13.1
Physical Properties of the Noble Gases
Overview of Group 18
The noble gases — Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), Radon (Rn) — are colourless, odourless, monoatomic gases at room temperature. They are sometimes called rare gases or inert gases, though these terms are not entirely accurate (some are not rare, and some do form compounds).
All noble gases are monoatomic — they exist as single atoms rather than diatomic molecules — because they have completely filled outer electron shells and have no tendency to form bonds with other atoms.
| Element | Z | Config | B.p. (°C) | Density (g/L) | % in atmosphere | First IE (kJ/mol) |
|---|---|---|---|---|---|---|
| Helium (He) | 2 | 1s² | −269 | 0.164 | 0.0005% | 2372 (highest of all) |
| Neon (Ne) | 10 | [He]2s²2p⁶ | −246 | 0.839 | 0.0018% | 2081 |
| Argon (Ar) | 18 | [Ne]3s²3p⁶ | −186 | 1.661 | 0.93% (most abundant) | 1521 |
| Krypton (Kr) | 36 | [Ar]3d¹⁰4s²4p⁶ | −153 | 3.479 | 0.00011% | 1351 |
| Xenon (Xe) | 54 | [Kr]4d¹⁰5s²5p⁶ | −108 | 5.458 | 0.000009% | 1170 |
| Radon (Rn) | 86 | [Xe]4f¹⁴5d¹⁰6s²6p⁶ | −62 | 9.23 | Trace (radioactive) | 1037 |
Trends in Physical Properties
- Boiling points — increase from He (−269°C) to Rn (−62°C). All are far below room temperature (all are gases at r.t.). The increase is due to increasing London dispersion forces as the number of electrons and atomic size increases going down.
- Atomic radius — increases down the group (more electron shells).
- Ionisation energy — decreases from He (highest of any element, 2372 kJ/mol) to Rn (1037 kJ/mol). Despite full shells, IE decreases as outer electrons are farther from the nucleus and better shielded.
- Density — increases from He (lightest gas, 0.164 g/L) to Rn (heavy, 9.23 g/L).
- Solubility in water — slightly increases going down (larger atoms, more polarisable — stronger induced dipole interactions with water).
He has the lowest boiling point of ANY substance (−269°C = 4 K)
Helium remains a liquid all the way to absolute zero at normal pressure — it must be pressurised to solidify. This is because He atoms are so tiny and light with such weak van der Waals forces that even at 4 K there is enough zero-point energy to prevent solidification. Used for cooling superconducting magnets.
13.2
Electronic Structure and Chemical Inertness
Why Are Noble Gases Inert?
Noble gases have completely filled outer electron shells:
He: 1s² (2 electrons — full n=1 shell)
Ne: [He]2s²2p⁶ (8 electrons in outer shell — full n=2)
Ar: [Ne]3s²3p⁶ (8 electrons in outer shell — full n=3)
Kr: [Ar]3d¹⁰4s²4p⁶ (full 4s and 4p — 8 outer electrons)
Xe: [Kr]4d¹⁰5s²5p⁶ (full 5s and 5p — 8 outer electrons)
Reasons for chemical inertness:
- Full outer shells — no tendency to gain or lose electrons. Already at the most stable electronic configuration (octet or duet for He).
- Very high ionisation energies — especially for He and Ne, it requires enormous energy to remove any electron. No tendency to form positive ions.
- Very low electron affinity — addition of an electron to a full shell would go into a new, higher-energy shell. No tendency to form negative ions.
- No unpaired electrons — no half-filled orbitals available for bond formation through electron sharing.
- No available low-energy empty orbitals for He and Ne — He and Ne are particularly inert as they have no accessible d orbitals to allow expansion of their small electron clouds.
Why Do Kr and Xe Form Compounds but He and Ne Do Not?
Although all noble gases are generally inert, Kr and Xe can form compounds under extreme conditions. The key factors are:
| Noble Gas | Can form compounds? | Reason |
|---|---|---|
| He, Ne | ❌ No known stable compounds | Very small atoms; very high IE (2372, 2081 kJ/mol); no accessible d orbitals; electrons too tightly held |
| Ar | ❌ Essentially no stable compounds (ArF compound only in matrix) | Still very high IE (1521 kJ/mol); very small |
| Kr | ✅ A few — KrF₂, KrF⁺ | Lower IE (1351 kJ/mol); larger atom with accessible 4d orbitals; very strong oxidants (F₂) can force reaction |
| Xe | ✅ Many — XeF₂, XeF₄, XeF₆, XeO₃, XeOF₄, XePtF₆ | Lower IE (1170 kJ/mol); large atom; accessible 5d orbitals; several compounds now well-characterised |
| Rn | ✅ Theoretically possible (RnF₂) | Lowest IE (1037 kJ/mol); difficult to study due to radioactivity |
Historical breakthrough — 1962
Neil Bartlett at University of British Columbia made the first noble gas compound: XePtF₆. He reasoned that since O₂ and Xe have similar ionisation energies, and O₂ + PtF₆ → O₂⁺PtF₆⁻ was known, Xe + PtF₆ should also react. The product Xe⁺PtF₆⁻ was the first noble gas compound ever made. This shattered the belief that noble gases are completely inert. Bartlett subsequently made XeF₂ and XeF₄.
13.3
Occurrence and Isolation of Noble Gases
Natural Occurrence
Atmospheric source: All noble gases except radon are present in the atmosphere. Argon is by far the most abundant (0.93% of air — the third most abundant atmospheric gas after N₂ and O₂). The others are present in trace amounts.
How did noble gases get into the atmosphere?
- Helium — mainly produced by α-decay of radioactive elements in Earth's crust (α particles = He nuclei → capture 2 electrons → He gas). Also primordial He from Earth's formation, but it escapes to space (too light). Natural gas wells in the USA and elsewhere contain significant He (trapped underground).
- Argon — produced by radioactive decay of 40K (potassium-40) in rocks: 40K + e⁻ → 40Ar. This is how most atmospheric Ar was generated over billions of years. Also 38Ar and 36Ar are primordial.
- Radon — produced continuously by radioactive decay of radium (226Ra → 222Rn + α). It seeps from rocks and soil. Radon-222 has a half-life of only 3.82 days — it does not accumulate significantly in the open atmosphere but can build up in enclosed spaces (homes, caves) to dangerous levels.
- Ne, Kr, Xe — primarily primordial, trapped in the atmosphere since Earth's formation.
Industrial Isolation
Noble gases are isolated from air by fractional distillation of liquid air — the same process used for N₂ and O₂. Air is first purified (CO₂ and water removed), then liquefied by cooling under pressure (Linde process). The liquid air is then fractionally distilled.
| Gas | Boiling Point | Isolation |
|---|---|---|
| N₂ | −196°C | Distils off first (lowest b.p.) — collected separately |
| Ar | −186°C | Distils with N₂ fraction — separated by further distillation |
| O₂ | −183°C | Remains as liquid longer — separated from N₂/Ar |
| Kr, Xe | −153, −108°C | Remain in liquid oxygen fraction — concentrated and purified separately |
| Ne, He | −246, −269°C | Very low b.p. — escape before other gases liquefy; concentrated from waste gas stream |
Helium is also isolated from natural gas wells (especially in the USA, Russia, Qatar), where it may constitute 0.3–7% of the gas stream. The natural gas is liquefied, He remains as a gas and is collected.
13.4
Noble Gas Compounds
Xenon Fluorides
The most important noble gas compounds are the xenon fluorides, made by direct reaction of Xe and F₂ under UV light or heating:
Xe + F₂ → XeF₂ (excess Xe, UV light or low pressure)
Xe + 2F₂ → XeF₄ (Xe:F₂ = 1:2 ratio, ~400°C, ~6 atm)
Xe + 3F₂ → XeF₆ (Xe:F₂ = 1:9 ratio, ~300°C, high pressure)
| Compound | Xe electron pairs | Lone pairs on Xe | Shape | Hybridisation |
|---|---|---|---|---|
| XeF₂ | 5 (2 bond + 3 lone) | 3 | Linear | sp³d |
| XeF₄ | 6 (4 bond + 2 lone) | 2 | Square planar | sp³d² |
| XeF₆ | 7 (6 bond + 1 lone) | 1 | Distorted octahedral | sp³d³ |
Xenon Oxides and Other Compounds
XeF₂ + H₂O → Xe + 2HF + ½O₂ (hydrolysis — Xe oxidises water)
2XeF₂ + 2H₂O → 2Xe + 4HF + O₂
XeF₄ + 2H₂O → XeO₂ + 4HF (partial hydrolysis)
6XeF₄ + 12H₂O → 4Xe + 24HF + 2XeO₃ + 3O₂ (disproportionation hydrolysis)
XeF₆ + 3H₂O → XeO₃ + 6HF (vigorous hydrolysis → xenon trioxide)
XeO₃: explosive solid; Xe in +6 state; strong oxidant
Xe + PtF₆ → Xe⁺PtF₆⁻ (Bartlett's 1962 discovery)
Noble gas compounds have been characterised primarily for Xe. They are all powerful oxidising agents and fluorinating agents used in research.
Clathrates — Physical Entrapment of Noble Gases
Noble gases (especially Ar, Kr, Xe) can be physically trapped within crystalline lattices of other compounds without forming true chemical bonds. These are called clathrates or inclusion compounds:
- Xe can form hydrate clathrates: Xe·nH₂O — water molecules form cage structures around Xe atoms (held by van der Waals forces only — no chemical bond)
- Noble gas clathrates form at high pressure and low temperature
- These are not true compounds — the noble gas can be released by warming or reducing pressure
- Used to study noble gas behaviour and in separation processes
13.5
Industrial and Scientific Uses of Noble Gases
| Noble Gas | Use | Reason |
|---|---|---|
| Helium (He) | Cooling superconducting magnets (MRI scanners, particle accelerators — LHC at CERN) | Lowest boiling point (4K) — only liquid that can cool to ~4K at normal pressure |
| Filling balloons and airships (dirigibles) | Very low density (2nd lightest gas), non-flammable — safer than H₂ | |
| Helium-oxygen breathing mixture (heliox) for deep-sea divers | Prevents nitrogen narcosis; does not dissolve in blood as much as N₂ | |
| Inert atmosphere for welding reactive metals (Ti, Al) | Chemically inert — prevents oxidation of hot metal | |
| Neon (Ne) | Neon signs and advertising lights | Emits bright red-orange light when electricity passes through low-pressure Ne gas |
| He-Ne laser (632.8 nm red) | Stimulated emission from excited Ne atoms; He pumps Ne to metastable state | |
| Plasma displays | Electrical discharge in Ne produces visible light | |
| Argon (Ar) | Inert atmosphere in arc welding (MIG/TIG) and metal casting | Most abundant cheap noble gas; prevents metal oxidation during high-T processing |
| Filling incandescent and fluorescent light bulbs | Inert — prevents tungsten filament from oxidising at high temperature; slows evaporation | |
| Inert atmosphere in production of reactive metals (Ti, Zr) and silicon for semiconductors | Chemically inert; cheaper than He or Kr | |
| Argon-40/Argon-39 isotope dating of rocks | ⁴⁰K → ⁴⁰Ar: ratio gives age of rock (K-Ar dating) | |
| Krypton (Kr) | Krypton fluoride (KrF₂) laser (248 nm UV) | High-energy UV laser used in photolithography (making microchips), LASIK eye surgery |
| High-intensity photographic flash lamps (krypton flash) | Intense white flash when current passes through Kr | |
| Xenon (Xe) | Xenon HID headlights and cinema projectors | Produces very bright white light spectrum similar to sunlight when electrically excited |
| Xenon ion thrusters (spacecraft propulsion — Dawn, Hayabusa2) | Xe atoms ionised and accelerated electrostatically → efficient propulsion in space | |
| General anaesthetic (XeF used in medicine) | Xe dissolves in cell membranes, blocking Na⁺ ion channels → anaesthesia; non-toxic, fast recovery | |
| Radon (Rn) | Radiotherapy for cancer (historical) — now rarely used | Alpha emitter — localised radiation treatment; replaced by safer alternatives |
Radon — a natural radiation hazard
Radon-222 (t½ = 3.82 days) is produced by decay of radium in rocks and soil, especially in granite-rich areas. It seeps into buildings through floors and walls. Being a gas, it can accumulate in poorly-ventilated ground-floor rooms. Radon decays to solid radioactive daughters that deposit in the lungs → alpha radiation → lung tissue damage → lung cancer. Radon is the second-leading cause of lung cancer after smoking. Test: radon detector kits; solution: improved ventilation.
Helium Supply Crisis
Helium is irreplaceable for cooling superconducting magnets — no other substance reaches 4 K as a liquid at normal pressure. However:
- Helium released into the atmosphere escapes to space (too light to be held by gravity)
- Earth's helium reserves (trapped underground, mainly in the USA) are finite and non-renewable
- Global helium supply has experienced shortages (2019–2023)
- Major producers: USA (depleting), Qatar, Russia, Algeria
- Conservation: MRI machines should recycle their He; research into He-free superconductors
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Exercises
- Explain why the noble gases are monoatomic and chemically inert. Give three specific reasons referring to their electronic structure.
(1) Completely filled outer electron shells (He: 1s²; others: ns²np⁶) — no tendency to gain or lose electrons; already at stable configuration → no ionic bonds. (2) No unpaired electrons — no half-filled orbitals available for covalent bond formation through electron sharing. (3) Very high ionisation energies (especially He: 2372 kJ/mol) — enormous energy needed to remove any electron → no tendency to form positive ions. (4) Very low electron affinity — adding an electron to a full shell would place it in a new higher-energy shell → unfavourable → no tendency to form negative ions. Noble gases exist as monoatomic gases because there is no energetic driving force to form X–X bonds.
- Explain why krypton and xenon can form compounds with fluorine, but helium and neon cannot.
Kr and Xe: (1) Lower ionisation energies (Kr 1351, Xe 1170 kJ/mol) — electrons can be removed / used in bonding more easily. (2) Larger atoms with accessible nd orbitals (4d for Kr, 5d for Xe) — can expand their octet beyond 8 electrons to accommodate 2, 4, or 6 F atoms. (3) Very strong oxidising agent F₂ (E° = +2.87V) provides sufficient driving force to overcome the energy of ionisation. He and Ne: (1) Very high ionisation energies (2372, 2081 kJ/mol) — virtually impossible to use these electrons in bonding. (2) No accessible low-energy d orbitals (Period 1 and 2 — no d orbitals). (3) Very small atoms — steric factors also prevent multiple bonds. No known stable He or Ne compounds.
- Using VSEPR theory, predict the shapes of XeF₂ and XeF₄. Give the hybridisation and bond angles for each.
XeF₂: Xe has 2 bond pairs (to 2 F) + 3 lone pairs = 5 electron pairs. VSEPR: 5 electron pairs → trigonal bipyramidal electron geometry. The 3 lone pairs go to equatorial positions (least repulsion). 2 F atoms in axial positions → shape = linear. Bond angle = 180°. Hybridisation = sp³d. XeF₄: Xe has 4 bond pairs + 2 lone pairs = 6 electron pairs. VSEPR: 6 pairs → octahedral electron geometry. 2 lone pairs go to opposite axial positions. 4 F atoms in equatorial plane → shape = square planar. Bond angles = 90°. Hybridisation = sp³d².
- Argon is used as an inert atmosphere in welding. Why is it preferred over nitrogen for welding titanium?
Argon is chemically inert — it does not react with hot titanium under any conditions. Nitrogen, although relatively unreactive at room temperature, reacts with titanium at high welding temperatures (>600°C): N₂ + 2Ti → 2TiN (titanium nitride — hard, brittle compound that weakens the weld). Ar (Group 18, full outer shell) forms no compounds at all under normal conditions. Ar is also preferred over He (also inert) because Ar is much cheaper and more abundant (0.93% of air vs 0.0005% for He) and has higher density — better shielding of the weld pool.
- Explain the trend in boiling points of noble gases from He (−269°C) to Xe (−108°C). Include the reason why helium has the lowest boiling point of any substance.
Boiling point increases He → Xe because London dispersion forces (instantaneous dipole–induced dipole) increase with the number of electrons and size of the electron cloud. Larger noble gas atoms are more polarisable — stronger temporary dipoles → stronger attractions between atoms → more energy needed to vaporise → higher boiling point. He has the lowest boiling point of any substance (−269°C = 4K) because: (1) He atoms have only 2 electrons — the smallest, least polarisable electron cloud → weakest London forces. (2) He atoms are so light that their zero-point kinetic energy (from quantum uncertainty) is large enough to prevent solidification even near absolute zero. He remains liquid down to 0K at normal pressure — it must be pressurised to solidify (solidifies at ~25 atm at 0K).
- Describe how radon poses a health risk. Include its source, how it enters buildings, and why it is dangerous.
Source: Rn-222 is produced by radioactive decay of radium-226 (itself a decay product of uranium-238) in granite and certain other rocks and soil. ²²⁶Ra → ²²²Rn + ⁴He (α decay). Entry into buildings: Rn seeps upward from soil and rock through foundations, cracks, and building materials → accumulates in ground-floor rooms and basements, especially in poorly ventilated buildings in granite-rich areas. Danger: Rn itself is a gas and is inhaled. It decays (t½ = 3.82 days) to solid radioactive daughters (Po-218, Pb-214, etc.), which are alpha and beta emitters. These deposit on lung tissue → ionising radiation → DNA damage → lung cancer. Radon is the second most common cause of lung cancer after smoking. Solution: improve ventilation, seal floors, use radon-resistant building materials.
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Multiple Choice Quiz — 25 Questions
Unit 13: Group 18 Noble Gases
25 QuestionsQ1
The outer electron configuration of all noble gases (except He) is:
Noble gases (Ne, Ar, Kr, Xe, Rn): ns²np⁶ — a full octet. He is ns² (1s²) — a full duet (only n=1 shell). The completely filled outer shells mean no tendency to form bonds — chemically inert.
Q2
Which noble gas is the most abundant in the atmosphere?
Argon (Ar) at 0.93% is by far the most abundant noble gas in the atmosphere — third most abundant gas after N₂ (78%) and O₂ (21%). It accumulated over geological time from radioactive decay of ⁴⁰K in Earth's rocks. He, Ne, Kr, Xe are all present in much smaller trace amounts.
Q3
Why do the boiling points of noble gases increase from He to Xe?
Noble gases are non-polar monoatomic — only London (dispersion) forces act between atoms. Going down: larger atoms with more electrons → greater polarisability → stronger instantaneous dipoles → stronger London forces → more energy needed to overcome these forces → higher boiling points. He (2e⁻, −269°C) → Xe (54e⁻, −108°C).
Q4
The first noble gas compound was made by Neil Bartlett in 1962. What was it?
Bartlett reacted Xe with PtF₆ → Xe⁺PtF₆⁻. He realised that Xe (IE = 1170 kJ/mol) and O₂ (IE = 1165 kJ/mol) have similar ionisation energies, and since O₂ + PtF₆ → O₂⁺PtF₆⁻ was known, Xe should do the same. This shattered 100 years of belief in complete noble gas inertness. He then made XeF₂ and XeF₄ shortly after.
Q5
The shape of XeF₂ is:
XeF₂: Xe has 2 bond pairs + 3 lone pairs = 5 electron pairs. Trigonal bipyramidal electron geometry. The 3 lone pairs occupy equatorial positions (120° apart, minimise repulsion). 2 F atoms are in axial positions (180° apart). Molecular shape = linear. Bond angle = 180°. This is an example of octet expansion using 3d orbitals of Xe (sp³d hybridisation).
Q6
Helium is used to fill weather balloons rather than hydrogen because:
He is chemically inert and non-flammable — completely safe. H₂ is flammable and forms explosive mixtures with air (H₂/O₂ = explosive, as demonstrated by the Hindenburg disaster in 1937). He is slightly denser than H₂ (4 vs 2 g/mol) so provides slightly less lift, but the safety advantage is overwhelming. He is more expensive but safety takes priority.
Q7
Why can xenon form compounds but neon cannot, even though both have full outer shells?
Two key factors: (1) IE: Xe (1170 kJ/mol) is much lower than Ne (2081 kJ/mol) — Xe's outer electrons are further from nucleus, less tightly held → can be used in bonding with very electronegative F₂. (2) Orbitals: Xe (Period 5) has accessible 5d orbitals for bond expansion → can accommodate 2, 4, or 6 F atoms. Ne (Period 2) has no d orbitals → maximum 4 bonds impossible → Ne cannot form XeF₂ analogue.
Q8
Argon is used as an inert atmosphere in light bulbs. What does this prevent?
Tungsten filament reaches ~2500°C in operation. In vacuum, W evaporates quickly (blackening bulb). In pure N₂, W eventually reacts. Ar (inert) prevents: (1) oxidation (2W + O₂ → 2WO₃) and (2) reduces W evaporation rate (collisions with Ar atoms return some W atoms to filament). This extends filament life. Modern LED bulbs have largely replaced this technology.
Q9
Radon (Rn-222) is considered a health hazard in buildings because:
Rn-222 (α emitter, t½ = 3.82 days) seeps from granite/rock into buildings. Inhaled Rn decays to solid Po-218 and Pb-214 etc. (also α/β emitters) which deposit on lung tissue. Ionising radiation damages lung cell DNA → mutations → lung cancer. 2nd leading cause of lung cancer. The gas itself is relatively harmless — the issue is its solid radioactive daughter products depositing in lungs.
Q10
Helium has the lowest boiling point of any substance (−269°C). This is because:
He has 2 electrons in 1s² — the smallest, lightest atom with the weakest London forces of any substance (just 2 electrons to polarise). The interatomic attractions are so weak that only at 4 K (−269°C) do atoms come close enough to condense. He remains liquid to 0 K at normal pressure — quantum zero-point energy keeps atoms in motion even at absolute zero. Must be pressurised to ~25 atm to solidify. Unique among all elements.
Q11
How is atmospheric argon produced naturally?
Most atmospheric ⁴⁰Ar was produced over geological time by electron capture decay of ⁴⁰K: ⁴⁰K + e⁻ → ⁴⁰Ar (t½ = 1.25×10⁹ years). The ⁴⁰Ar released from rocks accumulated in the atmosphere (too heavy to escape to space). This is why Ar makes up 0.93% of air — far more than expected for a primordial noble gas. ⁴⁰K/⁴⁰Ar ratio is also used in K-Ar radiometric dating of rocks.
Q12
The shape of XeF₄ (4 bond pairs + 2 lone pairs on Xe) is:
XeF₄: 4 bonding pairs + 2 lone pairs = 6 electron pairs → octahedral electron geometry. The 2 lone pairs go to opposite positions (axial) to minimise lone pair–lone pair repulsion (180° apart). The 4 F atoms occupy the equatorial plane → square planar molecular shape, 90° F–Xe–F angles. Hybridisation: sp³d².
Q13
Neon signs produce a characteristic red-orange glow because:
Electrical discharge ionises/excites Ne atoms → electrons promoted to higher energy levels (Ne* excited states). When electrons fall back to lower energy levels, they emit photons. Ne's characteristic emission lines are in the red-orange region of the visible spectrum → red-orange glow. Different gases give different colours: He (yellow/white), Ar (blue/violet), Kr (green/yellow), Xe (blue-white).
Q14
Helium is used in MRI scanners to cool superconducting magnets. Why is helium uniquely suited?
Superconducting magnets (used in MRI, LHC, NMR spectrometers) must be cooled below their critical temperature (~4–10 K for NbTi alloys). Only liquid helium (b.p. 4.2 K) can achieve this. No other substance exists as a liquid at these temperatures. At 4 K, electrical resistance of the superconductor drops to zero → persistent current → powerful magnetic field with zero energy loss. Without He, MRI scanners would require enormous electrical power to maintain the magnetic field.
Q15
Noble gases are isolated industrially from air by:
Noble gases are isolated by fractional distillation of liquid air (Linde process). Air is purified (CO₂ and H₂O removed), compressed, cooled until it liquefies, then fractionally distilled. Different gases boil off at different temperatures: N₂ (−196°C), Ar (−186°C), O₂ (−183°C), Kr (−153°C), Xe (−108°C). He and Ne (very low b.p.) are collected from the non-condensable fraction.
Q16
Xenon ion thrusters are used in spacecraft because:
Xe ion thrusters: Xe atoms are ionised (Xe → Xe⁺ + e⁻) then accelerated through an electric field → high-velocity Xe⁺ ions expelled → thrust (Newton's 3rd law). Advantages: very high specific impulse (fuel efficiency) — ~10× better than chemical rockets. Drawback: very low thrust (milli-newtons). Ideal for long missions where gradual acceleration is acceptable (Dawn spacecraft to Vesta and Ceres; Hayabusa2 to asteroid). Xe chosen because it is heavy (large mass/atom), easily ionised, inert, and storable as liquid.
Q17
Helium is produced naturally inside Earth primarily by:
Alpha particles (⁴He nuclei) emitted by U-238, U-235, Th-232 decay chains are slowed by rock, capture 2 electrons → become neutral He atoms trapped underground. Over millions of years, He accumulates in natural gas reservoirs (especially in porous sandstone above impermeable salt domes). USA (Kansas, Texas, Oklahoma, Wyoming) has the world's largest He reserves. He that reaches the atmosphere is so light it escapes to space → essentially non-renewable resource.
Q18
Which xenon fluoride, when hydrolysed, produces the explosive solid XeO₃?
XeF₆ + 3H₂O → XeO₃ + 6HF. XeO₃ is an explosive white solid (Xe in +6 oxidation state, powerful oxidant). It must be handled with extreme care. XeF₄ hydrolysis also produces some XeO₃ via disproportionation: 6XeF₄ + 12H₂O → 4Xe + 2XeO₃ + 24HF + 3O₂. XeF₂ hydrolysis gives Xe gas: XeF₂ + H₂O → Xe + 2HF + ½O₂.
Q19
The ionisation energy trend across Group 18 is:
IE decreases He (2372) > Ne (2081) > Ar (1521) > Kr (1351) > Xe (1170) kJ/mol — going down, outer electrons are in higher n shells, further from nucleus, better shielded → easier to remove. He has the highest IE of any element. Despite having full shells, IEs still decrease going down — outer electrons in larger shells experience less effective nuclear charge due to increased shielding.
Q20
What is a clathrate (inclusion compound) in the context of noble gases?
Clathrates: noble gas atoms are physically enclosed in cage-like structures (water ice clathrates, for example) by van der Waals forces — no chemical bonds formed. Xe·nH₂O: water forms hydrogen-bonded cages around Xe atoms. These are NOT true compounds — the noble gas can be released by warming. Clathrates form under high pressure and low temperature and are of interest in studying gas storage and noble gas behaviour.
Q21
Xenon can be used as a general anaesthetic because:
Xe is soluble in non-polar lipid cell membranes (large, polarisable atom — significant van der Waals interactions with lipids). In the membrane it physically blocks ion channels (particularly NMDA and AMPA receptors, Na⁺ channels) → inhibits nerve impulse transmission → anaesthesia. Properties: rapid onset and recovery, non-toxic, no metabolic breakdown required, mild cardiovascular effects. Disadvantage: expensive (scarce and costly to produce from air).
Q22
Why is helium use in industry described as "non-renewable"?
He is the second lightest element. At thermosphere temperatures, some He atoms exceed escape velocity and are permanently lost to space. Underground He (from α-decay of U/Th) has been accumulating for billions of years in geological traps — once mined and used, He released to atmosphere gradually escapes. We cannot economically recapture atmospheric He. The underground reserves are finite and depleting. This makes He a genuinely non-renewable resource — unlike most other gases that cycle through the atmosphere.
Q23
He-Ne lasers produce a red beam (632.8 nm). The role of helium in a He-Ne laser is to:
He-Ne laser mechanism: Electrical discharge excites He to metastable excited states (He*). He* collides with ground-state Ne → energy transfer → Ne excited to specific laser levels. Stimulated emission of Ne produces coherent 632.8 nm (red) photons. He acts as the "pump" that efficiently excites Ne. The He metastable level matches the Ne laser level — near-resonant energy transfer → very efficient. He is not the light-emitter; Ne is.
Q24
Which statement about noble gas compounds is correct?
He and Ne: essentially no stable compounds (Ne compounds exist only in matrix isolation at very low T). Ar: ArF has been detected but no stable Ar compound at room T. Kr: KrF₂ is stable (white solid), KrF⁺ known. Xe: rich chemistry — XeF₂ (linear, white solid), XeF₄ (square planar, colourless), XeF₆ (distorted octahedral), XeO₃ (explosive solid), XeOF₄, XePtF₆, etc. Rn: theoretically similar to Xe but radioactivity makes study difficult. All noble gas compounds are strong oxidising agents.
Q25
The KrF₂ laser (248 nm UV) is used in LASIK eye surgery because:
KrF excimer laser (248 nm, deep UV): each photon has energy ~5 eV — enough to break C–C and C–N bonds in corneal tissue directly (photoablation). The ablation is very precise (tissue removal per pulse is nanometres deep) with minimal thermal damage to surrounding tissue. Computer-guided pulses reshape the cornea to correct refractive errors (myopia, hyperopia, astigmatism). The KrF laser is also used in photolithography (making microchip circuits). Excimer = "excited dimer" — KrF forms only in the excited state, not the ground state.
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Unit Test — 50 Marks
Section A — Short Answer
30 marksQ1 [5 marks]
Explain why noble gases are chemically inert and exist as monoatomic gases. Why can xenon form compounds but helium cannot? [5]
Noble gas inertness: (1) Completely filled outer shells (1s² for He; ns²np⁶ for others) — no tendency to gain/lose electrons or form bonds. (2) No unpaired electrons → no covalent bonds possible. (3) Very high IEs (He 2372 kJ/mol) → very stable electron configuration. Monoatomic: no driving force to form X–X bonds; already at minimum energy configuration as single atoms. [2] He cannot form compounds: IE = 2372 kJ/mol (highest of any element); no accessible d orbitals (Period 1); 1s² shell is the smallest possible — extremely stable. [1.5] Xe can form compounds: IE only 1170 kJ/mol (much lower — outer 5p electrons less tightly held); has accessible 5d orbitals → can expand octet; large atomic radius allows multiple fluorine ligands; very strong oxidant F₂ provides sufficient driving force. [1.5]
Q2 [5 marks]
Using VSEPR theory, predict the shapes of XeF₂ and XeF₄. For each: draw the electron pair geometry, state the number of lone pairs on Xe, give the molecular shape, bond angle, and hybridisation. [5]
XeF₂: Total electron pairs on Xe = 2(bond) + 3(lone) = 5 → trigonal bipyramidal electron geometry. Lone pairs occupy equatorial positions (less repulsion at 120°). F atoms in axial positions. Molecular shape: linear (F–Xe–F = 180°). Hybridisation: sp³d. [2.5] XeF₄: Total electron pairs = 4(bond) + 2(lone) = 6 → octahedral electron geometry. Lone pairs in opposing axial positions (180° apart — minimise lp–lp repulsion). 4 F in equatorial plane. Molecular shape: square planar (F–Xe–F = 90°). Hybridisation: sp³d². [2.5]
Q3 [5 marks]
Describe the industrial isolation of noble gases from air. Which noble gas is most abundant in air, and why? How is helium isolated from a different source? [5]
Isolation from air: fractional distillation of liquid air. Air cleaned (CO₂/H₂O removed), compressed, cooled until it liquefies. Fractional distillation separates components: N₂ (−196°C) boils off first, then Ar (−186°C) with N₂ fraction, then O₂ (−183°C). Kr and Xe (−153, −108°C) remain in liquid O₂ fraction — extracted separately. Ne and He (very low b.p.) remain as gas and are collected separately. [3] Most abundant: Argon (0.93% of air). Reason: ⁴⁰Ar has accumulated over billions of years from radioactive decay of ⁴⁰K in Earth's rocks (⁴⁰K + e⁻ → ⁴⁰Ar, t½ = 1.25×10⁹ yr). [1] He from natural gas: He (from α-decay of U/Th trapped underground) makes up 0.3–7% of some natural gas wells. The natural gas is cooled to liquefy it — He (too low b.p. to liquefy at these conditions) remains as a gas and is collected. [1]
Q4 [5 marks]
Neil Bartlett's 1962 synthesis of the first noble gas compound was based on a key piece of chemical reasoning. Explain this reasoning, write the equation for his compound, and describe how he subsequently made XeF₂ and XeF₄. [5]
Bartlett's reasoning: He knew that O₂ + PtF₆ → O₂⁺PtF₆⁻ (oxygen molecule is oxidised). He noted that IE₁ of Xe (1170 kJ/mol) is almost identical to IE of O₂ (1165 kJ/mol). If O₂ can be oxidised by PtF₆, so should Xe. [2] Equation: Xe + PtF₆ → Xe⁺PtF₆⁻ (xenon hexafluoroplatinate — orange-yellow solid, forms when Xe and PtF₆ vapours mix at room temperature). [1] XeF₂ and XeF₄: reacting mixtures of Xe and F₂ under different conditions: Xe + F₂ (excess Xe, UV light) → XeF₂; Xe + 2F₂ (Xe:F₂ = 1:2, 400°C, ~6 atm) → XeF₄. Significance: ended 50+ years of belief that noble gases are completely inert — prompted research into noble gas chemistry. [2]
Q5 [5 marks]
Describe the uses of He, Ne, Ar, and Xe in technology and science. Give one specific use for each, explain the chemical/physical property exploited, and explain why no other substance could substitute for He in cooling superconducting magnets. [5]
He: cooling superconducting MRI magnets. Property: lowest b.p. (4.2 K at 1 atm) — liquid He cools superconducting NbTi coils below their critical temperature (~9 K) → zero electrical resistance → powerful persistent magnetic field. Irreplaceable: no other substance is liquid at 4 K — N₂ (b.p. 77 K), H₂ (b.p. 20 K) both too high and do not reach the required temperature. [1.5] Ne: neon signs. Property: electrical discharge excites Ne electrons → emit red-orange light (characteristic emission spectrum). [1] Ar: MIG/TIG welding inert atmosphere. Property: chemically inert even at welding temperatures (~1500°C), cheap (0.93% of air), higher density than He → better shielding of weld pool. [1] Xe: spacecraft ion thrusters (e.g. Dawn mission). Property: Xe atoms ionised and electrostatically accelerated → efficient low-thrust propulsion; Xe chosen for high mass (heavy atom → more momentum per particle), ease of ionisation (low IE 1170 kJ/mol), and ability to be stored as liquid. [1] He irreplaceable: only liquid at 4 K; NbTi and Nb₃Sn superconductors critical temperature ~9 K; liquid N₂ (77 K) is far too warm to cool them below critical temperature. No alternative cooling agent exists at these temperatures under normal pressure. Research into higher-temperature superconductors ongoing. [0.5]
Q6 [5 marks]
Explain the radon health hazard fully: how it forms, why it accumulates indoors, how it damages health, and what steps can reduce exposure. Write the nuclear equation for its formation. [5]
Formation: ²²⁶Ra → ²²²Rn + ⁴He (alpha decay; t½ of Ra-226 = 1600 yr; Rn-222 t½ = 3.82 days). Ra is itself a decay product of U-238 in uranium-containing rocks (granite, some soils). [1] Accumulation: Rn gas seeps upward through soil and rock; enters buildings through cracks in foundations, porous concrete, gaps around pipes. Being dense (9.23 g/L) and a gas, accumulates especially in basements and ground floors in poorly ventilated buildings in radon-prone areas (granite regions). [1] Health damage: Rn itself is inhaled as inert gas. Its solid decay daughters (Po-218, Pb-214, Bi-214, Po-214) are alpha and beta emitters and attach to airway surfaces. Alpha particles ionise lung tissue DNA → mutations → lung cancer (2nd cause after smoking; causes ~20,000 deaths/year in USA). [2] Reduction: improve ventilation (open windows; install sub-slab depressurisation); seal cracks in floors/foundations; use radon-resistant building materials; test regularly with radon detector kits. [1]
Section B — Extended Answer
20 marksQ7 [10 marks]
(a) Compare the noble gases with the halogens (Group 17) in terms of: electronic structure, reactivity, physical state at room temperature, and ability to form compounds. [5]
(b) Describe the trend in ionisation energies across Period 2 (Li to Ne). Explain the anomalies at B and O. Explain why Ne has the highest IE in Period 2 and how this connects to its complete chemical inertness. [5]
(a) Electronic structure: Noble gases — ns²np⁶ (full outer shell, 8 electrons, or 2 for He). Halogens — ns²np⁵ (7 valence electrons, 1 electron short of full shell). [1] Reactivity: Noble gases — essentially inert (only heaviest form compounds with F₂). Halogens — very reactive; all form compounds readily; powerful oxidising agents. [1] Physical state: Noble gases — all monoatomic gases at r.t. (no bonding between atoms). Halogens — F₂ and Cl₂ are gases; Br₂ is liquid; I₂ is solid (all diatomic molecules X₂ held by London forces which increase down group). [1.5] Compounds: Noble gases — only Kr (KrF₂) and Xe (XeF₂, XeF₄, etc.) with F₂ under forcing conditions; no ionic compounds. Halogens — form ionic halides with metals (Na⁺Cl⁻ etc.) and covalent compounds with non-metals; full range of oxidation states (Cl: −1 to +7); very large variety of compounds. The difference is explained by the single electron vacancy in halogens vs the completed shell in noble gases. [1.5] (b) Period 2 trend: general increase Li → Ne as nuclear charge increases with same n=2 shell → increasing Zeff → electrons held more tightly → higher IE. [1] Anomaly at B (IE lower than Be): Be = 2s², B = 2s²2p¹. B's 2p¹ electron is in a higher-energy subshell than Be's 2s² and is better shielded by the 2s² → easier to remove → lower IE. [1] Anomaly at O (IE lower than N): N = 2s²2p³ (half-filled 2p, each electron in its own orbital, extra stability from exchange energy). O = 2s²2p⁴ (one paired electron in 2p — extra electron repulsion → easier to remove → lower IE than expected). [1] Ne: highest IE in Period 2 (2081 kJ/mol): Z=10, full 1s²2s²2p⁶ — maximum Zeff with full shielding from 1s² and 2s² → outer 2p electrons experience the highest effective nuclear charge of any Period 2 element. Complete chemical inertness: removing any electron from Ne requires >2000 kJ/mol — energetically impossible under normal chemical conditions; no standard chemical oxidant can do this. Similarly, adding an electron goes into n=3 (much higher energy) — no thermodynamic driving force. Net result: Ne has absolutely no chemistry. [2]
Q8 [10 marks]
(a) Argon constitutes 0.93% of the atmosphere, making it far more abundant than the other noble gases. Explain why argon is so abundant. Compare this with why helium is so scarce in the atmosphere. [4]
(b) Describe the strategic and economic importance of helium in modern technology. Explain why some scientists warn of a "helium crisis" and suggest what measures could be taken to conserve supplies. [6]
(a) Argon abundance: ⁴⁰K (t½ = 1.25×10⁹ yr) has been decaying by electron capture to ⁴⁰Ar continuously throughout Earth's history. K is a very common element in rocks. The ⁴⁰Ar produced is trapped in the atmosphere (too heavy to escape — molecular mass 40 g/mol) and has accumulated for ~4.5 billion years. Also ³⁶Ar and ³⁸Ar from primordial nucleosynthesis. Total = 0.93% of atmosphere. [2] Helium scarcity in atmosphere: He (mass 4) is very light. Some He atoms at the top of the atmosphere (thermosphere) have enough thermal velocity to exceed escape velocity (~11 km/s) → permanently lost to space. He produced underground by α-decay accumulates in geological traps but does not reach the atmosphere in significant amounts. He present in atmosphere (0.0005%) is the balance between production (α-decay) and loss (space escape). [2] (b) Strategic importance: (1) MRI machines (hospitals worldwide — >40,000 MRI scanners globally, each uses ~1,700 L liquid He). (2) Scientific research: NMR spectrometers in chemistry/biochemistry labs; particle accelerators (LHC at CERN uses 120 tonnes of liquid He to cool 27 km of superconducting magnets). (3) Space programme: NASA uses He to pressurise rocket fuel tanks and purge rocket engines. (4) Semiconductor manufacturing: Ar/He atmosphere for Si crystal growth. (5) Fibre optic cable manufacturing. [3] Helium crisis: (1) Finite reserves — USA has largest proven reserves but has been selling from strategic reserve (US Federal Helium Reserve, Texas) since 1996; reserves declining. (2) Non-renewable — once in atmosphere, escapes to space over geological time; cannot be economically recovered. (3) No substitute for cooling below ~10 K. (4) Shortage events: multiple global shortages (2011–2013, 2019–2020, 2022) disrupted MRI, research, and industry. (5) Major producers: USA (depleting), Qatar (now world's 2nd largest), Russia, Algeria. [2] Conservation measures: (1) Recycle He from MRI scanners (install reliquefaction units — most modern MRI machines do this now). (2) Use He-free alternatives where possible — replace He-cooled NMR with higher-T superconductors or permanent magnets for some applications. (3) Develop high-temperature superconductors (YBCO, etc. that work at 77 K — cooled with cheap liquid N₂). (4) International treaty on He as a strategic resource — regulate export and pricing. (5) Increase He recovery from natural gas (currently much is vented). (6) Research into He substitutes for specific applications (e.g. turbine cooling). [1]