Helium at a glance
- Atomic number
- 2
- Symbol
- He
- Atomic mass
- 4.0026 u
- Category
- Noble gas
- Group / period
- 18 / 1
- Electron configuration
- 1s 2
- State at 25 °C
- Gas
- Melting point
- -272.2 °C
- Boiling point
- -268.9 °C
- Density
- 0.178 g/L
- Electronegativity
- n/a
- 1st ionization energy
- 2372 kJ/mol
- Oxidation states
- 0
- Discovered
- 1868
Atomic structure
A neutral helium atom has 2 protons and 2 electrons, and its most common isotope carries about 2 neutrons. One atom weighs about 6.65 × 10⁻²⁷ kg, so a mole of helium (6.022 × 10²³ atoms) weighs 4.0026 g. That nucleus holds 1 neutrons for every proton, close to the one-to-one balance that light nuclei prefer. The electrons fill 1 shell (K 2), which gives the configuration 1s 2. Its outer shell is full, which is why helium hardly reacts with anything. It has 2 stable isotopes: He-3 and He-4. No other element has a stable isotope with mass number 4, so He-4 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) helium is a gas. It melts at -272.2 °C and boils at -268.9 °C, so it stays liquid across a 3.27-degree range. To turn liquid it has to be cooled below -268.9 °C, colder than liquid hydrogen (−253 °C). Only 0 elements have a lower melting point. As a gas it weighs 0.178 g per litre, 6.7 times lighter than air. Its atoms have a radius of about 31 pm (Hydrogen next door is 25 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
It has no measured electronegativity, because it almost never forms bonds. Pulling off its outermost electron takes 2372 kJ/mol. It usually keeps an oxidation state of 0. Group 18 also holds Neon, Argon, Krypton, Xenon, Radon and Oganesson, which share its outer-shell pattern. Its Lewis symbol is He surrounded by 2 dots. Handle with care: as a pure element it is asphyxiant gas.
Where it comes from
It makes up 0.008 mg/kg of Earth's crust, the 72nd most abundant element there. It was first identified in 1868, making it the 63rd 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 587.6 nm, 667.8 nm and 447.1 nm.