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Which of the following has a **see-saw** molecular geometry around the central atom?
A$\mathrm{SF_6}$
B$\mathrm{SF_4}$
C$\mathrm{XeF_4}$
D$\mathrm{PF_5}$
Answer & Solution
Correct answer: B. $\mathrm{SF_4}$
VSEPR around the central S in $\mathrm{SF_4}$: S has 6 valence electrons, 4 are used in 4 S-F bonds, leaving 1 lone pair. Total electron domains: 5 (4 bond pairs + 1 lone pair), giving a trigonal-bipyramidal *electron* geometry but a **see-saw** *molecular* shape because the lone pair occupies an equatorial position.
Compare:
- $\mathrm{SF_6}$: 6 bond pairs, 0 lone pairs → octahedral.
- $\mathrm{XeF_4}$: 4 bond pairs + 2 lone pairs → square planar (lone pairs axial).
- $\mathrm{PF_5}$: 5 bond pairs + 0 lone pair → trigonal bipyramidal.
See-saw is the shape you get from a trigonal bipyramid when you delete one equatorial vertex.
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