Krypton at a glance
- Atomic number
- 36
- Symbol
- Kr
- Atomic mass
- 83.798 u
- Category
- Noble gas
- Group / period
- 18 / 4
- Electron configuration
- [Ar] 3d 10 4s 2 4p 6
- State at 25 °C
- Gas
- Melting point
- -157.4 °C
- Boiling point
- -153.2 °C
- Density
- 3.73 g/L
- Electronegativity
- 3
- 1st ionization energy
- 1351 kJ/mol
- Oxidation states
- 0, +2
- Discovered
- 1898
Atomic structure
A neutral krypton atom has 36 protons and 36 electrons, and its most common isotope carries about 48 neutrons. One atom weighs about 1.39 × 10⁻²⁵ kg, so a mole of krypton (6.022 × 10²³ atoms) weighs 83.798 g. That nucleus holds 1.33 neutrons for every proton. The electrons fill 4 shells (K 2, L 8, M 18, N 8), which gives the configuration [Ar] 3d 10 4s 2 4p 6. Its outer shell is full, which is why krypton hardly reacts with anything. It has 6 stable isotopes: Kr-78, Kr-80, Kr-82, Kr-83, Kr-84 and Kr-86. Its most common isotope, Kr-84, has the same mass number as Sr-84: isobars, nuclei of different elements with 84 nucleons each.
Physical properties
At room temperature (25 °C) krypton is a gas. It melts at -157.4 °C and boils at -153.2 °C, so it stays liquid across a 4.14-degree range. To turn liquid it has to be cooled below -153.2 °C, colder than dry ice (−78 °C). Only 7 elements have a lower melting point. As a gas it weighs 3.73 g per litre, 3.1 times heavier than air. Its atoms have a radius of about 88 pm (Bromine next door is 115 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 3 on the Pauling scale, the 5th highest of all elements, so it pulls hard on shared electrons. Pulling off its outermost electron takes 1351 kJ/mol, less than Argon above it (1521 kJ/mol). A bond between krypton and chlorine would be almost non-polar covalent, with an electronegativity difference of 0.16. Its common oxidation states are 0, +2, so in compounds you will usually meet it as Kr²⁺. Group 18 also holds Helium, Neon, Argon, Xenon, Radon and Oganesson, which share its outer-shell pattern. Its Lewis symbol is Kr surrounded by 8 dots. Handle with care: as a pure element it is asphyxiant gas.
Where it comes from
It makes up 0.0001 mg/kg of Earth's crust, the 82nd most abundant element there. It was first identified in 1898, making it the 78th 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 587.1 nm, 557.0 nm and 760.2 nm.