2.7 - Group 7 - The Halogens

The halogens and their trends

• The elements in Group 7 are non-metals called the halogens.
• Every halogen atom has seven electrons in its outer shell, so they all have similar chemical properties.
• Halogens exist as diatomic molecules (pairs of atoms joined by a single covalent bond): F₂, Cl₂, Br₂, I₂.
• All halogens are toxic and are handled in a fume cupboard.

Appearance at room temperature

• Fluorine (F₂) - Pale yellow gas.
• Chlorine (Cl₂) - Pale yellow-green gas.
• Bromine (Br₂) - Red-brown liquid that gives off an orange vapour.
• Iodine (I₂) - Dark grey crystalline solid that sublimes to a purple vapour when heated.

Trends going down the group

• Melting and boiling points increase - The state at room temperature changes from gas (fluorine, chlorine) to liquid (bromine) to solid (iodine).
• Relative molecular mass and density increase - Each halogen molecule is heavier than the one above it.
• Colour darkens - From pale yellow to dark grey/black.
• Reactivity decreases - Fluorine at the top is the most reactive, and astatine at the bottom is the least reactive.

Predicting properties of an unknown halogen

Because properties change steadily down the group, you can predict unknown properties from the trend: Astatine is below iodine, so it is predicted to be a black solid with a higher melting point and lower reactivity than iodine.

The reactions of the halogens

• Halogens react with metals to form metal halides.
• The halogen atom gains one electron to form a 1^- halide ion (F^-, Cl^-, Br^-, I^-).
• The metal atom loses electrons to form a positive ion, so the resulting compound is ionic.
• Metal halides are typically white solids.

Reactions with alkali metals

Halogens react vigorously with Group 1 alkali metals to form ionic salts:

2Na(s) + Cl₂(g) → 2NaCl(s) sodium + chlorine → sodium chloride

2K(s) + Br₂(l) → 2KBr(s) potassium + bromine → potassium bromide

Reactions with other metals

Halogens also react with other metals. For example, iron reacts with chlorine to form iron(III) chloride:

2Fe(s) + 3Cl₂(g) → 2FeCl₃(s) iron + chlorine → iron(III) chloride

Uses of chlorine and iodine

Chlorine

• Used to sterilise drinking water and swimming pools because it kills harmful bacteria and microorganisms.
• Used in the manufacture of bleach and other disinfectant cleaning products.

Iodine

• Used as an antiseptic on skin before surgery to prevent infection.
• Radioactive isotopes of iodine are used as medical tracers in nuclear medicine.