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Halogens as Diatomic Molecules

You rarely see a single atom of a Group 7 element floating around on its own. Halogens naturally exist as diatomic molecules, meaning they travel in pairs.

To achieve a stable, full outer shell of 8 electrons, two halogen atoms share one pair of electrons. This shared pair forms a single covalent bond between the atoms. Because of this, their chemical formulas are always written as $X_2$ (for example, $F_2$ , , , and ).

We can explain this trend step-by-step:

First, as you go down the group, the atoms have more electron shells, so the relative molecular mass increases.
Next, because the molecules are larger, the intermolecular forces between the molecules become stronger.
Finally, these stronger forces require more energy to overcome, which leads to higher melting and boiling points.

Reactions with Metals

When halogens react with metals, they form ionic compounds called metal halides (such as sodium chloride).

This reaction happens in distinct steps:

First, the halogen reacts with a metal atom.
Then, the metal atom transfers one electron to the halogen atom's outer shell.
This forms a halide ion with a stable noble gas structure and a 1- charge (e.g., $Cl^-$ ).
Finally, the oppositely charged metal and non-metal ions strongly attract each other to form a giant ionic lattice.

Example: Sodium reacting with chlorine

2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)

Reactions with Non-Metals

Halogens also react with other non-metals, like hydrogen or carbon, but the process is completely different. Because both elements are non-metals, they do not transfer electrons.

Instead, the reaction follows these steps:

First, the halogen atom comes into contact with another non-metal atom.
Then, the two atoms share a pair of electrons, forming a covalent bond.
This results in neutral molecules that form a simple molecular structure.

These covalent compounds, such as hydrogen chloride ( $HCl$ ), are typically gases or liquids at room temperature because they only have weak intermolecular forces holding the molecules together.

Example: Hydrogen reacting with chlorine

H_2(g) + Cl_2(g) \rightarrow 2HCl(g)

Students often state that covalent bonds are broken when halogens boil, but actually only the weak intermolecular forces between the molecules are overcome.

In 3-mark questions explaining the trend in boiling points, examiners expect you to explicitly link the increase in molecular mass to stronger intermolecular forces and the need for more energy.

Always write halogens as diatomic molecules (e.g., $Cl_2$ , not $Cl$ ) in balanced symbol equations unless they are part of a compound or acting as an ion.

When a halogen forms an ion in an ionic compound, remember to change the ending of the name from '-ine' to '-ide' (e.g., chlorine becomes chloride).

Molecules consisting of two atoms of the same or different elements chemically bonded together.

A strong chemical bond formed by the sharing of a pair of electrons between two non-metal atoms.

The sum of the relative atomic masses of all the atoms in a molecule.

Weak forces of attraction that exist between individual molecules.

Compounds formed by the electrostatic attraction between oppositely charged ions.

An ionic salt formed from the reaction of a metal and a halogen.

A halogen atom that has gained one electron, resulting in a 1- charge.

A structure consisting of small molecules held together by weak intermolecular forces.