Chemistry · Ch 5 — Chemical Bonding
Valence Shell Electron Pair Repulsion Theory (VSEPR)
Valence Shell Electron Pair Repulsion Theory (VSEPR)
Because a Lewis structure alone says nothing about a molecule's three-dimensional shape, the Valence Shell Electron Pair Repulsion (VSEPR) theory — proposed by Sidgwick and Powell — was developed specifically to predict molecular geometry (no single theory, the chapter notes, can describe every molecule's shape completely, but VSEPR handles a large number of cases well, especially for p-block-element compounds). Its central idea: every electron pair shown around a central atom in a Lewis structure — whether it is a bonding (shared) pair or a lone pair — repels every other electron pair on that same atom, and the pairs arrange themselves in space so as to MINIMISE that total repulsion, which in turn gives the molecule its minimum-energy, maximum-stability geometry. Four governing rules follow: electron pairs arrange for minimum mutual repulsion; this minimum-repulsion arrangement is also the molecule's minimum-energy, most stable arrangement; lone pairs, not just bonding pairs, help determine the final shape; and the STRENGTH of repulsion follows the order lone pair–lone pair > lone pair–bond pair > bond pair–bond pair, because a lone pair (attracted by only ONE nucleus) spreads out more broadly in space than a bonding pair (pulled in and confined by TWO nuclei). A direct consequence: a central atom with NO lone pairs gives the molecule a fully regular geometry (Table 5.3 — linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral, depending on the number of electron pairs), while a central atom carrying one or more lone pairs distorts that regular geometry and compresses …
Table 5.3 — number of electron pairs around the central atom, arrangement/molecular geometry (identical when there is no lone pair), and examples: 2 pairs — Linear — BeBr2, CO2. 3 pairs — Trigonal planar — BF3, BCl3, BH3. 4 pairs — Tetrahedral — CH4, NH4⁺, SiCl4. 5 pairs — Trigonal bipyramidal — PCl …
Table 5.4 — molecule type (A = central atom, B = bonded atom, E = lone pair), lone pairs, bonding pairs, shape, and examples: AB2E — 1 lone pair, 2 bonding pairs — Bent — SO2, O3. AB3E — 1 lone pair, 3 bonding pairs — Trigonal pyramidal — NH3, PCl3. AB2E2 — 2 lone pairs, 2 bonding pairs — Bent — H2O, OF2, H2S, SCl2. AB4E — 1 lone pair, 4 bonding pairs — See-saw — SF4. AB3E2 — 2 lone pairs, 3 bonding pairs — T-shape — ClF3, BrF3, ICl3. AB5E — 1 lone pair, 5 bonding pairs — Squa …
Worked out. In NH3 the central N has 8 valence-shell electrons in all — 6 involved in the three N–H bonds and the remaining pair forming one lone pair — so two kinds of repulsion operate: lone pair–bond pair (stronger) and bond pair–bond pair (weaker). Because the lone pair pushes the three bonding pairs closer together, the H–N–H bond angle is compressed from the ideal tetrahedral value of 109°28' down to about 107°18', and the resulting shape is pyramidal (trigonal p …
Worked out. In H2O the central O has 8 valence-shell electrons in its bonded state — two bonding pairs (to the two H atoms) and two lone pairs. The stronger lone pair–lone pair repulsion pushes the two lone pairs apart, which in turn compresses the two O–H bonding pairs even more than in NH3's single-lone-pair case; the H–O–H bond angle is reduced further, from the tetrahedral 109°28' down to about 104°35' (also written 104.5°), and the molecula …