More Molecular Shapes | 更多分子形状

📚 More Molecular Shapes | 更多分子形状

At A-level, students often master methane, ammonia and water, but exam questions increasingly probe less familiar geometries such as trigonal bipyramidal, octahedral, seesaw and square planar. This article extends VSEPR theory to molecules and ions with five or six electron domains, including expanded octet species and lone-pair placements.

在 A-level 阶段，学生通常掌握甲烷、氨和水，但考试越来越常考查不太熟悉的几何形状，例如三角双锥、八面体、跷跷板形和平面四方。本文将 VSEPR 理论扩展到具有五个或六个电子域的分子和离子，包括扩展八隅体物种以及孤对电子的放置。

1. VSEPR Recap: More Than Four Electron Domains | VSEPR 回顾：超过四个电子域

VSEPR treats both bonding pairs and lone pairs as electron domains that repel one another. The basic shapes met at AS Level – linear (2 domains), trigonal planar (3) and tetrahedral (4) – arise when only bonding pairs surround the central atom. When a Period 3 or heavier central atom expands its octet, five or six domains become possible, producing a new family of shapes.

VSEPR 将成键电子对和孤对电子都视为相互排斥的电子域。AS 阶段遇到的基本形状——直线形（2 个域）、三角平面（3 个）和四面体（4 个）——仅当中心原子周围只有成键对时出现。当第三周期或更重的中心原子扩展八隅体时，可能出现五个或六个域，从而产生一组新的形状。

2 domains → linear, 180°; 3 domains → trigonal planar, 120°; 4 domains → tetrahedral, 109.5°; 5 domains → trigonal bipyramidal, 90°/120°; 6 domains → octahedral, 90°

2. Trigonal Bipyramidal: Five Electron Domains | 三角双锥：五个电子域

With five bonding pairs and no lone pairs, the geometry is trigonal bipyramidal. Three equatorial positions lie in one plane at 120° to each other, while two axial positions sit above and below at 90° to the equatorial plane. PF₅ and PCl₅ are typical examples.

当中心原子周围有五个成键对且没有孤对时，几何形状为三角双锥。三个赤道位置位于同一平面内，彼此成 120°

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