Halogens

Part of the Atomic Structure section of Coddy's Chemistry journey. Lesson 34 of 45.

The halogens are group 17: F₂ and Cl₂ are gases, Br₂ a red-brown liquid, I₂ a gray-black solid.

The first five periods by family, with fluorine, chlorine, bromine and iodine ringed in group 17

The halogens: group 17

Each atom has seven outer electrons and gains one to form a halide ion: F⁻, Cl⁻, Br⁻, I⁻. The smaller the atom, the harder it pulls that electron in, so reactivity falls down the group.

Fluorine (2, 7) and chlorine (2, 8, 7) as shells, each with seven outer electrons ringed

Seven outer electrons: one short of a full shell

A more reactive halogen displaces a less reactive one from its halide: chlorine added to potassium bromide sets bromine free, and the solution turns orange-brown.

Challenge

A halogen atom reacts by pulling in one extra electron, and a small atom pulls hardest. Put fluorine, chlorine, bromine and iodine in order of atomic radius, the smallest first: that is also the most reactive first.

Try it yourself

Periodic tablePut the elements in order