Group 7: The Halogens

What the halogens are

• Group 7 contains the halogens: fluorine, chlorine, bromine, iodine and astatine
• They are reactive non-metals, all toxic, and many are coloured
• Each halogen atom has seven electrons in its outermost shell, which gives the group its family resemblance
• Halogens exist as diatomic molecules (F₂, Cl₂, Br₂, I₂) — a pair of atoms held together by one single covalent bond, sharing a pair of electrons

Physical state and appearance at room temperature

• The colour deepens as you go down the group (pale yellow → yellow-green → red-brown → dark grey/violet)

Trend in melting and boiling points

• Both the melting point and the boiling point increase steadily down the group
• F₂ boils at −188 °C, Cl₂ at −34 °C, Br₂ at 59 °C, I₂ at 184 °C
• The halogens are simple molecular structures, so it is the intermolecular forces between molecules that are broken on melting or boiling — not the strong covalent bond inside each molecule
• As the molecules get larger going down the group, the intermolecular forces become stronger and more energy is needed to overcome them — so the melting and boiling points rise