Argon at a glance
- Atomic number
- 18
- Symbol
- Ar
- Atomic mass
- 39.948 u
- Category
- Noble gas
- Group / period
- 18 / 3
- Electron configuration
- [Ne] 3s 2 3p 6
- State at 25 °C
- Gas
- Melting point
- -189.3 °C
- Boiling point
- -185.8 °C
- Density
- 1.78 g/L
- Electronegativity
- n/a
- 1st ionization energy
- 1521 kJ/mol
- Oxidation states
- 0
- Discovered
- 1894
Atomic structure
A neutral argon atom has 18 protons and 18 electrons, and its most common isotope carries about 22 neutrons. One atom weighs about 6.63 × 10⁻²⁶ kg, so a mole of argon (6.022 × 10²³ atoms) weighs 39.948 g. That nucleus holds 1.22 neutrons for every proton. The electrons fill 3 shells (K 2, L 8, M 8), which gives the configuration [Ne] 3s 2 3p 6. Its outer shell is full, which is why argon hardly reacts with anything. It has 3 stable isotopes: Ar-36, Ar-38 and Ar-40. Its most common isotope, Ar-40, has the same mass number as Ca-40: isobars, nuclei of different elements with 40 nucleons each.
Physical properties
At room temperature (25 °C) argon is a gas. It melts at -189.3 °C and boils at -185.8 °C, so it stays liquid across a 3.5-degree range. To turn liquid it has to be cooled below -185.8 °C, colder than dry ice (−78 °C). Only 6 elements have a lower melting point. As a gas it weighs 1.78 g per litre, 1.5 times heavier than air. Its atoms have a radius of about 71 pm (Chlorine next door is 100 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
It has no measured electronegativity, because it almost never forms bonds. Pulling off its outermost electron takes 1521 kJ/mol, less than Neon above it (2081 kJ/mol). It usually keeps an oxidation state of 0. Group 18 also holds Helium, Neon, Krypton, Xenon, Radon and Oganesson, which share its outer-shell pattern. Its Lewis symbol is Ar surrounded by 8 dots. Handle with care: as a pure element it is asphyxiant gas.
Where it comes from
It makes up 3.5 mg/kg of Earth's crust, the 43rd most abundant element there. It was first identified in 1894, making it the 76th 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 750.4 nm, 696.5 nm and 738.4 nm.