Group I Properties
Section: 8. The Periodic Table | Syllabus: Cambridge IGCSE Chemistry (0620)
The Alkali Metals: Lithium, Sodium, Potassium
Group I contains the alkali metals. The syllabus focuses on the first three: lithium (Li), sodium (Na) and potassium (K) - relatively soft metals that can be cut with a knife, freshly cut surfaces are shiny but tarnish quickly in air.
All have one outer-shell electron, which they lose readily to form a 1+ ion - this is why they are so reactive and why they are stored under oil.
Trends Down Group I
| Property | Trend down the group (Li → Na → K) |
|---|---|
| Melting point | decreases |
| Density | increases |
| Reactivity | increases |
Reactivity increases down the group because the outer electron is further from the nucleus (more shielded), so it is lost more easily.
Worked Example: Reaction with Water
2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g), where M = Li, Na or K
| Metal | Observation |
|---|---|
| Lithium | floats, fizzes steadily |
| Sodium | floats, melts into a ball, fizzes rapidly, moves across the surface |
| Potassium | floats, melts immediately, moves very fast, ignites with a lilac flame |
This increasing vigour (Li → Na → K) is the clearest evidence of increasing reactivity down the group. The metal hydroxide formed is alkaline (turns red litmus blue); the gas is hydrogen (tests with a lit splint - squeaky pop).
Predicting Properties of Other Group I Elements
Rubidium (Rb) and caesium (Cs) are further down Group I. Their properties can be predicted by extending the trends above, given data about the elements above them.
Worked Example
- Question: rubidium has a density of 1.53 g/cm³ and a melting point of 39°C. Which of these best matches caesium (directly below rubidium)? (A) density 1.20 g/cm³, m.p. 29°C (B) density 1.20 g/cm³, m.p. 49°C (C) density 1.90 g/cm³, m.p. 29°C (D) density 1.90 g/cm³, m.p. 49°C [1]
- Answer: C. Density must be higher than rubidium's (1.90 > 1.53) and melting point lower (29 < 39), following the Group I trends.
Common Mistakes
- ❌ Memorising specific data for rubidium and caesium
- ✓ the syllabus limits Group I content to lithium, sodium and potassium - Rb/Cs only appear as "given the data, predict..." style questions, not as facts to recall
- ❌ Thinking reactivity trends the same way as melting point down the group
- ✓ melting point decreases down Group I, while reactivity and density both increase - don't assume every property moves the same direction
Interactive revision notes, videos and practice questions load below.