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MHT-CET Chemistry · Elements of Group 16, 17 and 18

Group 18: Noble Gases, Their Uses and the Shapes of Xenon Compounds

The noble gases He, Ne, Ar, Kr, Xe and Rn have filled valence shells and few compounds, but xenon combines with fluorine and oxygen, and the shape of each xenon compound follows from its bond pairs and lone pairs exactly as for an interhalogen.

All Elements of Group 16, 17 and 18 notesMHT-CET Chemistry strategy

Why this matters

7 PYQs, none HARD, all from 2025 and 2026. Of the first three, two are xenon shapes — the compound with one lone pair on Xe, and the one shaped like ClF₅ (XeOF₄ both times); one is a use — helium in MRI. One card.

Concept 1 of 1: Noble Gas Uses and Xenon Compound Shapes

Xenon has eight valence electrons. Each Xe–F bond uses one of them, each Xe=O uses two, and what is left sits as lone pairs. XeF₂: 2 used, 6 left — three lone pairs, linear. XeF₄: 4 used — two lone pairs, square planar. XeF₆: 6 used — one lone pair, distorted octahedral. XeOF₄: 2 + 4 = 6 used — one lone pair, square pyramidal, the same shape as ClF₅ and BrF₅. XeO₃: 6 used — one lone pair, pyramidal. For uses, match the gas: liquid helium cools the superconducting magnets of MRI scanners.

Definition

  • Lone pairs on Xe: XeF₂ 3 (linear); XeF₄ 2 (square planar); XeOF₂ 2 (T-shaped); XeF₆ 1 (distorted octahedral); XeO₃ 1 (pyramidal); XeOF₄ 1 (square pyramidal).
  • Same shape as ClF₅ (square pyramidal): XeOF₄.
  • Uses: He — MRI magnet cooling, balloons, diving gas mixtures; Ne — discharge signs; Ar — bulbs, inert atmosphere for welding; Xe — flash and arc lamps; Rn — radiotherapy.

Lone pairs on xenon

lp=8−n(Xe-F)−2 n(Xe=O)2\text{lp} = \frac{8 - n(\text{Xe-F}) - 2\,n(\text{Xe=O})}{2}

Worked example

How many lone pairs does Xe carry in XeO₂F₂, and what is its electron geometry?
Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 23 April Shift I · Q100Moderate

Example 1 · Elements of Group 16, 17 and 18 · Group 18 Noble Gases and Xenon Compounds

Identify from following compounds where valence shell of 'Xe' consists one lone pair of electrons.

Counting an Xe=O bond as one electron

A double bond to oxygen uses two of xenon's electrons. XeOF₂ therefore has two lone pairs, not three.
Drill 2 more on noble gas uses and xenon compound shapes

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Watch out for (1)

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