Group 7: The Halogens (AQA GCSE Chemistry: Combined Science)

• 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

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

The properties of the Group 7 elements

Halogen	State & appearance at room temperature	Characteristics	Colour in solution
Fluorine	Yellow gas	Very reactive, poisonous gas	-
Chlorine	Pale yellow-green gas	Reactive, poisonous and dense gas	Pale green
Bromine	Red-brown liquid	Dense red-brown volatile liquid	Orange
Iodine	Grey solid	Shimmery, crystalline solid that sublimes to form a purple vapour	Dark brown

• 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 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 

	Chlorine (Cl2)	Bromine (Br2)	Iodine (I2)
Potassium chloride (KCl)	x	No reaction	No reaction
Potassium bromide (KBr)	Chlorine displaces the bromide ions Yellow-orange colour of bromine seen	x	No reaction
Potassium Iodide  (KI)	Chlorine displaces the iodide ions Brown colour of iodine is seen	Bromine displaces the iodide ions Brown colour of iodine is seen	x

• 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

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