Consider the covalent bond between two atoms, X and Y.

A covalent bond is a shared pair of electrons.

When the bond breaks, one electron could go with each atom, or both could go with just one of the atoms.

In this case both electrons go with Y.

Y has gained an electron, so it becomes a negatively charged ion, Y –.

X has lost an electron, so it becomes a positively charged ion, X +.

This is called heterolytic fission.

Curly arrows are often used to show the movement of a pair of electrons.

The electrons move in the direction of the arrow head. A full arrow head represents the movement of two electrons.

An electron-deficient species X + is formed when the bond breaks.

A species Y –, which has a lone pair of electrons, has also formed.

In heterolytic fission both electrons from the covalent bond are transferred to the more electronegative atom.

Chlorine is more electronegative than hydrogen and receives both electrons.

This forms a hydrogen ion H + …

… and a chloride ion Cl –.

Heterolytic fission also happens in organic compounds such as chloromethane.

When the positively charged species contains a carbon atom like this, it is called a carbocation.

Carbocations are important intermediates in organic reactions.

Consider the covalent bond between two atoms, X and Y.

A covalent bond is a shared pair of electrons.

When the bond breaks, one electron could go with each atom, or both could go with just one of the atoms.

In this case one electron goes with X and the other goes with Y.

Each atom has an unpaired electron. They are called free radicals.

This is called homolytic fission.

In homolytic fission one electron from the broken covalent bond goes to each atom.

Unlike the ions formed by heterolytic fission, the radicals formed by homolytic fission are uncharged.

Homolytic fission can also happen in organic compounds, such as methane CH 4.

A methyl radical and a hydrogen radical are formed in this example.

This is a diagram of methane, CH 4.

Below is a diagram of chlorine, Cl 2.

Ultraviolet light provides energy to break the covalent bonds, but which bond will break first?

The mean bond energy for the Cl–Cl bond is 242 kJ mol –1 so it breaks more easily.

The mean bond energy for the C–H bond is 435 kJ mol –1 so it breaks less easily.

The formation of chlorine radicals is the first step in the chlorination of methane, called the initiation stage.

The chlorination of methane occurs by a free radical mechanism.

Ultraviolet light provides the energy needed to break the Cl–Cl bond in chlorine.

The bond breaks by homolytic fission to produce two chlorine radicals.

It is easier to write the equation like this.

The chlorine radicals are highly reactive.

They may collide with methane molecules.

Stable hydrogen chloride molecules and reactive methyl radicals are formed.

The methyl radicals may collide with chlorine molecules.

Stable chloromethane molecules and reactive chlorine radicals are formed.

Although chlorine radicals are used up in these reactions, they are also formed, so continuing the reactions.

When two radicals collide their unpaired electrons can pair to make a covalent bond.

If a methyl radical and a chlorine radical collide, a molecule of chloromethane is formed.

Other termination steps are possible, leading to many by-products that must be removed.

For example, two methyl radicals collide to form a molecule of ethane. The number of by-products can be reduced by using an excess of methane.