Potassium – facts and properties
- Symbol
- K
- Atomic number
- 19
- Atomic mass
- 39.098 u
- Period
- 4
- Group
- 1
- Category
- alkali metal
- Electron configuration
- [Ar] 4s¹
- Electronegativity
- 0.82
- Atomic radius
- 243 pm
- Valence electrons
- 1
- State at standard conditions
- solid
- Melting point
- 63.38 °C
- Boiling point
- 759 °C
- Density
- 0.89 g/cm³
- Discovered
- 1807
Potassium is a shiny, silvery alkali metal with atomic number 19. It is soft enough to cut with a knife and reacts even more violently than sodium. Drop a piece onto water and the hydrogen it releases catches fire at once, burning with a lilac flame. In compounds, though, potassium is indispensable: plants need it as a major nutrient, and in our cells potassium ions control nerves, muscles and the beating of the heart.
Position in the periodic table
Occurrence and extraction
Potassium is one of the ten most abundant elements in the Earth's crust. It is found in feldspars such as orthoclase and in micas. As these minerals weather, potassium is released into the soil, where plants take it up. The economically important sources are potash salts left behind when ancient seas evaporated, above all sylvite, which is potassium chloride, and carnallite. Potash has been mined in Germany since the nineteenth century, but the largest deposits today are in Canada, Russia and Belarus.
The English name comes from potash, the potassium carbonate that people once leached from plant ashes in pots. The symbol K comes from the Latin name kalium, which like the word alkali goes back to an Arabic word for plant ash. Humphry Davy first isolated the metal in 1807 by passing an electric current through molten potassium hydroxide. Potassium thus became the first metal to be isolated by electrolysis, a few days before sodium. Today it is usually made without electrolysis, by reducing molten potassium chloride with sodium vapour. Because potassium evaporates more easily than sodium, it escapes as a vapour and is collected.
Properties and reactions
Potassium has the electron configuration [Ar] 4s¹ and a single valence electron, which sits far from the nucleus and is only weakly held. It belongs to period 4 and group 1, directly below sodium, and its electronegativity of 0.82 is very low. In its compounds potassium almost always has the oxidation state plus one.
The metal has a density of 0.89 g/cm³, so it floats on water. It melts at just 63.38 °C and boils at 759 °C. A freshly cut surface shines like silver but turns dull within seconds in air. Potassium reacts with water even more violently than sodium, forming potassium hydroxide solution and hydrogen. The heat released is enough to ignite the hydrogen, so the metal skims across the water in flames, and larger pieces react explosively. The potassium hydroxide formed is highly corrosive. For these reasons potassium is stored under paraffin oil. During long storage a yellow layer of potassium superoxide can form on the surface, which may explode when the metal is cut, so old potassium is regarded as especially hazardous.
Potassium compounds colour a flame pale lilac. Because even traces of sodium swamp this colour with bright yellow, chemists view the flame through blue cobalt glass when testing for potassium. Natural potassium also contains a small proportion of the radioactive isotope potassium-40. That is why every person, and every banana, is very slightly radioactive.
Uses in everyday life and technology
By far the largest share of mined potash goes into fertilisers. Together with nitrogen and phosphorus, potassium is one of the three main plant nutrients. It controls the water balance of plant cells, helps crops withstand drought and frost, and supports the formation of sugars and starch.
Other potassium compounds are widely used as well. Potassium nitrate, also called saltpetre, is an ingredient of gunpowder and fireworks. Potassium hydroxide is used to make soft soaps and serves as the electrolyte in alkaline batteries. Potassium carbonate is a traditional raising agent in gingerbread. Potassium iodide tablets protect the thyroid gland from radioactive iodine after an accident at a nuclear power plant. The metal itself is needed only in small quantities, for example as a strong reducing agent in the laboratory.
In the human body, potassium is the most important positive ion inside cells. Together with sodium, which is found mainly outside cells, it creates the electrical voltage across cell membranes. This voltage underlies nerve impulses and muscle contraction, including the heartbeat.
Molecules containing this element
- Potassium chloride KCl
- Potassium iodide KI
- Potassium hydroxide KOH
Quick question
Which element has the electron configuration [Ar] 4s1?
FAQ
Why does potassium react more violently with water than sodium?
Potassium's valence electron sits in a shell further from the nucleus than sodium's, so it is held more weakly. Potassium gives it up more easily and therefore reacts faster. Potassium also melts at a lower temperature, so the heat of the reaction quickly melts the metal and the hydrogen ignites straight away.
Which foods are high in potassium?
Bananas, potatoes, pulses such as beans and lentils, nuts, spinach and dried fruit such as apricots are all rich in potassium. A varied diet usually provides enough. People with kidney disease, on the other hand, often have to watch their potassium intake, because too much potassium in the blood can disturb the heart rhythm.