Potassium Element Properties
Complete reference for Potassium (K, element 19): atomic data, electron configuration, isotopes, physical constants, and chemical behavior.
Potassium
Alkali Metal — Period 4, Group 1
Atomic Identity
Electron Configuration
One unpaired electron in 4s
Key Isotopes of Potassium
| Isotope | Symbol | Protons | Neutrons | Mass (u) | Natural Abundance | Stability |
|---|---|---|---|---|---|---|
| Potassium-38 | ³⁸K | 19 | 19 | 37.9690812 | Radioactive |
Unstable
β⁺ decay, t½ = 7.636 min |
| Potassium-39 | ³⁹K | 19 | 20 | 38.9637064 | 93.258% | Stable |
| Potassium-40 | ⁴⁰K | 19 | 21 | 39.9639982 | 0.0117% |
Unstable
β⁻ / EC / β⁺, t½ = 1.248 Gyr |
| Potassium-41 | ⁴¹K | 19 | 22 | 40.9618252 | 6.730% | Stable |
| Potassium-42 | ⁴²K | 19 | 23 | 41.9624022 | Radioactive |
Unstable
β⁻ decay, t½ = 12.355 hr |
Potassium-39 and potassium-41 are stable. Potassium-40, though radioactive, is used in potassium-argon (K-Ar) radiometric dating because it decays to stable argon-40 with a known half-life of 1.248 billion years.
Physical Properties
Chemical Properties
Ground State Quantum Numbers
Notable Emission Lines
Potassium produces a pale violet (lilac) flame in a flame test, dominated by lines near 766 nm and 770 nm. The color is often difficult to see with the naked eye due to the intense yellow of sodium contamination.
Common Potassium Compounds
| Compound | Formula | Common Name | Key Uses |
|---|---|---|---|
| Potassium chloride | KCl | Muriate of potash | Fertilizers, salt substitutes, medicine, food additive |
| Potassium hydroxide | KOH | Caustic potash | Soap making, battery electrolytes, chemical synthesis |
| Potassium nitrate | KNO₃ | Saltpeter | Fertilizers, food preservation, gunpowder, fireworks |
| Potassium carbonate | K₂CO₃ | Potash | Glass making, soap production, fire suppression agents |
| Potassium permanganate | KMnO₄ | Condy's crystals | Water treatment, disinfection, wound care, chemical oxidizer |
| Potassium sulfate | K₂SO₄ | Sulfate of potash | Fertilizers (chloride-sensitive crops), alum production |
| Potassium iodide | KI | — | Radiation protection, iodized salt additive, photography |
| Potassium bicarbonate | KHCO₃ | Potassium acid carbonate | Baking, wine making, fire extinguisher propellant |
Key Facts About Potassium
Essential for Life
Potassium ions (K⁺) are the dominant cation inside living cells and are critical for nerve impulse transmission, muscle contraction including heartbeats, and maintaining fluid and pH balance. The Na⁺/K⁺-ATPase pump actively maintains the concentration gradient, consuming about 20–40% of cellular ATP.
Violet Flame Test
Potassium produces a pale lilac-violet flame in a flame test due to emission lines at 766 nm and 770 nm. The color is often obscured by trace sodium impurities; a cobalt blue glass filter blocks yellow sodium emission, allowing the violet potassium color to be seen clearly.
Essential Plant Nutrient
Potassium is one of the three primary macronutrients for plants (N-P-K), essential for enzyme activation, osmoregulation, and transport of sugars. Potassium fertilizers (mainly KCl and K₂SO₄) account for the majority of potassium mining output worldwide.
Radiometric Dating
Potassium-40 decays to argon-40 with a half-life of 1.248 billion years, making the K-Ar dating method invaluable for determining the age of volcanic rocks and minerals ranging from thousands to billions of years old. This technique has been pivotal in establishing the geological timescale.
Discovered by Electrolysis
Humphry Davy first isolated potassium in 1807 by passing electricity through molten potassium hydroxide. It was the first metal ever isolated by electrolysis, and Davy described the tiny globules of metal bursting into violet flame on contact with air as one of the most beautiful sights in chemistry.
Softer and Lighter than Water
With a density of only 0.862 g/cm³ — less than water — potassium floats before reacting violently with water. Its Mohs hardness is 0.4, softer than sodium, and it can be easily cut with a knife, revealing a shiny metallic surface that tarnishes within seconds.
Summary
Complete reference for Potassium (K, element 19): atomic data, electron configuration, isotopes, physical constants, and chemical behavior.
How it works
- Browse the atomic identity section for symbol, atomic number, and standard atomic weight.
- Check the electron configuration panel for orbital notation and quantum numbers.
- Review the isotopes table for potassium-39, potassium-40, and potassium-41 including natural abundance.
- Consult the physical properties panel for melting point, density, and state at STP.
- Use the chemical properties section to understand alkali metal behavior and ionization energies.
- Explore common potassium compounds and their everyday applications.
Use cases
- Look up potassium constants for chemistry homework or exams.
- Verify atomic data when writing lab reports.
- Reference isotope data for nuclear chemistry or radiometric dating research.
- Check thermodynamic constants for materials science or engineering.
- Teach or learn alkali metal properties using potassium as an example.
- Confirm electron configuration before writing molecular orbital diagrams.
- Research potassium compounds for agricultural, pharmaceutical, or industrial applications.
- Quick-reference ionization energy for electrochemistry or spectroscopy work.