Salam Academy
Sign inCreate account
Learn0620Papers 2, 4 · ExtendedThe Periodic TableGroup I and group VII
The Periodic Table · Subtopic 2 of 3

Group I and Group VII

Group I (alkali metals): soft, low-density metals that react with water to make an alkaline hydroxide and hydrogen. Reactivity increases down the group because the outer electron is further from the nucleus and lost more easily. Group VII (halogens): diatomic non-metals (F2\mathrm{F_2}, Cl2\mathrm{Cl_2}, Br2\mathrm{Br_2}, I2\mathrm{I_2}) that get darker and less reactive down the group. A more reactive halogen displaces a less reactive one from its salt.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe the reactions of group I metals with water.
  • Describe and explain the trends in group I and group VII.
  • Predict and write displacement reactions of halogens.
Pause and think

Why is sodium stored under oil, while iron is left in the open?

Section 2 of 14

Core idea

Start hereExplosive families

Drop lithium in water and it fizzes politely. Drop caesium and the beaker shatters. Same group, same outer electron, very different drama. Distance from the nucleus is the reason.

The big idea

Group I metals lose one electron to react, and do it more easily further down. Halogens gain one electron, and do it less easily further down.

1. Alkali metals

Worked examplePotassium and water

Describe and explain what happens when potassium is added to water.

Goal
Observations, equation, explanation.
Strategy
Link to group I pattern.
  1. 2K+2H2O→2KOH+H2\mathrm{2K+2H_2O\rightarrow2KOH+H_2}
    Alkaline hydroxide + hydrogen.
  2. floats, melts, lilac flame\text{floats, melts, lilac flame}
    Low density, low mp, heat ignites the hydrogen.
  3. faster than Na\text{faster than Na}
    Outer electron further from the nucleus, more shielded, lost more easily.
AnswerVigorous reaction with a lilac flame, forming KOH(aq) and H2\mathrm{H_2}.
✓ CheckUniversal indicator turns purple ✓.

2. Halogens

Halogen displacement

A halogen displaces anything below it.

Key takeawayColours in water Chlorine water: pale green/colourless. Bromine water: orange. Iodine solution: brown.
Exam tipRedox In Cl2+2Br−→2Cl−+Br2\mathrm{Cl_2+2Br^-\rightarrow2Cl^-+Br_2}, chlorine is reduced (gains electrons) and bromide is oxidised.
Pause and think

Why does iodine not react with potassium chloride solution?

Summary

  • Group I: more reactive down.
  • Group VII: less reactive down.
  • Displacement: higher halogen wins.
Key words
Alkali metal
A group I element.
Halogen
A group VII element.
Displacement
A more reactive element takes the place of a less reactive one.
HistoryHumphry Davy. Davy first isolated sodium and potassium in 1807 by electrolysing molten hydroxides. He reportedly danced around the lab when globules of potassium burst into flame.
Fun factFluoride. Fluorine is so reactive it even reacts with glass, yet fluoride ions in toothpaste are perfectly safe and strengthen your teeth.
Keep reading for free

Create a free account to read the rest of this lesson: key formulae, worked examples, explore the graph, common mistakes, exam tip, guided practice and more. Your answers are checked and your progress is saved.

Create free accountSign in