Q.Find and draw the structures of SeF$_4$ and SCl$_2$.

SeF$_4$ has four bond pairs and one lone pair on Se (the lone pair equatorial in a trigonal bipyramidal arrangement), while SCl$_2$ has two bond pairs and two lone pairs on S. …

Step 1. SeF$_4$. Selenium (group 16) has six valence electrons; four form Se-F bonds, leaving one lone pair. Five electron pairs adopt a trigonal bipyramidal arrangement, and the lone pair takes an equatorial position -- the molecular shape is a see-saw, exactly the tetrahalide structure section 7.6.4 describes ("trigonal bipyramidal geometry with one equatorial position occupied by a lone pair").

Step 2. SCl$_2$. Sulfur also has six valence electrons; two form S-Cl bonds, leaving two lone pairs. Four electron pairs adopt a tetrahedral arrangement with the two lone pairs on sulfur, so the molecule is bent (angular), like H$_2$O. …

Applying VSEPR to each central atom: count bonding pairs and lone pairs (SeF4 = 4+1, SCl2 = 2+2), pick the parent electron geometry, place lone pairs to m …

• Drawing SeF4 as a perfect tetrahedron: the lone pair makes it AX4E -- a see-saw, not tetrahedral.
• Putting SeF4's lone pair in an axial position; lone pairs occupy the roomier equatorial sites of a trigonal bipyrami …