4.3 - Elements in Group 18

Noble Gases and Electron Stability

• Group 18 contains helium, He; neon, Ne; argon, Ar; krypton, Kr; xenon, Xe; radon, Rn; and oganesson, Og.
• These elements are noble gases or inert gases.
• Their valence shells are fully filled: helium has a stable duplet, while the other noble gases have a stable octet.
• They are chemically unreactive because their atoms do not need to donate, accept or share electrons.
• Noble gases exist as monoatomic particles.

Physical Trends Down Group 18

Down the group:

• atomic radius increases because the number of electron-filled shells increases;
• melting point and boiling point increase;
• the larger atoms have stronger attraction forces between them, so more heat energy is needed to overcome those forces;
• density generally increases.

Element	Atomic radius (nm)	Melting / boiling point (${}^{\circ }\mathrm{C}$)
He	0.050	$-270/-269$
Ne	0.070	$-248/-246$
Ar	0.094	$-189/-186$
Kr	0.109	$-156/-152$
Xe	0.130	$-122/-108$
Rn	—	$-71/-62$

Element	Density ($\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$)
He	0.00017
Ne	0.00080
Ar	0.00170
Kr	0.00350
Xe	0.00550

Uses in Daily Life

Element	Uses
Helium	Weather balloons; divers’ oxygen tanks
Neon	Advertising board lights
Argon	Electric bulbs; inert atmosphere for high-temperature welding
Krypton	Camera flashlights; lasers for retinal treatment
Xenon	Lighthouse lamps; anaesthesia
Radon	Cancer treatment