Group 7: The Halogens (AQA GCSE Chemistry)

Atomic structure of Group 7 elements

• The elements in Group 7 are known as the halogens

  • These are fluorine, chlorine, bromine, iodine and astatine

• These elements are non-metals that are poisonous

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

• Halogens are diatomic, meaning they form molecules made of pairs of atoms sharing electrons (forming a single covalent bond between the two halogen atoms) such as F₂, Cl₂, etc

• When halogen atoms gain an electron during reactions, they form -1 ions called halide ions

Atomic structure of Group 7 elements

The atoms of the elements of Group 7 all have 7 electrons in their outer shell

Properties of Group 7 Elements

• At room temperature, the halogens exist in different states and colours, with different characteristics

The properties of the Group 7 elements

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

• This is due to increasing intermolecular forces as the atoms become larger, so more energy is required to overcome these forces

Boiling points of the Group 7 elements

This graph shows the melting and boiling points of the Group 7 elements

Group 7 elements state at room temperature

• At room temperature (20 °C), the physical state of the halogens changes as you go down the group

  • Fluorine and chlorine are gases, bromine is a liquid and iodine is crumbly solid

• The colours of the halogens also change as you descend the group - they become darker

The appearance and state of the Group 7 elements

The physical states and colours of chlorine, bromine and iodine at room temperature 

Reactivity of the Halogens

• Reactivity of Group 7 non-metals decreases as you go down the group

  • As you go down Group 7, the number of shells of electrons increases, the same as with all other groups

• However, halogen atoms form negative ions when they gain an electron to obtain a full outer shell

  • This means that the increased distance from the outer shell to the nucleus as you go down a group makes the halogens become less reactive

• Fluorine is the smallest halogen, which means its outermost shell is the closest to the positive nucleus of all the halogen

  • Therefore, the ability to attract an electron is strongest in fluorine making it the most reactive

  • As you move down the group, the forces of attraction between the nucleus and the outermost shell decreases

  • This makes it harder for the atoms to gain electrons as you descend the group

  • Therefore, the halogens are less reactive the further down the group you go

Displacement Reactions

• A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide

• The reactivity of Group 7 elements decreases as you move down the group

• You only need to learn the displacement reactions with chlorine, bromine and iodine

  • Chlorine is the most reactive and iodine is the least reactive

Chlorine with Bromine & Iodine

• If you add chlorine solution to colourless potassium bromide or potassium iodide solution a displacement reaction occurs:

  • The solution becomes orange as bromine is formed or

  • The solution becomes brown as iodine is formed

• Chlorine is above bromine and iodine in Group 7 so it is more reactive

• Chlorine will displace bromine or iodine from an aqueous solution of the metal halide

chlorine + potassium bromide →  potassium chloride + bromine

Cl₂ + 2KBr → 2KCl + Br₂

chlorine + potassium iodide →  potassium chloride + iodine

Cl₂ + 2KI → 2KCl + I₂

Bromine with iodine

• Bromine is above iodine in Group 7 so it is more reactive

• Bromine will displace iodine from an aqueous solution of the metal iodide

bromine + potassium iodide →  potassium bromide + iodine

Br₂ + 2KI → 2KBr + I₂

Summary table of the displacement reactions of the halogens: chlorine, bromine and iodine 

• You could be asked at Higher Tier to provide ionic equations to show what is happening during displacement reactions of the halogens

• These are:

Cl₂ + 2Br^- → 2Cl^- + Br₂

Cl₂ + 2I^- → 2Cl^- + I₂

Br₂ + 2I^- → 2Br^- + I₂

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:

    • 2Na + Cl₂ → 2 NaCl

• Calcium is a group 2 metal:

  • Ca + Br₂ → CaBr₂

• The halogens decrease in reactivity moving down the group, but they still form halide salts with some metals including iron

• The rate of reaction is slower for halogens which are further down the group such as bromine and iodine

Formation of sodium chloride

Sodium donates its single outer electron to a chlorine atom and an ionic bond is formed between the positive sodium ion and the negative chloride ion

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)

• Reactivity decreases down the group, so iodine reacts less vigorously with hydrogen than chlorine (which requires light or a high temperature to react with hydrogen)

• Fluorine is the most reactive (reacting with hydrogen at low temperatures in the absence of light)

• Other compounds formed include CCl₄, HF and PCl₅