Noble Gases (Group 18 / Group VIII)

Noble gases are a family of elements on the far right of the Periodic Table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). They are all gases at room temperature and pressure.

Key Properties

• Unreactive (inert): they rarely form compounds.
• Monatomic: they exist as single atoms, not molecules.
• Colorless, odorless gases; non-flammable.

Why are they so unreactive?

Atoms react to get a full outer electron shell. Noble gases already have this.

• Helium has a full first shell: $2$.
• Neon: $2,8$; Argon: $2,8,8$.
• Because their outer shells are full, they do not need to gain, lose, or share electrons.

Note: In some syllabuses this group is called Group VIII; in others, Group 18. Both mean the noble gases.

Trends down the group

• Density increases (heavier atoms lower down).
• Boiling and melting points increase (they liquefy more easily lower down), but all are gases at room temperature.
• Reactivity stays very low. Heavier ones like xenon can form rare compounds under special conditions.

Monatomic vs diatomic

Noble gases are monatomic (single atoms). This contrasts with halogens like chlorine, which are diatomic ($\text{Cl}_2$) because they need to share electrons. Noble gases do not.

Everyday uses

• Helium: balloons and airships (low density, non-flammable).
• Neon: advertising signs (red–orange glow; other colours often use argon or other gases).
• Argon: fills light bulbs and provides a safe, unreactive atmosphere for welding.
• Krypton/Xenon: bright lamps, camera flashes, car headlights.
• Radon: radioactive gas from some rocks; high levels are a health risk, so buildings may be tested and ventilated.

Common misconceptions

• Helium is not the same as hydrogen; helium is non-flammable, hydrogen is flammable.
• “Full outer shell” for helium means $2$ electrons (duplet), not $8$.
• They are atoms (monatomic), not pairs or molecules.