Extreme Halogens: Predicting the Unknown
Extrapolating Group VII Trends • Astatine · Fluorine · Displacement · Reactivity
Group VII — Trend Visualiser
Astatine — The Unknown Halogen
Astatine (At) is the heaviest element in Group VII. Using the trend in physical states of the halogens, predict the physical state of astatine at room temperature (25 °C). Give a reason for your prediction.
Predict the colour of astatine at room temperature.
Predict whether astatine would displace iodide ions from a solution of potassium iodide. Write an ionic equation for the reaction, or explain why no reaction occurs.
The boiling points of the halogens are: F₂ = −188 °C, Cl₂ = −35 °C, Br₂ = +59 °C, I₂ = +184 °C. Estimate the boiling point of astatine. Explain what structural feature causes this trend.
Astatine forms a compound with hydrogen. (a) Write the molecular formula of this compound. (b) Predict whether a solution of this compound in water would be a strong or weak acid, compared to hydrochloric acid.
Fluorine — The Most Reactive Halogen
Fluorine is the first element in Group VII. State its physical state and colour at room temperature. Explain how you used the group trend to arrive at your answer.
Will fluorine displace iodine from a solution of potassium iodide (KI)? Write a balanced symbol equation (with state symbols) and identify the oxidising and reducing agents.
Explain, in terms of electron configuration and nuclear attraction, why fluorine is the most reactive halogen.
A student adds a small amount of chlorine water to a solution of potassium fluoride. Describe what you would observe and explain your answer.
Unlike other hydrogen halides, hydrofluoric acid (HF) is a weak acid despite fluorine being the most reactive halogen. Suggest a reason for this apparent contradiction.
The Complete Trend — Ranking & Comparing
Rank fluorine, chlorine, and bromine in order of INCREASING boiling point (lowest first). Give a reason for the order.
The table shows data for some halogens. Complete the missing entries for iodine (I₂): Colour at room temperature = ?, Physical state = ?, Relative molecular mass = ?, Reactivity compared to Br₂ = ?
Use the reactivity data of halogens to predict the outcome of each reaction. Write 'reaction' or 'no reaction' and explain. (a) Cl₂ + KBr(aq) (b) I₂ + KCl(aq) (c) Br₂ + KI(aq)
A student has four unlabelled solutions: potassium fluoride (KF), potassium chloride (KCl), potassium bromide (KBr), and potassium iodide (KI). Describe a simple test using one reagent that could distinguish all four solutions. State what you would observe for each.
SYNOPTIC QUESTION: Explain, using the concept of electron gain and atomic structure, why the oxidising ability of halogens DECREASES down Group VII. In your answer, refer to atomic radius and electron shielding.
📋 Quick Reference — The 5 Halogens at a Glance
| Element | Symbol | State (25°C) | Colour | Boiling Pt | Reactivity |
| Fluorine | F₂ | Gas | Pale yellow | −188 °C | Highest ⬆ |
| Chlorine | Cl₂ | Gas | Pale green | −35 °C | High |
| Bromine | Br₂ | Liquid | Red-brown | +59 °C | Medium |
| Iodine | I₂ | Solid | Grey-purple | +184 °C | Low |
| Astatine | At | Solid (predicted) | Black (predicted) | ~300 °C? | Lowest ⬇ |
Cambridge IGCSE Chemistry 0620 · Topic 8.3.4 · Paper 4 Prediction Skills
Sign In to Keep Going
You've reached the end of the free preview. Sign in — and upgrade to Plus or Pro — to keep going with Chemistry.
Full Access
Unlock all 320 lessons