Chemistry · Ch 14 — The p-Block Elements

Occurrence

14.23.1

Occurrence

Where the Noble Gases Are Found

With the exception of radon and the synthetic element oganesson, every noble gas occurs naturally in the atmosphere. Together they make up roughly 1% by volume of dry air, and within that share, argon is by far the most abundant of the group.

Helium — and, less commonly, neon — is also trapped inside certain minerals of radioactive origin, such as pitchblende, monazite and cleveite; these gases build up there as decay products of radioactive elements. Commercially, though, the main source of helium is natural gas deposits, from which it is extracted.

Xenon and radon are the rarest members of the group. Radon does not occur primordially — it is generated continuously as a decay product of radium-226:

^{226}_{88}Ra \rightarrow \, ^{222}_{86}Rn + \, ^{4}_{2}He

Oganesson, the heaviest member of the group, does not occur in nature at all — it has only ever been made synthetically, by colliding californium-249 nuclei with calcium-48 ions:

^{249}_{98}Cf + \, ^{48}_{20}Ca \rightarrow \, ^{294}_{118}Og + 3n

Oganesson carries the symbol Og, atomic number 118, atomic mass 294, and the electronic configuration $[Rn]\,5f^{14}\,6d^{10}\,7s^2\,7p^6$ . Only an extremely small number of Og atoms have ever been produced, and its half-life is a mere 0.7 milliseconds — far too short-lived for its chemistry to be studied experimentally, so essentially everything known about how it should behave is theoretical prediction.

Atomic and Physical Properties (Table 7.12)

Property	He	Ne	Ar	Kr	Xe	Rn*
Atomic number	2	10	18	36	54	86
Atomic mass / g mol⁻¹	4.00	20.18	39.95	83.80	131.30	222.00
Electronic configuration	1s²	[He]2s²2p⁶	[Ne]3s²3p⁶	[Ar]3d¹⁰4s²4p⁶	[Kr]4d¹⁰5s²5p⁶	[Xe]4f¹⁴5d¹⁰6s²6p⁶
Atomic radius / pm	120	160	190	200	220	–
Ionisation enthalpy / kJ mol⁻¹	2372	2080	1520	1351	1170	1037
Electron gain enthalpy / kJ mol⁻¹	48	116	96	96	77	68
Density (at STP) / g cm⁻³	1.8×10⁻⁴	9.0×10⁻⁴	1.8×10⁻³	3.7×10⁻³	5.9×10⁻³	9.7×10⁻³
Melting point / K	–	24.6	83.8	115.9	161.3	202
Boiling point / K	4.2	27.1	87.2	119.7	165.0	211
Atmospheric content (% by volume)	5.24×10⁻⁴	1.82×10⁻³	0.934	1.14×10⁻⁴	8.7×10⁻⁶	–

Table 7.12Atomic and Physical Properties of Group 18 Elements [Footnote: * radioactive]

Property	He	Ne	Ar	Kr	Xe	Rn*
Atomic number	2	10	18	36	54	86
Atomic mass/ g mol⁻¹	4.00	20.18	39.95	83.80	131.30	222.00
Electronic configuration	1s²	[He]2s²2p⁶	[Ne] 3s²3p⁶	[Ar]3d¹⁰4s²4p⁶	[Kr]4d¹⁰5s²5p⁶	[Xe]4f¹⁴5d¹⁰6s²6p⁶
Atomic radius/pm	120	160	190	200	220	–
Ionisation enthalpy /kJmol⁻¹	2372	2080	1520	1351	1170	1037
Electron gain enthalpy /kJmol⁻¹	48	116	96	96	77	68
Density (at STP)/gcm⁻³	1.8×10⁻⁴	9.0×10⁻⁴	1.8×10⁻³	3.7×10⁻³	5.9×10⁻³	9.7×10⁻³
Melting point/K	–	24.6	83.8	115.9	161.3	202

Start your 14-day free trial to unlock the full solution →

← Previous · 14.23Group 18 Elements Next · 14.23.1 →Example 7.20 — Why are the elements of Group 18 known as noble gases ?