分子构型Molecular geometry

中心原子周围的电子对（成键电子对和孤对电子）相互排斥，尽可能远离。先数出电子对区域数（双键、三键各算一个区域），确定电子对的排布；再看其中有几个是孤对电子，得到原子的排布即分子构型。排斥强弱为：孤对–孤对 > 孤对–成键 > 成键–成键，所以孤对电子会压缩键角。

Electron pairs around the central atom, bonding and lone pairs alike, repel and get as far apart as possible. Count the regions of electron density (a double or triple bond counts as one) to fix their arrangement; then note how many are lone pairs to get the arrangement of atoms, the molecular shape. Repulsion decreases in the order lone–lone > lone–bonding > bonding–bonding, so lone pairs squeeze bond angles.

2 对，无孤对：直线形 180°（CO₂、BeCl₂）。3 对，无孤对：平面三角形 120°（BF₃）。4 对：无孤对为四面体 109.5°（CH₄），1 对孤对为三角锥 107°（NH₃），2 对孤对为 V 形 104.5°（H₂O）。5 对无孤对：三角双锥 90° 和 120°（PCl₅）。6 对无孤对：八面体 90°（SF₆）。

Two regions, no lone pairs: linear, 180° (CO₂, BeCl₂). Three, none: trigonal planar, 120° (BF₃). Four: tetrahedral 109.5° with none (CH₄), trigonal pyramidal 107° with one lone pair (NH₃), bent 104.5° with two (H₂O). Five, none: trigonal bipyramidal, 90° and 120° (PCl₅). Six, none: octahedral, 90° (SF₆).

分子构型决定分子是否有极性，进而影响分子间作用力、熔沸点和溶解性，如 CO₂ 为直线形、非极性，H₂O 为 V 形、极性。构型还决定分子的对称性（点群）、光谱性质和生物活性，酶与底物、药物与受体都需要形状匹配。

Shape decides whether a molecule is polar and therefore its intermolecular forces, melting and boiling points and solubility: linear CO₂ is non-polar, bent H₂O polar. Shape also fixes symmetry (point group), spectroscopic properties and biological activity, since enzymes and substrates, and drugs and receptors, must fit together.