Valence-shell electron-pair repulsion trigonal bipyramidal arrangement

The valence shell of the sulfur atom contains ten electrons six from the sulfur and one each from the four fluorine atoms. There are four bonding pairs and one lone pair. The basic shape adopted by the electron pairs In the molecule Is trigonal bipyramidal. In this arrangement, the electron pairs at the equatorial positions experience less repulsion compared to axial electron pairs. Flence the lone pair occupies an equatorial position and thus the shape of the molecule Itself resembles a see-saw (Figure 14.5). As a general rule, for a molecule where the electron domains adopt a trigonal bipyramidal structure, any lone pairs will occupy equatorial positions. 

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