond cleavage, breaks the bond to give two separated atoms with one electron each. Such an atom with a single electron is called a free radical. Being electron-deficient by one, they are unstable and reactive.
The second method of heterolytic bond cleavage is where the two electrons of the bond break unequally so that both bond electrons go to one atom of the bond. This creates two species, one electron rich by two electrons and the other electron-deficient by two. The deficient specie is assigned a positive, and the electron excess specie is assigned a negative sign. Since they are charged, these species are also reactive and unstable.
These types of bond breakages can happen when energy is supplied to the atoms of the bond to undergo violent vibrations and cleavage. Often heat, and light is used to provide energy. Sometimes, solvent, and radical initiators are also used to increase the rate of bond cleavage.
Now, if a molecule is made of similar type atoms with close electronegativity values, like carbon, hydrogen, or oxygen- oxygen, the bond breaking would most likely be homolytic.
However, in the case of dissimilar heteroatoms like carbon-oxygen or carbon-halogen, or hydrogen-halogen, one atom of the bond already has more share of the bond electrons. So, even in the bonded state, there is unequal sharing. In such scenarios, bond breaking will occur heterolytically.