Xenon at a glance
- Atomic number
- 54
- Symbol
- Xe
- Atomic mass
- 131.29 u
- Category
- Noble gas
- Group / period
- 18 / 5
- Electron configuration
- [Kr] 4d 10 5s 2 5p 6
- State at 25 °C
- Gas
- Melting point
- -111.7 °C
- Boiling point
- -108.1 °C
- Density
- 5.89 g/L
- Electronegativity
- 2.6
- 1st ionization energy
- 1170 kJ/mol
- Oxidation states
- 0, +2, +4, +6
- Discovered
- 1898
Atomic structure
A neutral xenon atom has 54 protons and 54 electrons, and its most common isotope carries about 78 neutrons. One atom weighs about 2.18 × 10⁻²⁵ kg, so a mole of xenon (6.022 × 10²³ atoms) weighs 131.29 g. That nucleus holds 1.44 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 5 shells (K 2, L 8, M 18, N 18, O 8), which gives the configuration [Kr] 4d 10 5s 2 5p 6. Its outer shell is full, which is why xenon hardly reacts with anything. It has 9 stable isotopes: Xe-124, Xe-126, Xe-128, Xe-129, Xe-130, Xe-131, Xe-132, Xe-134 and Xe-136. Its most common isotope, Xe-132, has the same mass number as Ba-132: isobars, nuclei of different elements with 132 nucleons each.
Physical properties
At room temperature (25 °C) xenon is a gas. It melts at -111.7 °C and boils at -108.1 °C, so it stays liquid across a 3.63-degree range. To turn liquid it has to be cooled below -108.1 °C, colder than dry ice (−78 °C). Only 8 elements have a lower melting point. As a gas it weighs 5.89 g per litre, 4.9 times heavier than air. Its atoms have a radius of about 108 pm (Iodine next door is 140 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 2.6 on the Pauling scale. Pulling off its outermost electron takes 1170 kJ/mol, less than Krypton above it (1351 kJ/mol). A bond between xenon and chlorine would be polar covalent, with an electronegativity difference of 0.56. Its common oxidation states are 0, +2, +4, +6, so in compounds you will usually meet it as Xe²⁺, Xe⁴⁺ and Xe⁶⁺. Group 18 also holds Helium, Neon, Argon, Krypton, Radon and Oganesson, which share its outer-shell pattern. Its Lewis symbol is Xe surrounded by 8 dots. Handle with care: as a pure element it is asphyxiant gas.
Where it comes from
It makes up 0.00003 mg/kg of Earth's crust, the 83rd most abundant element there. It was first identified in 1898, making it the 79th 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 467.1 nm, 462.4 nm and 473.4 nm.