Group 18 contains the noble gases: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). Except He (1s2), all have the stable closed-shell configuration ns2np6, which gives them their famous chemical inertness.
Occurrence and general trends
Noble gases are present in trace amounts in the atmosphere; argon is the most abundant (~0.9% of air). Helium is found in natural gas (from radioactive α-decay) and the universe at large. They are obtained by the fractional distillation of liquid air (He by separation from natural gas). All are monatomic, colourless, odourless gases with very low boiling points. Down the group, atomic radius and boiling point increase (stronger van der Waals forces) while ionisation enthalpy decreases — the falling ionisation enthalpy of the heavier members is what finally allows xenon to react.
Reactivity and xenon compounds
For a long time noble gases were thought to be completely inert. In 1962 Bartlett prepared the first compound, and only xenon (and to a lesser extent Kr and Rn) forms true compounds, because its large size gives a low enough ionisation enthalpy. Xenon reacts directly with the most electronegative element, fluorine, under different conditions:
- XeF2: Xe + F2 → XeF2 (excess Xe, ~673 K, 1 bar).
- XeF4: Xe + 2F2 → XeF4 (1:5 ratio, ~873 K, ~7 bar).
- XeF6: Xe + 3F2 → XeF6 (excess F2, ~573 K, ~60–70 bar).
Xenon also forms oxides and oxofluorides by hydrolysis: XeF6 + 3H2O → XeO3 + 6HF, and partial hydrolysis gives XeOF4. XeO3 is an explosive solid and XeF6 is a strong fluorinating agent.
Structures (VSEPR)
The shapes follow VSEPR by counting lone pairs on Xe: XeF2 has 3 lone pairs and 2 bond pairs (sp3d) → linear; XeF4 has 2 lone pairs and 4 bond pairs (sp3d2) → square planar; XeF6 has 1 lone pair and 6 bond pairs (sp3d3) → distorted octahedral. XeO3 is pyramidal (one lone pair, sp3).
Uses
Helium fills weather balloons and provides an inert atmosphere for welding and a coolant for superconducting magnets (very low boiling point); a He–O2 mixture is used by deep-sea divers. Neon gives the red glow of discharge tubes and advertising signs. Argon provides an inert filling for electric bulbs and an inert atmosphere for arc welding of reactive metals. Krypton and xenon are used in high-efficiency and photographic flash lamps. Radon was once used in radiotherapy.
Why are the noble gases chemically inert, and why is xenon the most reactive among them?
Solution- Noble gases (except He) have a completely filled ns2np6 octet, which is a very stable arrangement.
- Because the shell is full, they have very high ionisation enthalpies and almost zero electron-gain enthalpy, so they neither lose nor gain electrons easily.
- Down the group ionisation enthalpy decreases as atomic size increases.
- Xenon, being large, has a low enough ionisation enthalpy to be oxidised by the most electronegative element, fluorine, so it is the most reactive noble gas.
Answer: A stable filled octet and high ionisation enthalpy make noble gases inert; large Xe has a low ionisation enthalpy, so it alone reacts with F2.
Predict the shape and hybridisation of XeF4 using VSEPR theory.
Solution- Xe has 8 valence electrons; 4 are shared with F atoms in 4 bonds, leaving 4 electrons as 2 lone pairs.
- Total electron pairs = 4 bond pairs + 2 lone pairs = 6, giving sp3d2 hybridisation.
- The 6 pairs adopt an octahedral arrangement; the 2 lone pairs occupy opposite (trans) axial positions to minimise repulsion.
- The 4 F atoms then lie in one plane, giving a square planar shape.
Answer: XeF4 is sp3d2 hybridised and square planar (2 lone pairs trans, 4 Xe–F bonds coplanar).
Write the equations for the complete and partial hydrolysis of XeF6.
Solution- Complete hydrolysis gives xenon trioxide: XeF6 + 3H2O → XeO3 + 6HF.
- XeO3 is a colourless, explosive solid.
- Partial (limited) hydrolysis replaces only some fluorine atoms: XeF6 + H2O → XeOF4 + 2HF.
- Further partial hydrolysis: XeF6 + 2H2O → XeO2F2 + 4HF.
Answer: Complete: XeF6 + 3H2O → XeO3 + 6HF; partial: XeF6 + H2O → XeOF4 + 2HF.
Why is XeF2 linear whereas XeF4 is square planar, despite both being xenon fluorides?
Solution- XeF2: Xe forms 2 bonds and keeps 3 lone pairs, so there are 5 electron pairs (sp3d).
- The 3 lone pairs occupy the equatorial plane of a trigonal bipyramid, forcing the 2 F atoms axial → linear.
- XeF4: Xe forms 4 bonds and keeps 2 lone pairs, so there are 6 electron pairs (sp3d2).
- The 2 lone pairs go trans-axial in an octahedron, leaving the 4 F atoms coplanar → square planar.
Answer: The different number of lone pairs (3 in XeF2, 2 in XeF4) places them so that XeF2 is linear and XeF4 is square planar.
State two uses each of helium and argon, linking each use to the relevant property.
Solution- Helium has very low density and is non-flammable, so it is used to fill weather balloons and airships safely (unlike hydrogen).
- Helium has an extremely low boiling point, so liquid He is used as a cryogenic coolant for superconducting magnets (e.g. in MRI).
- Argon is chemically inert, so it provides an inert filling for electric light bulbs, preventing oxidation of the filament.
- Argon also gives an inert atmosphere for the arc welding of reactive metals such as stainless steel and titanium.
Answer: He — balloons (low density, non-flammable) and cryogenic coolant (very low b.p.); Ar — bulb filling and welding atmosphere (chemical inertness).
Helium is found in natural gas though it is a noble gas. Explain its origin and why it is not obtained from liquid air like the other noble gases.
Solution- Helium is continuously produced underground by the radioactive α-decay of heavy elements such as uranium and thorium, since an α-particle is a He nucleus.
- This helium becomes trapped in porous rocks along with natural gas, so natural gas can contain a few per cent helium.
- Liquid air contains only a tiny fraction of helium and helium has so low a boiling point that it would not condense at the temperatures used for fractional distillation of air.
- It is therefore far more economical to recover helium from helium-rich natural gas than from liquid air.
Answer: Helium comes from radioactive α-decay trapped in natural gas; its very low boiling point and low atmospheric abundance make natural gas the practical source rather than liquid air.