Group I properties
The alkali metals: physical properties, reactions with water, and the trend in reactivity.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 8.2.1Describe the physical properties and reactions with water of lithium, sodium and potassium.
- 8.2.2Explain the trend in reactivity down Group I in terms of electron loss.Supplement
6 minute read
Group I: the alkali metals
Group I contains the alkali metals: lithium, sodium, potassium, and others further down the group. All have one electron in their outer shell, which they lose readily to form a +1 ion — this single fact explains almost everything about how they behave.
Physical properties
Compared with typical metals, the alkali metals are unusually soft (can be cut with a knife) and have low density (lithium, sodium and potassium all float on water) and low melting points.
Reaction with water
All three react vigorously with water, producing a metal hydroxide and hydrogen gas:
2Na + 2H₂O → 2NaOH + H₂
Observations: the metal floats, moves around the surface, may melt into a ball from the heat released, fizzes as hydrogen is produced, and may ignite (especially potassium, which burns with a lilac flame). The resulting solution turns universal indicator purple/blue, showing it is alkaline — consistent with the hydroxide formed.
Trend in reactivity
Reactivity increases down the group (lithium < sodium < potassium). Going down the group, the outer electron is in a shell further from the nucleus, and shielded by more inner shells of electrons. This weakens the attraction between the nucleus and the outer electron, so it is lost more easily — and losing that outer electron is the reaction.
Think of it like this
Losing the single outer electron in Group I is like letting go of a helium balloon: the further down the group you go, the more "layers of hands" (shielding shells) stand between the nucleus and that outer electron, and the weaker the grip becomes, so the electron drifts away more easily — explaining why potassium reacts more violently than lithium.
Common misconceptions
- Thinking alkali metals become less reactive down the group, by wrongly assuming heavier atoms are always less reactive. Reactivity actually increases down Group I.
- Confusing the trend reasoning for Group I with Group VII — for Group I, more shielding down the group means the outer electron is lost MORE easily (more reactive); for Group VII, it means an extra electron is gained LESS easily (less reactive). These are opposite consequences of the same physical trend.
- Believing the alkali metals are typical "hard, shiny, unreactive" metals like iron or gold. They are soft, low density, and among the most reactive metals known — they are stored under oil to prevent reaction with air and moisture.
In the exam
- Explaining the reactivity trend needs three linked ideas: outer electron further from nucleus, more shielding by inner shells, weaker attraction so it is lost more easily — all three together, not just one.
- Describing the reaction with water, mention the float, the movement, and the gas produced — a full description usually needs at least two distinct observations.