Group I Alkali Metals and Group VII Halogens
Two groups with opposite reactivity trends — and understanding why they differ is worth more than memorising either.
1. Group I — the alkali metals
Lithium, sodium, potassium, rubidium, caesium
Physical properties:
Soft (cut with a knife), shiny when freshly cut but tarnish quickly, low density (lithium, sodium and potassium float on water), and low melting points for metals.
All have ONE outer-shell electron, which they lose to form 1+ ions.
Reaction with water
metal + water → metal hydroxide + HYDROGEN 2Na + 2H₂O → 2NaOH + H₂
Observations, in increasing vigour:
| Metal | Observations |
|---|---|
| Lithium | floats, fizzes steadily, moves slowly |
| Sodium | melts into a ball, moves rapidly, fizzes vigorously |
| Potassium | bursts into a LILAC flame, moves very fast, may explode |
The solution formed is alkaline — it turns universal indicator purple/blue and red litmus blue. That is why they are called alkali metals.
When asked for observations, write what you can SEE — bubbles/effervescence, the metal dissolving, moving, melting, a flame and its colour. Tutors flagged this directly.
The trend
Reactivity INCREASES DOWN Group I.
The explanation — give all three steps:
1. Going down the group, the atoms have MORE electron shells, so the outer electron is FURTHER from the nucleus and SHIELDED by more inner shells. 2. So the force of attraction between the nucleus and the outer electron is WEAKER. 3. So the outer electron is LOST MORE EASILY — and reactivity increases.
Other trends down Group I: melting point decreases, density generally increases.
2. Group VII — the halogens
Fluorine, chlorine, bromine, iodine
All exist as DIATOMIC molecules (F₂, Cl₂, Br₂, I₂) and have SEVEN outer-shell electrons, gaining one to form 1− ions (halides).
Physical appearance at room temperature:
| Halogen | State | Colour |
|---|---|---|
| Fluorine | gas | pale yellow |
| Chlorine | gas | pale green |
| Bromine | LIQUID | red-brown |
| Iodine | solid | grey-black (purple vapour) |
Down the group: colour gets DARKER, and the state goes gas → liquid → solid (melting and boiling points increase, because the larger molecules have stronger intermolecular forces).
Bromine is one of only two liquid elements at room temperature (with mercury).
The trend
Reactivity DECREASES DOWN Group VII — the opposite of Group I.
The explanation:
Going down, the atoms are LARGER with more shielding, so the nucleus attracts an incoming electron LESS strongly — it is HARDER to GAIN an electron, so reactivity decreases.
This is the single most examined trend in the topic, and it was recorded wrong. The key is that Group I loses an electron (easier when attraction is weak) while Group VII gains one (harder when attraction is weak). Same cause, opposite effect.
3. Halogen displacement reactions
A MORE reactive halogen DISPLACES a LESS reactive halide from solution.
Since reactivity decreases down the group: chlorine > bromine > iodine.
| Reaction | Result |
|---|---|
| Chlorine + potassium bromide | displaces → solution turns orange (bromine formed) |
| Chlorine + potassium iodide | displaces → turns brown (iodine formed) |
| Bromine + potassium iodide | displaces → turns brown |
| Bromine + potassium chloride | NO reaction |
| Iodine + potassium bromide | NO reaction |
Example equation:
Cl₂ + 2KBr → 2KCl + Br₂
Ionic: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
Displacement is a REDOX reaction — the halogen is reduced (gains electrons), the halide ion is oxidised.
The colour change is the observation — state the new colour the solution turns.
4. Uses
Group I compounds — sodium chloride (table salt), sodium hydroxide (industry).
Halogens:
Chlorine — sterilising water supplies, bleach, making PVC Iodine — antiseptic Fluoride — added to toothpaste and water to protect teeth
5. Group VIII — the noble gases
Helium, neon, argon, krypton, xenon — unreactive (inert) because they have a FULL OUTER SHELL.
They are monatomic and colourless.
Uses: helium in balloons (low density, non-flammable), argon in lamps and welding (inert atmosphere), neon in advertising signs.
6. Mistakes that cost marks
Saying reactivity increases down Group VII.
Giving only one step of the reactivity explanation.
Forgetting the hydroxide and hydrogen products with water.
Not naming the flame colour for potassium.
Wrong halogen colours or states.
Predicting a displacement that doesn’t happen (bromine with chloride).
Forgetting halogens are diatomic in equations.
Not stating the colour change as the observation.
Frequently asked questions
What do Group I metals have in common? One outer electron, forming 1+ ions.
What are the products of Group I + water? Metal hydroxide + hydrogen.
What happens with potassium and water? It reacts violently and burns with a lilac flame.
Why does reactivity increase down Group I? The outer electron is further away and shielded, so it is lost more easily.
Why does reactivity decrease down Group VII? It is harder to gain an electron into a larger, more shielded atom.
What colour and state is bromine? A red-brown liquid.
What happens when chlorine is added to potassium iodide? Chlorine displaces iodine — the solution turns brown.
Will bromine displace chloride? No — bromine is less reactive than chlorine.
Why are the halogens diatomic? Two atoms share a pair of electrons to complete their outer shells.
Why are noble gases unreactive? They have a full outer shell.
Quick revision checklist
- I know Group I properties and that they have one outer electron
- I know the water equation and products
- I can give observations for lithium, sodium and potassium
- I know potassium’s lilac flame
- I can explain the Group I trend in three steps
- I know the halogens’ colours and states
- I know they are diatomic with seven outer electrons
- I know reactivity decreases down Group VII
- I can explain why the two groups trend oppositely
- I can predict displacement reactions and their colour changes
- I can write full and ionic displacement equations
- I know the uses of chlorine, iodine and fluoride
- I know why noble gases are inert
These notes cover Group I and Group VII in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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