VSEPR Shapes and Bond Angles

Let A be the central atom, X a bonded neighbor and E a central lone pair. Count a multiple bond as one X direction.

In five-domain systems, lone pairs preferentially occupy equatorial sites because an equatorial site has two 90° contacts rather than an axial site's three. In six-domain AX₄E₂ systems, placing the two lone pairs opposite reduces their close repulsion and leaves a square of bonded atoms. The geometry table supplies reference angles; lone pairs and ligand effects can distort them.

A lone pair is attracted to only one nucleus, so its electron cloud usually spreads wider than a bonding pair and compresses nearby bond angles. Methane has ideal 109.5° angles, ammonia drops to about 107° with one lone pair, and water drops to about 104.5° with two. This is a useful trend, not a rule that explains every measured angle: PH₃ has an H–P–H angle near 93° because its P–H bonds are largely p-like, whose directions are close to 90°, unlike the more hybridized N–H bonds in NH₃.

The commonly used repulsion pattern is lone pair–lone pair greater than lone pair–bonding pair greater than bonding pair–bonding pair, within a comparable local framework. It supports qualitative compression, not a fixed “degrees per lone pair” subtraction. A bent label also does not specify one angle: AX₂E from a trigonal-planar framework differs from AX₂E₂ from a tetrahedral framework. Determine the electron framework before choosing an angle reference.

A molecule is polar when polar bonds fail to cancel. CO₂ is linear, so its two bond dipoles cancel and the molecule is nonpolar. Water is bent, so its dipoles add and the molecule is strongly polar. Symmetric shapes such as tetrahedral CCl₄ and square planar XeF₄ cancel; asymmetric shapes such as trigonal pyramidal NH₃ and seesaw SF₄ do not.

Polarity depends on vectors in three dimensions. Four equal tetrahedral C–Cl dipoles cancel in CCl₄, but replacing one Cl by H removes that cancellation. In square-planar XeF₄, each Xe–F dipole has an equal opposite partner. For NH₃ the horizontal components of the three N–H dipoles cancel while their components along the pyramidal axis add. It is inaccurate to say the three bond arrows literally point in the same direction.

For XeF₂, the valence budget is 8 + 2(7) = 22 electrons. Two Xe–F bonds and completed F octets leave six electrons as three Xe lone pairs. Five domains give a trigonal-bipyramidal electron framework; three equatorial lone pairs leave the two F atoms axial, producing a linear molecule. Equal opposing Xe–F dipoles cancel despite the central lone pairs.

For SF₄, four bonded neighbors and one lone pair give the same five-domain framework but a seesaw atom arrangement; its dipoles do not cancel. Thus “has lone pairs” alone neither establishes polarity nor supplies the shape name. Identify the framework, place the lone pairs, name the atom arrangement and then add the bond dipoles.