The Periodic Table – Group I Alkali Metals (Li, Na, K)
Key Facts at a Glance
- 🔹 Soft, shiny metals – can be cut with a knife.
- 🔹 One valence electron – makes them highly reactive.
- 🔹 Group 1 (alkali metals) – leftmost column of the periodic table.
- 🔹 Common uses: batteries, fireworks, glass, soaps, fertilizers, medicine.
Trend (a): Decreasing Melting Point Down the Group
Think of a soft sponge that melts as it gets warmer. The alkali metals melt more easily the lower you go in the group:
- Li – 180.5 °C
- Na – 97.7 °C
- K – 63.5 °C
The outermost electron is farther from the nucleus and shielded by more inner electrons, so it’s easier to pull away and the metal “softens” faster.
Trend (b): Increasing Density Down the Group
Imagine a heavy backpack that gets heavier as you add more items. The density of alkali metals rises from Li to Na, and then K is slightly lighter than Na – the trend is generally upward:
- Li – 0.534 g cm⁻³
- Na – 0.971 g cm⁻³
- K – 0.862 g cm⁻³ (still higher than Li)
Formula for density: \$\\rho = \\frac{m}{V}\$ – mass over volume.
Trend (c): Increasing Reactivity Down the Group
Picture a firecracker that gets more explosive as it gets bigger. The alkali metals become more reactive because their single valence electron is farther from the nucleus and more shielded, so it’s easier to lose that electron:
- Li – reacts slowly with water.
- Na – reacts vigorously, producing hydrogen gas.
- K – reacts violently, often with a bright flame.
The reaction with water can be written as:
\$2\,\\text{M} + 2\,\\text{H}2\\text{O} \\rightarrow 2\,\\text{MOH} + \\text{H}2\$
where M = Li, Na, or K.
| Element | Melting Point (°C) | Density (g cm⁻³) | Reactivity (Water) |
|---|
| Li | 180.5 | 0.534 | Slow |
| Na | 97.7 | 0.971 | Vigorous |
| K | 63.5 | 0.862 | Very violent |
Exam Tips for Group I
- 📝 Memorise the trend: Melting point ↓, density ↑, reactivity ↑.
- 🧪 Typical reaction: \$\\text{M} + \\text{H}2\\text{O} \\rightarrow \\text{MOH} + \\text{H}2\$.
- ⚡️ Use analogies: sponge for melting, backpack for density, firecracker for reactivity.
- 📚 Recall uses: batteries (Li‑ion), fireworks (Na/K), glass (Na), soaps (NaOH), medicine (Li).
- 🔍 Check the periodic table: Group 1, left column, one valence electron.