You can’t always guess a molecular shape from its chemical formula; even quite simple molecules can have surprising shapes. Predict the shape of a sulfur tetrafluoride molecule, SF₄.

ANTICIPATE You might guess that SF₄ has a tetrahedral geometry. However, you need to be aware that sulfur is capable of having an expanded valence shell. You need to follow the steps in Toolbox 2E.1 to predict the shape of SF₄.

PLAN Use the procedure in Toolbox 2E.1.

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Toolbox 2E.1 HOW TO USE THE VSEPR MODEL CONCEPTUAL BASIS Regions of high electron concentration-bonds and lone pairs attached to a central atom in a molecule-arrange themselves in such a way as to minimize mutual repulsions. PROCEDURE The general procedure for predicting the shape of a molecule follows the four VSEPR rules for predicting the shape of a molecule: Step 1 Decide how many atoms and lone pairs are present on the central atom by writing a Lewis structure for the molecule. Step 2 Identify the electron arrangement, including lone pairs and atoms, and treating a multiple bond as equivalent to a single bond (see Fig. 2E.2). Step 3 Locate the atoms and identify the molecular shape (according to Fig. 2E.1). The molecular shape describes only the positions of the atoms, not the lone pairs. Step 4 Allow the molecule to distort so that lone pairs are as far from one another and from bonding pairs as possible. The repulsions are in the order Lone pair-lone pair > lone pair-atom > atom-atom Example 2E.3 shows how this procedure is used.