2.1 Group 1 (Alkali Metals)Edexcel IGCSE Chemistry: Revision notes
Section 1
How does reaction with water show lithium, sodium and potassium are a family?
Lithium, sodium and potassium (the alkali metals) all react with water in a similar way, producing a metal hydroxide and hydrogen gas:
alkali metal + water → metal hydroxide + hydrogen
For example: 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
All three metals: float on water, move around the surface, fizz/effervesce as hydrogen gas is released, and gradually dissolve/disappear, leaving a colourless alkaline solution (which turns universal indicator purple). This shared pattern of reactivity is evidence they belong to the same chemical family (Group 1).
Section 2
How do differences in reactions with air and water show the trend in reactivity?
Although lithium, sodium and potassium react similarly, the vigour of their reactions increases down the group:
| Metal | Reaction with water | Reaction with air |
|---|---|---|
| Lithium | Fizzes gently, moves slowly | Tarnishes slowly |
| Sodium | Fizzes more vigorously, may melt into a ball | Tarnishes faster |
| Potassium | Reacts violently, may ignite (lilac flame) | Tarnishes very quickly, must be stored under oil |
These differences (increasing vigour of reaction, increasing rate of tarnishing) provide evidence of a trend of increasing reactivity down Group 1.
Students often say potassium 'explodes' with water — the correct description is that it reacts violently, may ignite and burn with a lilac flame, which is more precise exam language.
Section 3
How can trends be used to predict properties of other alkali metals?
Since reactivity increases down Group 1 (Li → Na → K), this trend can be used to predict the properties of alkali metals further down the group, such as rubidium (Rb) and caesium (Cs):
- They will react even more vigorously with water and air than potassium.
- They will have lower melting points than potassium (melting point decreases down the group).
- They will still form a hydroxide + hydrogen with water, following the same general pattern.
Caesium is expected to react explosively with water at room temperature, more violently than potassium, based on the trend of increasing reactivity down the group.
Section 4
Why does reactivity increase down Group 1?
Alkali metal atoms all have one electron in their outer shell, which they lose to form a 1+ ion during reactions.
Going down the group, atoms have more electron shells, so the outer electron is further from the nucleus and is shielded by more inner shells of electrons. This means the outer electron is less strongly attracted to the nucleus and is therefore easier to lose as you go down the group.
Since reactivity of alkali metals depends on how easily the outer electron is lost, reactivity increases down Group 1.
Full-mark explanation needs: more shells → outer electron further from nucleus AND more shielding → weaker attraction → electron lost more easily → more reactive. Missing any one link loses marks.
Must Know
- Alkali metal + water → metal hydroxide + hydrogen (all Group 1 metals react this way)
- Reactions with water and air get more vigorous going down the group: Li < Na < K
- Trends can be used to predict properties of other alkali metals (e.g. rubidium, caesium react even more vigorously)
- Reactivity increases down Group 1 because atoms have more shells, so the outer electron is further from the nucleus and more shielded, making it easier to lose
- All Group 1 metals have one electron in their outer shell, lost to form a 1+ ion
That's the notes covered.
Carry on to the next subtopic.