Noble Gases

Electronic configurations.

• He: $2$ (full first shell).
• Ne: $2, 8$.
• Ar: $2, 8, 8$.
• Kr: $2, 8, 18, 8$ (4th shell starts to fill differently).

Why this matters. Noble gases have FULL OUTER SHELLS. This is the most stable arrangement. Other elements lose, gain, or share electrons EXACTLY to achieve a noble-gas configuration.

Consequence. Noble gases:

• Don't form ions (no need to lose or gain).
• Rarely form molecules with other elements (no driving force).
• Are MONATOMIC — exist as single atoms, not pairs.

Worked qualitative. Why doesn't argon form ions or compounds? It already has $2, 8, 8$ — full outer shell, the same configuration that other ions strive to attain. There's no chemical motivation to change.

Edge cases (beyond IGCSE). Heavy noble gases (Kr, Xe) can form a few compounds with the most reactive non-metals (e.g. XeF₂, XeF₄, XeF₆). These are exotic and not part of IGCSE.

Trends down the group.

• Atomic radius INCREASES.
• m.p. and b.p. INCREASE (heavier atoms, stronger van der Waals forces).
• Density INCREASES (more massive atoms).
• Reactivity barely changes — all essentially inert at IGCSE level.

Helium (He).

• Less dense than air → BALLOONS, AIRSHIPS (replaced flammable hydrogen).
• Cryogenics — liquid He at $\sim 4\,\text{K}$ used to cool MRI superconducting magnets.
• Mixture with oxygen for deep-sea diving (heliox) — reduces nitrogen narcosis.

Neon (Ne).

• Bright RED-ORANGE glow when current passes — neon advertising signs.
• Other noble gases give different colours in similar tubes (Ar pale lavender, He pale orange-pink, Kr white-blue, Xe lavender-blue).

Argon (Ar).

• Inert atmosphere in incandescent / fluorescent light bulbs (prevents tungsten filament from oxidising and burning out).
• Inert atmosphere in welding (prevents oxidation of hot metal).
• Used in some lasers.

Krypton (Kr) and xenon (Xe).

• Specialty lamps — high-intensity discharge for car headlights, photography flashes.
• Anaesthetics (xenon — works without binding chemically).

Radon (Rn).

• RADIOACTIVE — health concern in some homes (built on uranium-bearing rocks). Not used industrially because of its radioactivity.

Worked qualitative. Why is argon used in light bulbs rather than nitrogen? Both are unreactive at filament temperatures, but argon is even less reactive — won't slowly nitride the tungsten over years of use.