Potassium at a glance
- Atomic number
- 19
- Symbol
- K
- Atomic mass
- 39.098 u
- Category
- Alkali metal
- Group / period
- 1 / 4
- Electron configuration
- [Ar] 4s 1
- State at 25 °C
- Solid
- Melting point
- 63.38 °C
- Boiling point
- 758.9 °C
- Density
- 0.862 g/cm³
- Electronegativity
- 0.82
- 1st ionization energy
- 419 kJ/mol
- Oxidation states
- +1
- Discovered
- 1807
Atomic structure
A neutral potassium atom has 19 protons and 19 electrons, and its most common isotope carries about 20 neutrons. One atom weighs about 6.49 × 10⁻²⁶ kg, so a mole of potassium (6.022 × 10²³ atoms) weighs 39.098 g. That nucleus holds 1.05 neutrons for every proton, close to the one-to-one balance that light nuclei prefer. Its symbol K comes from kalium. The electrons fill 4 shells (K 2, L 8, M 8, N 1), which gives the configuration [Ar] 4s 1. With 1 electron in its outer shell, it tends to lose electrons and form positive ions. It has 2 stable isotopes: K-39 and K-41. K-40 is radioactive but lasts billions of years. No other element has a stable isotope with mass number 39, so K-39 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) potassium is a solid. It melts at 63.38 °C and boils at 758.9 °C, so it stays liquid across a 695.5-degree range. It stays solid in your hand (body temperature) but would melt in boiling water. Its density is 0.862 g/cm³, lighter than water. Its atoms have a radius of about 220 pm (Calcium next door is 180 pm). In the solid, its atoms pack into a body-centred cubic lattice.
Chemical behaviour
Its electronegativity is 0.82 on the Pauling scale, one of the lowest, so it gives up electrons very easily. Pulling off its outermost electron takes 419 kJ/mol, less than Sodium above it (496 kJ/mol). A bond between potassium and chlorine would be mostly ionic, with an electronegativity difference of 2.3. It releases 48 kJ/mol when it gains an extra electron. Its common oxidation states are +1, so in compounds you will usually meet it as K⁺. Group 1 also holds Hydrogen, Lithium, Sodium, Rubidium, Caesium and Francium, which share its outer-shell pattern. Its Lewis symbol is K surrounded by 1 dot. Handle with care: as a pure element it is flammable or reacts with water.
Where it comes from
It makes up 2.09 % of Earth's crust, the 8th most abundant element there. In the human body it accounts for about 0.4 % of body mass. It was first identified in 1807, making it the 43rd element to be discovered (counting the 11 known since antiquity together). When excited it glows at its own set of colours; its brightest visible lines are at 766.5 nm, 769.9 nm and 404.4 nm. In a flame test its compounds colour the flame lilac.