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Chemistry Question 61 – JEE-MAIN 2025

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A molecule with the formula AX4Y has all it's elements from p-block. Element A is rarest, monoatomic, non-radioactive from its group and has the lowest ionization enthalpy value among A, X and Y. Elements X and Y have first and second highest electronegativity values respectively among all the known elements. The shape of the molecule is :

Identify elements A, X, and Y based on the given properties like p-block, rarity, monoatomic nature, non-radioactivity, ionization enthalpy, and electronegativity.

● Video Walkthrough
Step 1: Identify the elements and the molecular formula✦ Active

Element A is described as the rarest, monoatomic, non-radioactive from its p-block group, and has the lowest ionization enthalpy among A, X, and Y. This strongly indicates Xenon (Xe). Elements X and Y have the first and second highest electronegativity values, respectively, among all known elements. This identifies X as Fluorine (F) and Y as Oxygen (O). The ionization enthalpies are: Xe (1170 kJ/mol), O (1314 kJ/mol), F (1681 kJ/mol), confirming Xe has the lowest. Thus, the molecule is XeF4O.

Step 2: Determine the steric number and electron geometry of XeF4O○ Expand

The central atom is Xe, which has 8 valence electrons. It forms four single bonds with F atoms (using 4 electrons) and one double bond with an O atom (using 2 electrons). Total electrons used in bonding = 4+2=6. Remaining electrons on Xe = 8−6=2, which form 1 lone pair. The number of electron domains (steric number) is the sum of bond pairs (treating a double bond as one domain) and lone pairs: 4 (Xe-F)+1 (Xe=O)+1 (lone pair)=6. An electron geometry with a steric number of 6 is Octahedral.

💡 Teacher's Secret Hint

Remember that for VSEPR theory, a double bond counts as one electron domain, similar to a single bond.

Step 3: Determine the molecular geometry (shape)○ Expand

For an octahedral electron geometry with 5 bond pairs (4 Xe-F, 1 Xe=O) and 1 lone pair (AX5E type), the molecular geometry is Square Pyramidal. The lone pair occupies one of the axial positions to minimize repulsion, and the five atoms (four F and one O) occupy the remaining positions, forming a square pyramid.

💡 Teacher's Secret Hint

The lone pair typically occupies a position that minimizes repulsion with other electron domains, often an axial position in octahedral geometry.

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