3.2.11 · D5p-Block
Question bank — Group 18 (Noble gases) — discovery, isolation, compounds of Xe (XeF₂, XeF₄, XeF₆, XeO₃) — structure and bonding
True or false — justify
They were called "inert gases," so noble gases can never form any compound.
False. "Inert" was a historical guess based on full shells; it was renamed "noble" because Xe forms XeF₂, XeF₄, XeF₆ and XeO₃ — a full shell means a high energy cost to react, not an infinite one.
Helium was discovered on Earth before it was found anywhere else.
False. It was first seen as a yellow line in the Sun's spectrum (1868, hence helios = sun) and only later found on Earth in uranium minerals that emit α-particles (He nuclei).
XeF₂ is linear because it has only two bonds, just like CO₂.
False in cause, true in shape. XeF₂ is linear because it has 3 equatorial lone pairs in a trigonal-bipyramidal frame forcing the two F's axial; CO₂ is linear with zero lone pairs. Same shape, opposite reasons.
XeF₄ should be tetrahedral because it has four Xe–F bonds.
False. Four bonds plus two lone pairs give six electron domains → octahedral base; the two lone pairs sit trans (opposite), leaving the four F's in a square plane.
XeF₆ is a perfect octahedron like SF₆.
False. SF₆ has no lone pair; XeF₆ has one stereochemically active lone pair (7 domains) that pushes the bonds, giving a distorted octahedron.
XeO₃ has the same shape as XeF₄ because both have Xe surrounded by three-or-four atoms.
False. XeO₃ has 3 Xe=O bonds + 1 lone pair = 4 domains → trigonal pyramidal (like NH₃, sp³); XeF₄ has 6 domains → square planar. Different domain counts, different geometries.
Argon reacts with fluorine about as readily as xenon does.
False. Ar sits higher in the group with a much higher ionization enthalpy, so its electrons are held too tightly; only Xe (and marginally Kr) has a low-enough IE for F to extract electrons.
Radon is the most reactive and most studied noble gas because it is largest.
False. Radon is largest and lowest in IE, but it is intensely radioactive with short-lived isotopes, so its chemistry is hard to study — Xe remains the workhorse.
All noble gases exist as diatomic molecules like N₂ and O₂.
False. They are monatomic — a full valence shell means no unpaired electrons to pair up, so single atoms drift apart, held only weakly by London Dispersion Forces.
Krypton, being higher than Xe, forms no fluorides at all.
False. Kr sits right at the borderline: it forms KrF₂ (linear, like XeF₂) under forcing conditions, but its higher IE means it stops there — no KrF₄ or KrF₆ analogues of the xenon series.
Spot the error
"Xe reacts with F₂ because fluorine gives electrons to xenon."
Error: direction is reversed. Fluorine is the oxidiser; it pulls Xe's loosely held electrons toward itself, giving Xe a partial positive character in polar Xe–F bonds.
"To make XeF₆ instead of XeF₂, just raise the temperature."
Error: the controlling variable is the F₂ : Xe ratio and pressure, not temperature alone. XeF₆ needs a large F₂ excess (≈1:20) at 60–70 bar to force six F onto Xe.
"XeF₄'s two lone pairs sit next to each other (cis) to stay out of the way of the bonds."
Error: two lone pairs minimise lone-pair–lone-pair repulsion by going trans — 180° apart, the maximum possible separation in an octahedron (recall the pecking order: lp–lp repulsion is the strongest and gets the most room).
"The lone pairs in XeF₂ go axial in the trigonal bipyramid."
Error: lone pairs need the roomiest sites. In a TBP the equatorial positions have more angular space (120° neighbours vs 90° for axial), so the three lone pairs go equatorial.
"Complete hydrolysis of XeF₆ gives XeOF₄, a mild liquid."
Error: complete hydrolysis gives XeO₃, an explosive solid. XeOF₄ is the product of partial hydrolysis (one water molecule).
"Bartlett tried Xe with PtF₆ because Xe and O₂ are similar in size."
Error: the matching quantity was ionization enthalpy (Xe ≈ 1170, O₂ ≈ 1175 kJ/mol), not size. Since PtF₆ had already stripped an electron from O₂, it should manage Xe too.
"Noble gases were discovered by reacting air with metals to see what was left."
Error: they were found as the unreactive residue after all known components (N₂, O₂, CO₂, H₂O) were removed — argon appeared because atmospheric "N₂" was denser than chemically made N₂.
"KrF₂ must be bent, since it has lone pairs just like water."
Error: KrF₂ has 2 bonds + 3 lone pairs = 5 domains (TBP base); the 3 lone pairs go equatorial and force the F's axial → linear, exactly like XeF₂ — not bent like water (which is only 4 domains).
Why questions
Why can F and O — but not, say, Cl or S — force xenon into compounds?
F and O are the most electronegative, electron-hungry partners; only they exert enough pull to overcome the energy cost of prying electrons from Xe's full shell.
Why does adding one lone pair turn a clean octahedron (XeF₆-would-be) into a distorted shape?
The lone pair is stereochemically active — it occupies a real volume and repels the six bond pairs unevenly (lp–bp repulsion > bp–bp), warping the bond angles away from the ideal 90°/180°.
Why is helium obtained mainly from natural gas rather than from air?
Underground, radioactive decay of heavy elements has produced and trapped helium over geological time, so natural gas is far richer in He than the atmosphere is.
Why do the noble gases have such low boiling points that fractional distillation can separate them?
Being monatomic with full shells, they attract each other only through weak London Dispersion Forces; little energy is needed to pull the atoms apart, giving very low, well-separated boiling points (see Fractional Distillation of Liquid Air).
Why does XeF₆ need sp³d³ hybridisation while XeF₄ needs only sp³d²?
XeF₆ has 7 electron domains (6 bonds + 1 lone pair) requiring 7 hybrid arms (sp³d³, so 3 d orbitals mixed in); XeF₄ has 6 domains, satisfied by 6 arms (sp³d², 2 d orbitals). Xe can supply those d orbitals because period 5 has accessible empty 5d levels.
Why is XeO₃ drawn with Xe=O double bonds rather than single bonds like XeF₂?
Oxygen forms two bonds to complete its octet, allowing π-overlap with Xe; this Xe=O double bonding is characteristic of the oxides and oxyfluorides (see Interhalogen and Oxyfluoride compounds).
Why is a period-2 atom like nitrogen unable to form an "NF₆" the way Xe forms XeF₆?
Nitrogen's valence shell (n = 2) has only s and p orbitals — no d exists there to hybridise — so it cannot expand past 8 electrons; Xe's period-5 shell has empty 5d orbitals to borrow.
Edge cases
Does Xe react directly with oxygen gas to form XeO₃?
No. XeO₃ is made indirectly by hydrolysing XeF₆; Xe and O₂ do not combine directly because O₂ cannot extract Xe's electrons the way F₂ can.
Is there a noble gas with a higher ionization enthalpy than any other element?
Yes — helium. Its two electrons sit in a tiny, tightly bound 1s shell right next to the nucleus, giving it the highest ionization enthalpy of all elements (see Ionization Enthalpy trends).
If XeF₆ had no lone pair, what shape would it take?
A perfect octahedron (like SF₆). The single lone pair is precisely what distinguishes real XeF₆ from that idealised octahedron.
What happens to reactivity as you move down Group 18 from He to Rn?
It rises: atomic size increases, IE falls, so outer electrons become easier to remove — hence Xe reacts, Kr barely (only KrF₂), and He/Ne/Ar essentially not at all (see p-Block general trends).
Would you expect a stable ArF₄ under ordinary conditions?
No. Argon's ionization enthalpy is far too high for fluorine to extract its electrons, so no ordinary Ar–F compound analogous to XeF₄ is expected.
Is the shape of XeOF₄ the same as XeF₄?
No. XeOF₄ has 4 Xe–F bonds + 1 Xe=O + 1 lone pair = 6 domains giving square pyramidal (sp³d²); XeF₄ has 6 domains too but with 2 lone pairs, giving square planar.
Where does Kr sit on the "reactive or not" line, and what one compound proves it?
Kr is the exact borderline: essentially inert except that its slightly-too-high IE still lets fluorine make KrF₂ under harsh conditions — the mid-group proof that reactivity fades smoothly from Xe up to Ar.
Connections
- VSEPR Theory — the counting rule behind every "spot the error" on shapes
- Hybridisation (sp3d, sp3d2, sp3d3) — why sp³d / sp³d² / sp³d³ appear
- Ionization Enthalpy trends — the low-IE reason Xe reacts at all
- London Dispersion Forces — why noble gases are monatomic, low-boiling
- Fractional Distillation of Liquid Air — how the gases are separated
- Interhalogen and Oxyfluoride compounds — XeOF₄ and Xe=O bonding context
- p-Block general trends — group-wide size/IE/reactivity logic
- Hinglish version of the parent topic