Noble Gases: Group 18 of the Periodic Table

Group 18 is the noble gases

Group 18 is the far-right column of the periodic table, and its elements are called the noble gases: helium, neon, argon, krypton, xenon, radon and oganesson. If your textbook is older it may label the same column group 0 or group VIIIA, and it may call them the inert gases. Same seven elements.

The name is borrowed from the noble metals, gold and platinum, which resist reacting with things around them. Noble gases do the same, and for one reason that runs through the whole group: their outer electron shell is already full.

The seven elements of group 18

Two things stand out. Boiling points climb steadily down the group, from helium at −269 °C, the coldest boiling point of any substance, to radon at −62 °C: bigger atoms hold each other more strongly. And the glow colour column is what a low-pressure discharge tube shows when you run current through it. The gases themselves are all colourless; the colour is light emitted as excited electrons fall back down, which is why a “neon” sign filled with argon glows lilac rather than orange.

Argon is worth a second look. At 0.93% of dry air it is the third most abundant gas in the atmosphere, after nitrogen and oxygen, and there is roughly twenty times more of it than carbon dioxide.

Why a full outer shell means no reactions

Chemical reactions are about valence electrons, the electrons in an atom’s outermost shell. Most atoms react because that shell is unfinished: they gain, lose or share electrons until it is full. A noble gas is already there, so there is no energetic reward for reacting.

Two consequences follow, and both show up in exam questions:

• Their ionisation energies are the highest in their periods. Removing an electron would break the full shell, and that costs a lot of energy.
• They exist as single atoms. Every other gas in the air travels as a molecule (N₂, O₂, CO₂). Noble gases are monatomic, because they have no reason to bond even to each other.

That same full shell is what every other element is imitating when it bonds. The octet rule is really the rule “get to the nearest noble gas”, which is why sodium loses one electron to look like neon and chlorine gains one to look like argon.

Helium is the exception

Helium has 2 electrons in its outer shell, not 8, and that trips up more students than anything else on this page.

There is nothing irregular about it. The first shell holds a maximum of two electrons, so for helium two is full. That full pair is called a duet, and it makes helium exactly as unreactive as the others: helium has never been persuaded into a stable compound. Every other noble gas has a shell that holds eight, so they fill it with an octet.

So if a question asks which noble gas does not have eight outer electrons, the answer is helium, and the reason is the size of the first shell, not any weakness in helium’s stability.

Which noble gas is it?

Worksheets like to describe one and ask you to name it.

• Two electrons, two protons, a full duet: helium.
• The one in period 2: neon, with 10 electrons arranged 2, 8.
• The one with 18 protons: argon, which happens to sit in group 18 as well.
• The most abundant in air: argon again, at 0.93%.
• The heaviest that occurs naturally: radon. The heaviest of all is oganesson, but only a handful of atoms have ever existed.
• The radioactive ones: radon and oganesson.
• The one that really does form compounds: xenon.

Do noble gases form ions?

Not in ordinary chemistry. A noble gas has an oxidation number of 0 and no tendency to become an ion, because gaining or losing an electron would break the full shell it already has. You will not find Ne⁺ or Ar²⁻ in a chemical formula the way you find Na⁺ and Cl⁻.

They can still be ionised if you push hard enough, and that is exactly what a discharge tube does: the current strips electrons off, and the light you see is them dropping back. Those ions last microseconds and form no compounds.

Not perfectly inert

“Noble”, not “inert”, and the distinction is real. The heavier atoms are bigger, so their outer electrons sit further from the nucleus and are held less tightly. Push hard enough with something aggressive enough and they will bond.

Xenon is the one that matters. Xenon difluoride (XeF₂) and xenon tetrafluoride (XeF₄) are stable compounds you can put in a bottle, and their shapes are worth looking at: XeF₂ is linear and XeF₄ is square planar, both consequences of xenon carrying lone pairs it does not use. Krypton manages KrF₂ under harsher conditions. Helium, neon and argon form nothing stable at all.

The radioactive two

Radon is radioactive and natural. It is produced continuously as uranium and thorium decay in rock and soil, seeps into buildings from the ground, and is tracked as an indoor-air hazard for that reason. It is the only noble gas that is dangerous to be around in ordinary life.

Oganesson is element 118, synthetic, and intensely radioactive: its known isotope has a half-life well under a millisecond, and only a few atoms have ever been made. It sits in group 18 by position, but calling it a noble gas is a prediction rather than an observation. Relativistic effects on such a heavy atom are expected to make it a solid at room temperature and possibly reactive, which would make it the least noble member of the group. The boiling point in the table above is calculated, not measured.

Explore further

Open any gas’s full page from the explorer, browse the whole elements index, or start with helium, neon, or argon. To see how a full shell is built up in the first place, step through the electron configuration builder.

Using this with a class

Project the explorer and ask students to predict, before clicking, how many outer electrons each gas has, then reveal the duet or octet. A good follow-up: ask why sodium and magnesium form Na⁺ and Mg²⁺ rather than Na²⁺, and let them find neon in the answer. It is free to embed on your own site or LMS using the snippet below.