The Noble Gases: Why Group 18 Is (Almost) Unreactive

The noble gases — helium, neon, argon, krypton, xenon, radon and the superheavy oganesson — fill the last column of the periodic table, group 18. For decades they were called the “inert gases”, because nobody could make them react with anything. That turned out to be not quite true, but they’re still the least reactive family of elements.

Full outer shells

Helium has two electrons, completely filling its first shell (1s²). Every other noble gas has eight electrons in its outer shell (ns² np⁶). A full shell is extremely stable:

• It takes a lot of energy to remove an electron — noble gases have the highest ionization energies in each period. Helium’s, 24.6 eV, is the highest of any element.
• There’s no room to add one without starting a new, higher-energy shell.

So noble gas atoms have almost no tendency to lose, gain or share electrons. They exist as single atoms (they’re monatomic), not molecules.

Properties and trends (from our element data)

Element	Boiling point (°C)	Density at 0 °C, 1 atm (g/L)	First ionization energy (eV)
Helium	−268.9	0.18	24.59
Neon	−246.1	0.90	21.56
Argon	−185.9	1.78	15.76
Krypton	−153.2	3.73	14.00
Xenon	−108.1	5.89	12.13
Radon	−61.7	9.73	10.75

Down the group, atoms get bigger, dispersion forces grow, and boiling points rise. Ionization energy falls, which matters for the story below.

All are colourless, odourless gases at room temperature, and all glow in characteristic colours when electricity passes through them.

How they were discovered

The noble gases were missed entirely by the chemists who built the first periodic tables — Mendeleev’s 1869 table had no column for them.

• Helium was first detected in 1868 as an unexplained yellow line in the spectrum of the Sun during a solar eclipse — which is why it’s named after the Greek sun god, Helios. It wasn’t found on Earth until 1895.
• Argon was discovered in 1894 by Lord Rayleigh and William Ramsay, who noticed that nitrogen extracted from air was slightly denser than nitrogen made chemically. The difference was about 1% of argon hiding in the “nitrogen”. The name means “lazy” in Greek.
• Neon, krypton and xenon were found by Ramsay and Morris Travers in 1898 by distilling liquefied air. Their names mean “new”, “hidden” and “stranger”.
• Radon was discovered around 1899–1900 as a radioactive gas given off by radium and thorium.

Ramsay received the 1904 Nobel Prize in Chemistry for adding an entire new group to the periodic table.

The noble gas compounds surprise

In 1962 Neil Bartlett, working in Canada, reasoned that since platinum hexafluoride could strip an electron from oxygen, and xenon’s ionization energy was almost the same as oxygen’s, PtF₆ might react with xenon. It did, forming an orange-yellow solid — the first noble gas compound. Within months, others made xenon tetrafluoride (XeF₄) and xenon difluoride (XeF₂).

Today hundreds of xenon compounds are known, plus some of krypton and radon, and even argon forms a fragile compound (HArF) at very low temperatures. The heavier noble gases react because their outer electrons are further from the nucleus and easier to share with extremely electronegative partners like fluorine and oxygen. Helium and neon still have no known stable neutral compounds.

This is why many modern textbooks prefer “noble gases” to “inert gases”.

Uses

• Helium: balloons and airships; cooling superconducting magnets in MRI scanners; a breathing gas for deep-sea divers (mixed with oxygen, it avoids nitrogen narcosis); leak detection.
• Neon: the classic orange-red glow of neon signs (other colours in “neon” signs usually come from different gases or coatings).
• Argon: the most abundant — nearly 1% of air. It shields welding from oxygen, fills incandescent bulbs, and protects valuable documents.
• Krypton: high-performance lighting and insulated windows.
• Xenon: bright car headlights and cinema projectors, a general anaesthetic, and propellant for spacecraft ion thrusters.
• Radon: mainly a health hazard — radon seeping into homes from the ground is a major cause of lung cancer. Testing and ventilation reduce the risk.

Quick answers

Are noble gases completely unreactive? No. Xenon, krypton and radon form compounds, mainly with fluorine and oxygen. Helium and neon have no known stable neutral compounds.

Why are they called “noble”? By analogy with “noble metals” like gold, which also resist reacting.

Which noble gas is most abundant? In the atmosphere, argon. In the universe, helium — the second most abundant element of all.