Alkali Metals – IGCSE Chemistry Definition
IGCSE Chemistry definition of alkali metals: Group I elements. Covers properties, trends, reactions with water, and exam tips for Cambridge 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Group I is one of the most frequently examined groups in the periodic table. Examiners test trends in reactivity, melting point, and density, and expect candidates to explain these trends using atomic structure. At least one Paper 4 question per session involves Group I.
The 0620 definition
The alkali metals are the elements in Group I of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). They all have one electron in their outer shell.
Physical properties and trends
| Property | Trend down the group | Reason |
|---|---|---|
| Melting point | Decreases | Metallic bonding weakens as atoms get larger |
| Density | Generally increases | Atomic mass increases faster than volume |
| Hardness | Decreases (can be cut with a knife) | Weaker metallic bonds |
All alkali metals are soft, shiny when freshly cut (tarnish rapidly in air), and have low densities — Li, Na, and K all float on water.
Chemical properties
All alkali metals:
- Have an electron configuration ending in one outer-shell electron
- Form +1 ions (e.g. Na → Na⁺ + e⁻)
- React vigorously with water
- React with oxygen and halogens
- Are stored under oil to prevent reaction with air and moisture
Reaction with water
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
General equation: 2M + 2H₂O → 2MOH + H₂
| Metal | Observation |
|---|---|
| Lithium | Floats, fizzes steadily, dissolves slowly |
| Sodium | Floats, melts into a ball, fizzes vigorously, moves on surface |
| Potassium | Floats, melts, lilac flame, fizzes violently, may ignite hydrogen |
Reactivity increases down the group because the outer electron is easier to lose (further from nucleus, more shielding).
Worked exam question
Potassium reacts more vigorously with water than sodium. (a) Write the balanced equation for potassium reacting with water. [1] (b) Explain why potassium is more reactive than sodium. [2]
Mark scheme
(a) 2K + 2H₂O → 2KOH + H₂ [1]
(b) Potassium has more electron shells / the outer electron is further from the nucleus [1]; there is more shielding, so the outer electron is lost more easily / less attraction between the nucleus and outer electron [1]
Common exam mistakes
- Confusing alkali metals (the Group I elements) with alkalis (soluble hydroxides). Sodium is an alkali metal; sodium hydroxide is an alkali.
- Saying reactivity decreases down the group — it increases because the outer electron is easier to lose.
- Forgetting the 2 in front of K and KOH when balancing the equation with water.
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