Predict the properties of other elements in Group VII, given information about the elements

IGCSE Chemistry 0620 – The Periodic Table: Group VII (Halogens)

Group VII elements (the halogens) occupy the seventh column of the periodic table. They are highly reactive non‑metals. The core syllabus covers fluorine (F), chlorine (Cl), bromine (Br) and iodine (I); astatine (At) is included for extended work.

1. Physical & Chemical Data for the Core Halogens (and At – extended)

Data source: CRC Handbook of Chemistry and Physics (2023) and IUPAC recommendations.

2. Trends Down the Group (with Quantitative Links)

• Physical state: Gas → liquid → solid.
  Reason: Atomic radius increases down the group; larger atoms experience stronger London‑dispersion forces, so more energy is required to keep them in the gaseous state.
• Melting & boiling points: Both rise down the group.
  Reason: More electrons → stronger instantaneous dipoles → higher dispersion forces. The trend can be visualised in the graph below.
• Density: Increases with atomic mass.
  Reason: Mass grows faster than the increase in atomic volume, giving a higher mass per unit volume.
• Colour: Gases are pale, liquids are vividly coloured, solids become dark or metallic‑grey.
  Reason: As the atom gets larger, the energy gap between the ground state and excited states narrows, allowing absorption of visible light of lower energy.
• Electronegativity (Pauling scale): F > Cl > Br > I > At.
  Reason: Valence electrons are farther from the nucleus and increasingly shielded, reducing the nucleus’s pull on bonding electrons.
• Oxidising power (reactivity): Decreases down the group.
  Reason: Two related factors:
  1. Bond‑dissociation energy (X–X) falls because the bond length (r_X‑X) increases. Approximate relationship:
     DX‑X ≈ A / rX‑X where A is a constant for the halogen series. Larger r → smaller D → easier to break the X–X bond, so the halogen is a weaker oxidiser.
  2. Electronegativity falls, so the halogen’s ability to attract electrons from other species diminishes.

3. Reactivity Series & Displacement Reactions (Core)

Because a more reactive halogen is a stronger oxidising agent, it can displace a less reactive halogen from its compounds.

General displacement equation

where X is the more reactive halogen and Y the less reactive one.

Worked examples

1. Cl₂ displaces Br⁻ (as already given):
   Cl₂(g) + 2 Br⁻(aq) → 2 Cl⁻(aq) + Br₂(l)
2. I₂ displaces Cl⁻ (illustrating the opposite direction – no reaction):
   I₂(s) + 2 Cl⁻(aq) → 2 I⁻(aq) + Cl₂(g) (does not occur – iodine is less reactive than chlorine)
3. Br₂ displaces I⁻ (common exam question):
   Br₂(l) + 2 I⁻(aq) → 2 Br⁻(aq) + I₂(s)

The driving force is the difference in oxidising power (or electronegativity) between the two halogens.

4. Predicting Properties of Unfamiliar Group VII Elements (Extended)

1. Locate the element in the periodic table (group VII, period n).
2. Apply the trends from section 2 to estimate:
   • State at 20 °C (gas, liquid, or solid).
   • Melting & boiling points – interpolate linearly between the two known neighbours.
   • Density – roughly proportional to relative atomic mass.
   • Colour – follow the pale‑gas → coloured‑liquid → dark‑solid sequence.
   • Reactivity – weaker oxidising ability than the element directly above it.
3. Bond‑dissociation‑energy (BDE) estimate (optional quantitative step):

   For a halogen X, the X–X BDE can be approximated by

   D_X‑X ≈ A / r_X‑X

   where r_X‑X is the covalent radius (or bond length) and A is a constant that is the same for the whole series (≈ 1.5 × 10⁻⁸ J m). As r increases down the group, D decreases, explaining the fall in oxidising power.

4. Worked calculation – estimate the boiling point of a hypothetical element “U” between Br and I:
   1. Boiling point of Br₂ = 58.8 °C, of I₂ = 184.3 °C.
   2. Atomic numbers: Br = 35, I = 53. Suppose “U” has Z = 44 (mid‑point).
   3. Linear interpolation:
      T_U = T_Br + (Z_U‑Z_Br) / (Z_I‑Z_Br) × (T_I‑T_Br)
      = 58.8 + (44‑35)/(53‑35) × (184.3‑58.8) ≈ 58.8 + 9/18 × 125.5 ≈ 58.8 + 62.8 ≈ 121.6 °C
   4. Thus the predicted boiling point of “U” would be ≈ 122 °C.
5. Check consistency – does the predicted state, colour and reactivity fit the trend? If not, revise the estimate.

5. Real‑World Applications (Core + Environmental/Health Links)

• Fluorine (F₂) / Fluorides – Used to make PTFE (Teflon®) and other polymers; fluoride ions are added to drinking water to prevent dental caries, but excess intake can cause fluorosis (a health concern).
• Chlorine (Cl₂) – Powerful oxidiser; employed for water disinfection, bleaching of paper and textiles, and production of PVC. Chlorine gas is toxic and can form harmful chlorinated by‑products (e.g., trihalomethanes) in drinking water.
• Bromine (Br₂) – Component of fire‑retardant chemicals and some pharmaceuticals. Organobromine compounds can be persistent in the environment and bioaccumulate, raising ecological concerns.
• Iodine (I₂) – Essential micronutrient for thyroid hormones; used as an antiseptic (tincture of iodine) and in iodised salt to prevent iodine deficiency disorders.
• Astatine (At) – Rare, short‑lived; research suggests potential use in targeted alpha‑particle radiotherapy, but practical applications are still experimental.

6. Safety Considerations (Core)

7. Connections to Other Syllabus Topics

• Acids, bases and salts (Section 7) – Preparation of hydrogen halides (HF, HCl, HBr, HI) and corresponding halide salts.
• Electrochemistry (Section 9) – Electrolysis of aqueous halide solutions yields the free halogen at the anode.
• Organic chemistry (Section 10) – Halogenation reactions of alkanes and alkenes rely on the halogen’s oxidising power.
• Periodic trends (Section 8) – The halogen trends illustrate how atomic size, London dispersion forces, bond‑energy and electronegativity influence physical and chemical behaviour.

8. Typical IGCSE Examination Questions

1. State two physical properties that change down Group VII and explain why they change.
2. Given that chlorine reacts with sodium to give NaCl, predict the product when bromine reacts with sodium and write a balanced equation.
3. Explain why fluorine is a stronger oxidising agent than iodine.
4. Using the trend in electronegativity, predict which hydrogen halide (HF, HCl, HBr, HI) has the strongest H–X bond.
5. Write the balanced equation for the displacement reaction when chlorine gas is added to a potassium bromide solution.
6. Estimate the boiling point of a hypothetical halogen that lies halfway between bromine and iodine (show your reasoning).

9. Suggested Diagram for Revision