What Are the Noble Gases?
The noble gases are the six elements in Group 18 (the last column) of the periodic table. They are all colourless, odourless gases at room temperature. In order of atomic number, they are:
- Helium (He) — atomic number 2
- Neon (Ne) — atomic number 10
- Argon (Ar) — atomic number 18
- Krypton (Kr) — atomic number 36
- Xenon (Xe) — atomic number 54
- Radon (Rn) — atomic number 86 (radioactive)
Noble gases are also sometimes called inert gases because they are so unreactive.
Why Are Noble Gases Unreactive?
The key to understanding noble gases is their electron arrangement. Every noble gas (except helium) has 8 electrons in its outer shell — a completely full shell. Helium has 2 electrons, which fills its only (first) shell.
A full outer shell is the most stable arrangement for an atom. Because noble gas atoms already have this stable arrangement, they have no tendency to gain, lose, or share electrons with other atoms. This means they do not form chemical bonds and exist as single atoms (they are monatomic).
All other elements react to try to achieve a full outer electron shell, which drives all of chemistry. Noble gases already have this full shell, so they simply do not need to react. This is the foundation of the octet rule — atoms tend to gain, lose, or share electrons to end up with 8 electrons in their outer shell.
Properties of Noble Gases
| Property | Description |
|---|---|
| State at room temp. | Gas |
| Colour/smell | Colourless and odourless |
| Reactivity | Extremely low — do not normally react with other elements |
| Boiling points | Very low (helium − 269 °C; argon −186 °C) |
| Exist as | Single atoms (monatomic) |
Uses of Noble Gases
Their unreactivity makes noble gases very useful:
| Gas | Use | Reason |
|---|---|---|
| Helium | Weather balloons, airships | Lifts because it is lighter than air; safe (not flammable, unlike hydrogen) |
| Neon | Glowing advertising signs | Produces a red/orange glow when an electric current passes through it |
| Argon | Inside light bulbs; welding | Prevents the hot filament or weld from reacting with oxygen in the air |
| Krypton/Xenon | High-performance lights, lasers | Produce bright, specific colours of light |
Neon signs do not have to contain neon — different noble gases (and some other gases) produce different colours. True neon gas glows red-orange, argon glows blue-violet, krypton glows white, and xenon gives a blue glow.
Discovery of the Noble Gases
The noble gases were unknown before 1894. Their existence was predicted by gaps in the periodic table. Argon was the first to be discovered (by Lord Rayleigh and William Ramsay), and the others were found in the following years. Helium was actually detected on the Sun (via its light spectrum) before it was found on Earth.
Real world: Helium in party balloons and airships
Helium is used in balloons and airships because it is much less dense than air (so it floats) and is completely non-flammable. The 1937 Hindenburg airship disaster happened because it was filled with hydrogen instead of helium — hydrogen is flammable and ignited catastrophically. Modern airships and blimps all use helium instead.
Real world: Argon protecting light bulb filaments
Inside a traditional incandescent light bulb, the tungsten filament reaches temperatures above 2,500 °C. Without protection, it would react with oxygen and burn away instantly. Argon is pumped in to fill the bulb — its complete unreactivity prevents the filament from oxidising, extending the bulb's life significantly.
Real world: Neon in advertising signs
Neon signs work by passing an electric current through sealed glass tubes containing noble gases. True neon gas glows red-orange. Other colours are produced by argon (blue-violet), krypton (white), or xenon (bright blue). These signs last years because noble gases do not react with anything — the glass simply glows without any chemical change.
Where do you see this in real life? Helium balloons at parties, glowing signs in shop windows, and lightbulbs all involve noble gases. Their unreactivity is what makes them so useful — they do the job without interfering with anything else.
Practice Problems
Explain why noble gases are monatomic (exist as single atoms) rather than forming diatomic molecules the way oxygen (O₂) and nitrogen (N₂) do.
Show answer
Why is helium used in weather balloons rather than hydrogen, even though hydrogen is even lighter?