Trends in Group 7 Properties Revision Notes | GCSE AQA Higher Chemistry

1. Why Group 7 Elements Follow Patterns

Picture the periodic table as a map. Elements in the same column share the same number of outer electrons, so they behave similarly. Group 7 elements all have 7 outer electrons — one short of a full shell.

But here's where it gets interesting. As you go down Group 7, the atoms get bigger and heavier. This changes everything about how they behave — their melting points, boiling points, and even how reactive they are.

2. Physical Property Trends Down Group 7

Group 7 elements exist as diatomic molecules — F₂, Cl₂, Br₂, I₂. As you go down the group, each atom gets more protons and neutrons, making the molecules heavier.

Worked example: Calculating Relative Molecular Mass for Halogens

Calculate the relative molecular mass of chlorine gas (Cl₂). Relative atomic mass of Cl = 35.5

1. Chlorine gas = Cl₂ — Halogens exist as diatomic molecules
2. 2 chlorine atoms per molecule — The subscript 2 tells us this
3. Mr = 2 × 35.5 = 71 — Relative molecular mass has no units

Answer: 71

This increasing mass affects melting and boiling points. Heavier molecules need more energy to break apart from each other. Think of it like trying to move a heavy suitcase versus a light one — you need more energy for the heavy one.

3. Reactivity Trends — The Surprising Twist

Here's where Group 7 breaks the pattern you might expect. Unlike Group 1 metals that get MORE reactive going down, halogens get LESS reactive as you go down the group.

**Trap**: Many students think bigger atoms are always more reactive. Actually, fluorine (the smallest) is the most reactive halogen because its outer shell is closest to the nucleus, so it attracts electrons most strongly.

We can prove this reactivity order using displacement reactions. When you add chlorine water to potassium bromide solution, the chlorine displaces the bromine because chlorine is more reactive.

Worked example: Writing Displacement Reaction Equations

Write the equation for chlorine displacing bromine from potassium bromide solution.

1. Cl₂ + KBr → — Chlorine gas + potassium bromide solution
2. Bromine gets displaced by chlorine — More reactive displaces less reactive
3. Cl₂ + KBr → KCl + Br₂ — Potassium chloride + bromine formed
4. Cl₂ + 2KBr → 2KCl + Br₂ — 2 bromide ions needed to make 1 Br₂ molecule

Answer: Cl₂ + 2KBr → 2KCl + Br₂

4. Predicting Properties Using Trends

Once you know the patterns, you can predict properties of any Group 7 element. The key is understanding that physical properties (melting/boiling points) increase down the group, but chemical reactivity decreases.

Examiners love testing this by giving you data for some halogens and asking you to predict values for others. The trick is spotting whether they want a physical property (goes up) or reactivity (goes down).

Let's see how this works with real data. If chlorine boils at -34°C and iodine boils at 184°C, you can predict that bromine (between them) boils somewhere in the middle — around 59°C in fact.

5. Displacement Reactions in Action

Displacement reactions are your evidence for reactivity order. Think of it like a competition — the more reactive halogen always wins and kicks out the less reactive one from its compound.

The visual clues are important for exams. When bromine displaces iodine, the solution turns orange-brown (bromine colour). When iodine is displaced, you see a brown/black colour appear.

6. Exam Technique and Common Mistakes

Group 7 questions often test whether you can distinguish between physical and chemical properties. Physical properties (melting/boiling points) increase down the group. Chemical reactivity decreases down the group.

**Trap**: Students often confuse Group 1 and Group 7 trends. Group 1 metals get MORE reactive down the group, but Group 7 halogens get LESS reactive. Don't mix them up.

Exam precision note: In Trends in Group 7 Properties - Group 7 - The periodic table, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

Halogen displacement reactions are observed using aqueous solutions of halide salts. For example, chlorine water added to potassium bromide solution displaces bromine because chlorine is more reactive than bromine. The word aqueous matters because the ions must be free to move in solution for the displacement reaction to happen.