Noble Gases (Group 18)
Helium, neon, argon, krypton, xenon, radon and oganesson: group 18, with filled outer shells and very little urge to react.
What they have in common
Every noble gas has a complete outer shell: 1s² for helium and ns² np⁶ for the rest. That gives them the highest ionization energies in their periods and almost no tendency to gain, lose or share electrons, so they exist as single atoms.
“Inert” turned out to be too strong. In 1962 Neil Bartlett made the first noble gas compound, a xenon hexafluoroplatinate, and xenon fluorides and oxides are now well known. The lighter noble gases are used for lighting (neon signs, argon in bulbs) and helium for cryogenics.
- Full valence shell (8 electrons, 2 for helium)
- Monatomic, colourless, odourless gases
- Highest ionization energy in each period
- Very low boiling points (helium boils at 4 K)
Noble gases at a glance
| Element | Z | Mass | Configuration | Valence e⁻ | Charges | EN | Radius (pm) |
|---|---|---|---|---|---|---|---|
| Helium (He) | 2 | 4.003 | 1s2 | 2 | 0 | — | 32 |
| Neon (Ne) | 10 | 20.18 | [He] 2s2 2p6 | 8 | 0 | — | 69 |
| Argon (Ar) | 18 | 39.948 | [Ne] 3s2 3p6 | 8 | 0 | — | 97 |
| Krypton (Kr) | 36 | 83.798 | [Ar] 3d10 4s2 4p6 | 8 | 0 | — | 110 |
| Xenon (Xe) | 54 | 131.293 | [Kr] 4d10 5s2 5p6 | 8 | 0 | — | 130 |
| Radon (Rn) | 86 | [222] | [Xe] 4f14 5d10 6s2 6p6 | 8 | 0 | — | 145 |
| Oganesson (Og) | 118 | [294] | [Rn] 5f14 6d10 7s2 7p6 | 8 | 0 | — | — |
Questions students ask
Why are noble gases unreactive?
Their outer shell is full, so gaining or losing an electron costs a lot of energy.
Do noble gases form compounds?
The heavier ones do. Xenon forms XeF₂, XeF₄, XeF₆ and oxides; krypton forms KrF₂.
How many valence electrons do noble gases have?
Eight, except helium, which has two.