Molecular formulae and shapes of some molecules are given below. Choose the incorrect match
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Molecules with lone pairs have shapes that differ from their electronic geometry. $NH_3$ is tetrahedral geometry but pyramidal shape; $SF_4$ is trigonal bipyramidal geometry but see-saw shape!
Concept: The shapes of molecules are determined by the number of bond pairs (BP) and lone pairs (LP) around the central atom according to VSEPR theory.
• Steric Number: (Number of valence electrons of central atom + Number of monovalent atoms -- Charge on cation + Charge on anion) 2.
• Geometry vs. Shape: Geometry depends on the total electron pairs; shape refers only to the positions of atoms (ignoring lone pairs).
Step 1: Analyze the electronic structure of SF\textsubscript{4}.
In $SF_4$, Sulfur is the central atom ($Z=16$, valence electrons = 6). It is bonded to 4 fluorine atoms.
Steric Number = $(6 + 4) / 2 = 5$.
This indicates $sp^3d$ hybridization with 5 electron pairs.
Distribution: 4 Bond Pairs + 1 Lone Pair.
Step 2: Identify the correct shape.
For a steric number of 5 with one lone pair, the lone pair occupies an equatorial position to minimize repulsion. This results in a See-saw shape.
Tetrahedral shape (Option B) requires a steric number of 4 with 0 lone pairs. Thus, (B) is the incorrect match.
