Sodium vs Potassium
Both are alkali metals, so they share the same broad chemistry.
The differences in plain English
- They sit in the same column, group 1, so they have the same number of outer electrons and react in similar ways; the heavier one, Potassium, is usually the more reactive of the two if they are metals, and the less reactive if they are non-metals.
- Both are solids at room temperature.
- Sodium is denser, only slightly more than Potassium (0.968 against 0.862 g/cm³).
- Sodium melts at 98 °C, Potassium at 64 °C.
- An atom of Potassium is about 70 per cent heavier than an atom of Sodium (39.10 u against 22.99 u).
- Sodium pulls electrons more strongly in a bond (electronegativity 0.93 against 0.82).
Side by side
| Sodium | Potassium | |
|---|---|---|
| Atomic number | 11 | 19 |
| Symbol | Na | K |
| Family | Alkali metal | Alkali metal |
| Group / period | 1 / 3 | 1 / 4 |
| State at room temperature | Solid | Solid |
| Atomic mass | 22.990 u | 39.098 u |
| Density | 0.968 g/cm³ | 0.862 g/cm³ |
| Melting point | 98 °C | 64 °C |
| Boiling point | 883 °C | 759 °C |
| Electronegativity | 0.93 | 0.82 |
| First ionisation energy | 495.8 kJ/mol | 418.8 kJ/mol |
| Electron configuration | [Ne] 3s1 | [Ar] 4s1 |
| Discovered by | Humphry Davy | Humphry Davy |
Sodium
Sodium is a metal so soft that it can be cut with a butter knife, and so reactive that a small piece dropped into water fizzes, melts and often bursts into flame. Freshly cut, it is bright silver. Within seconds the surface dulls as it reacts with the air. Nobody meets sodium metal outside a laboratory, but everyone meets its compounds, because one of them is table salt.
- Sodium is soft enough to cut with a knife and floats on water, where it fizzes and often catches fire.
- Sodium street lamps glow orange because excited sodium atoms emit light at a wavelength close to 589 nanometres.
Potassium
Drop a piece of potassium into water and it skates about, hisses and bursts into a lilac flame. Yet you need potassium every day to stay alive. An adult body holds well over 100 grams of it, and every heartbeat and every nerve impulse depends on potassium ions moving across cell membranes.
- About one atom in every eight thousand of natural potassium is radioactive potassium-40, and the potassium in an adult body produces thousands of radioactive decays every second.
- Bananas are rich enough in potassium that the informal banana equivalent dose is sometimes used to put small radiation exposures into perspective.
Colour the bubbles by density or melting point and the difference jumps out.
More comparisons
Data from Periodic-Table-JSON (CC BY-SA); temperatures rounded to the nearest degree. Written by Anthony.