Reactivity and displacement in Group 7

Halogens react by gaining one electron, giving each atom a complete outer electron shell. The more strongly a halogen atom attracts an incoming electron, the more readily it reacts.

Reactivity decreases down Group 7. An atom lower down the group has more electron shells, so its outer shell is farther from the nucleus. The inner electrons also shield the incoming electron from the nucleus. These effects weaken the attraction between the nucleus and the incoming electron, making it harder for the atom to gain that electron.

Displacement of halogens

A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. In the solution, the less reactive halogen is present as negative ions. The more reactive halogen gains electrons from these ions, forming its own negative ions and releasing the less reactive halogen.

For example, chlorine is more reactive than bromine, so it displaces bromine from a bromide solution:

Chlorine can also displace iodine from an iodide solution. Bromine is less reactive than chlorine but more reactive than iodine, so bromine can displace iodine from an iodide solution.

The reverse displacements do not occur: a less reactive halogen cannot displace a more reactive halogen from its salt solution.

Halogens as a group

The halogens are the elements in Group 7 of the periodic table. They are non-metals and form a family: their atoms have the same number of electrons in their outer shell, so they often react in similar ways.

Each halogen atom has seven outer-shell electrons. By gaining one electron, it can achieve a full outer shell. This shared tendency to gain an electron is a key reason for the similarities in halogen chemistry.

Halogen molecules

In their elemental form, halogens exist as simple molecules containing pairs of atoms. A molecule made of two atoms is called diatomic. For example, elemental chlorine, bromine and iodine exist as $$\ce{Cl2}$$, $$\ce{Br2}$$ and $$\ce{I2}$$ respectively. In each molecule, the two atoms are of the same halogen.

Physical trends down Group 7

As you move down Group 7, the halogen molecules become larger and their relative molecular mass increases. Their melting points and boiling points also increase.

The order from fluorine to iodine is:

fluorine → chlorine → bromine → iodine

In this direction, both melting point and boiling point rise. The trend continues to heavier halogens below iodine.

Why melting and boiling points increase

Larger halogen molecules have stronger attractions between them. More energy is needed to overcome these attractions when the substance melts or boils, so the melting and boiling points are higher. These attractions are between molecules; the bonds within the molecules are not broken during melting or boiling.

Predicting properties from position

A halogen lower in Group 7 is expected to have larger molecules, a higher relative molecular mass and higher melting and boiling points than a halogen above it. For example, bromine is below chlorine but above iodine, so its melting point and boiling point are expected to be higher than chlorine’s but lower than iodine’s.

Halogen compounds with metals and non-metals

Chlorine, bromine and iodine can react with both metals and non-metals. The type of compound formed depends on how the atoms use electrons: reactions with metals form ionic compounds, while reactions with non-metals usually form covalent compounds.

Halogens reacting with metals

When a halogen reacts with a metal, electrons transfer from metal atoms to halogen atoms. Each halogen atom gains one electron and becomes a halide ion with a charge of $$1-$$. The metal atoms lose electrons and become positive ions. The attraction between the positive metal ions and negative halide ions holds the ionic compound together.

These ionic compounds are called halide salts. Examples include sodium chloride, $$\ce{NaCl}$$; potassium bromide, $$\ce{KBr}$$; and magnesium iodide, $$\ce{MgI2}$$. In magnesium iodide, one magnesium ion has a $$2+$$ charge, so two iodide ions, each with a $$1-$$ charge, balance it.

Halogens reacting with non-metals

When a halogen reacts with a non-metal, the atoms usually share electrons rather than transfer them completely. The shared electrons form covalent bonds, producing a covalent molecular compound. For example, hydrogen and chlorine form hydrogen chloride molecules, $$\ce{HCl}$$, in which the hydrogen and chlorine atoms share electrons.

The same overall pattern applies to chlorine, bromine and iodine: with metals they form ionic halide salts; with non-metals they usually form covalent molecular compounds.