group 7 - the halogens

What are the group 7 elements?

  • there are elements in group 7 - fluorine, chlorine, bromine, iodine and astatine (we don’t do much about astatine in detail but you still need to know a bit about it)

  • These elements are non-metals that are poisonous

  • All halogens have similar reactions as they have seven electrons in their outer shell

  • Halogens are diatomic, meaning they form molecules made of pairs if atoms sharing electrons (forming a single covalent bond)

  • When halogens gain an electron during reactions they form -1 ions called halide ions, this is because they now have one more electron than they do protons

Properties of the group 7 elements

Fluorine

  • yellow gas at room temp

  • Very reactive, poisonous gas

  • Colourless in solution

Chlorine

  • pale yellow - green gas at room temp

  • Reactive, poisonous and dense gas

  • Pale green in solution

Bromine

  • red/brown liquid at room temp

  • Dense red-brown volatile (will easily become a gas) liquid

  • Orange in solution

Iodine

  • grey solid at room temp

  • Shimmery, crystalline solid that sublimes to form a purple vapour

  • Dark brown in solution

Tip to remember; going down the group they get more and more solid and dark in solution

Boiling points

  • The melting and boiling points of the halogens increase as you go down the group

  • This is due to the increasing intermolecular forces (remember they are diatomic, meaning covalent) as the atoms get large

  • Meaning more energy is required to over come these forces

Reactivity and displacements reactions of the halogens

  • the atoms in group 7 get larger as you go down the group because there are more electrons and therefore energy levels

  • All group 7 metals are reactive since they have 7 electrons in their outer shell and it does not require much energy to gain only one more

  • However, the smaller atoms are more reactive than the larger ones. This is because in, for example, fluorine the outer shell that the electrons is going to is very close to the positive nucleus making it easier to attract the negative electron.

  • Whereas in iodine the outer shell is very far from the nucleus making it more difficult to attract the electron.

  • However, whilst iodine is the least reactive of the halogens it is still a reactive element compared to others.

Displacement reactions

You only need to know about the displacement reactions between chlorine bromine and iodine

A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide. So for example, chlorine could displace bromine but iodine could not.

Reactions of the halogens

Metal halides

  • chlorine, bromine and iodine react with metals and non-metals to form compounds

  • The halogens react with some metals to form ionic compounds which are metal halide salts

  • The halide ion carries a -1 charge so the ionic compound formed will have different numbers of halogen atoms, depending on the valency of the metal

E.g Sodium is a group 1 metal: 2 Na + Cl 2 —> 2 NaCl

Calcium is a group 2 metal: Ca + Br 2 —→ CaBr2

Formation of sodium chloride

Non-metal halides

  • the halogens react with non-metals to form simple molecular covalent structures

  • For example, the halogens react with hydrogen to form hydrogen halides (e.g hydrogen chloride)