Potassium
Potassium is a chemical element; it has symbol K (from Neo-Latin kalium) and atomic number 19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to form flaky white potassium peroxide in only seconds of exposure. It was first isolated from potash, the ashes of plants, from which its name derives. In the periodic table, potassium is one of the alkali metals, all of which have a single valence electron in the outer electron shell, which is easily removed to create an ion with a positive charge (which combines with anions to form salts). In nature, potassium occurs only in ionic salts. Elemental potassium reacts vigorously with water, generating sufficient heat to ignite hydrogen emitted in the reaction, and burning with a lilac-colored flame. It is found dissolved in seawater (which is 0.04% potassium by weight), and occurs in many minerals such as orthoclase, a common constituent of granites and other igneous rocks.
Potassium pearls (in paraffin oil, ~5 mm each) | ||||||||||||||||||||||||||||||
| Potassium | ||||||||||||||||||||||||||||||
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| Appearance | silvery white, faint bluish-purple hue when exposed to air | |||||||||||||||||||||||||||||
| Standard atomic weight Ar°(K) | ||||||||||||||||||||||||||||||
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| Potassium in the periodic table | ||||||||||||||||||||||||||||||
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| Atomic number (Z) | 19 | |||||||||||||||||||||||||||||
| Group | group 1: hydrogen and alkali metals | |||||||||||||||||||||||||||||
| Period | period 4 | |||||||||||||||||||||||||||||
| Block | s-block | |||||||||||||||||||||||||||||
| Electron configuration | [Ar] 4s1 | |||||||||||||||||||||||||||||
| Electrons per shell | 2, 8, 8, 1 | |||||||||||||||||||||||||||||
| Physical properties | ||||||||||||||||||||||||||||||
| Phase at STP | solid | |||||||||||||||||||||||||||||
| Melting point | 336.7 K (63.5 °C, 146.3 °F) | |||||||||||||||||||||||||||||
| Boiling point | 1030.793 K (757.643 °C, 1395.757 °F) | |||||||||||||||||||||||||||||
| Density (near r.t.) | 0.8590 g/cm3 | |||||||||||||||||||||||||||||
| when liquid (at m.p.) | 0.82948 g/cm3 | |||||||||||||||||||||||||||||
| Critical point | 2223 K, 16 MPa | |||||||||||||||||||||||||||||
| Heat of fusion | 2.33 kJ/mol | |||||||||||||||||||||||||||||
| Heat of vaporization | 76.9 kJ/mol | |||||||||||||||||||||||||||||
| Molar heat capacity | 29.6 J/(mol·K) | |||||||||||||||||||||||||||||
| Atomic properties | ||||||||||||||||||||||||||||||
| Oxidation states | −1, +1 (a strongly basic oxide) | |||||||||||||||||||||||||||||
| Electronegativity | Pauling scale: 0.82 | |||||||||||||||||||||||||||||
| Ionization energies |
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| Atomic radius | empirical: 227 pm | |||||||||||||||||||||||||||||
| Covalent radius | 203±12 pm | |||||||||||||||||||||||||||||
| Van der Waals radius | 275 pm | |||||||||||||||||||||||||||||
| Spectral lines of potassium | ||||||||||||||||||||||||||||||
| Other properties | ||||||||||||||||||||||||||||||
| Natural occurrence | primordial | |||||||||||||||||||||||||||||
| Crystal structure | body-centered cubic (bcc) (cI2) | |||||||||||||||||||||||||||||
| Lattice constant | a = 532.69 pm (at 20 °C) | |||||||||||||||||||||||||||||
| Thermal expansion | 83.3 µm/(m⋅K) (at 25 °C) | |||||||||||||||||||||||||||||
| Thermal conductivity | 102.5 W/(m⋅K) | |||||||||||||||||||||||||||||
| Electrical resistivity | 72 nΩ⋅m (at 20 °C) | |||||||||||||||||||||||||||||
| Magnetic ordering | paramagnetic | |||||||||||||||||||||||||||||
| Molar magnetic susceptibility | +20.8×10−6 cm3/mol (298 K) | |||||||||||||||||||||||||||||
| Young's modulus | 3.53 GPa | |||||||||||||||||||||||||||||
| Shear modulus | 1.3 GPa | |||||||||||||||||||||||||||||
| Bulk modulus | 3.1 GPa | |||||||||||||||||||||||||||||
| Speed of sound thin rod | 2000 m/s (at 20 °C) | |||||||||||||||||||||||||||||
| Mohs hardness | 0.4 | |||||||||||||||||||||||||||||
| Brinell hardness | 0.363 MPa | |||||||||||||||||||||||||||||
| CAS Number | 7440-09-7 | |||||||||||||||||||||||||||||
| History | ||||||||||||||||||||||||||||||
| Discovery and first isolation | Humphry Davy (1807) | |||||||||||||||||||||||||||||
| Symbol | "K": from New Latin kalium | |||||||||||||||||||||||||||||
| Isotopes of potassium | ||||||||||||||||||||||||||||||
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Potassium is chemically very similar to sodium, the previous element in group 1 of the periodic table. They have a similar first ionization energy, which allows for each atom to give up its sole outer electron. It was first suggested in 1702 that they were distinct elements that combine with the same anions to make similar salts, which was demonstrated in 1807 when elemental potassium was first isolated via electrolysis. Naturally occurring potassium is composed of three isotopes, of which 40
K is radioactive. Traces of 40
K are found in all potassium, and it is the most common radioisotope in the human body.
Potassium ions are vital for the functioning of all living cells. The transfer of potassium ions across nerve cell membranes is necessary for normal nerve transmission; potassium deficiency and excess can each result in numerous signs and symptoms, including an abnormal heart rhythm and various electrocardiographic abnormalities. Fresh fruits and vegetables are good dietary sources of potassium. The body responds to the influx of dietary potassium, which raises serum potassium levels, by shifting potassium from outside to inside cells and increasing potassium excretion by the kidneys.
Most industrial applications of potassium exploit the high solubility of its compounds in water, such as saltwater soap. Heavy crop production rapidly depletes the soil of potassium, and this can be remedied with agricultural fertilizers containing potassium, accounting for 95% of global potassium chemical production.