Potassium (K) — Atomic Number 19

Potassium is a chemical element with the symbol K (from Latin kalium) and atomic number 19. It is a soft, silvery alkali metal — more reactive than sodium. It was first isolated in 1807 by Sir Humphry Davy by electrolysis of molten potash, just days after he discovered sodium.

At a Glance

Chemical Properties

Potassium’s 4s¹ electron is further from the nucleus and more shielded than sodium’s 3s¹ — so it is lost even more easily. This makes potassium more reactive than sodium.

Key Reactions

1. With water: $2\text{K}+2{\text{H}}_2\text{O}\to 2\text{KOH}+{\text{H}}_2$ — violent reaction, lilac flame, hydrogen ignites immediately
2. With oxygen: $4\text{K}+{\text{O}}_2\to 2{\text{K}}_2\text{O}$ (limited O₂)
3. With chlorine: $2\text{K}+{\text{Cl}}_2\to 2\text{KCl}$

Characteristic Test

Flame test: Lilac/purple — use cobalt blue glass to filter out sodium’s yellow

Compounds & Uses

• NPK fertilisers — the “K” in NPK
• Fireworks — purple flames

Biological Role

Potassium is the primary cation inside cells. The Na⁺/K⁺ pump maintains the resting potential of nerves. It is essential for nerve transmission, muscle contraction, and heart function.

IGCSE Essentials

1. Flame test: lilac — use cobalt glass if Na is present
2. More reactive than Na — trend: Li < Na < K
3. Stored under oil
4. Forms K⁺ (loses 1 electron → 2,8,8)
5. The “K” in NPK fertiliser

Sources

• Potassium — Wikipedia
• Potassium — Ptable.com