Hydrogen Bonding | 氢键

📚 Hydrogen Bonding | 氢键

Hydrogen bonding is one of the most important intermolecular forces in A-Level chemistry. It explains why water is a liquid at room temperature, why ice floats, and why biological molecules such as DNA keep their shape. Understanding hydrogen bonding helps you link structure, bonding and physical properties across the whole syllabus.

氢键是 A-Level 化学中最重要的分子间作用力之一。它能解释为什么水在室温下是液体、为什么冰会浮在水面上，以及为什么 DNA 等生物分子能保持特定形状。理解氢键有助于你把结构、化学键和物理性质联系起来。

1. What Is Hydrogen Bonding? | 什么是氢键？

A hydrogen bond is a strong type of permanent dipole-dipole attraction. It forms between a hydrogen atom that is covalently bonded to a very electronegative atom (nitrogen, oxygen or fluorine) and a lone pair of electrons on another electronegative atom. The bond can be represented as X-H···Y, where X and Y are N, O or F.

氢键是一种较强的永久偶极-偶极吸引力。它形成于共价键合在强电负性原子（氮、氧或氟）上的氢原子，与另一个电负性原子上的孤对电子之间。氢键可表示为 X-H···Y，其中 X 和 Y 是 N、O 或 F。

The hydrogen atom carries a partial positive charge (δ⁺) because the N/O/F atom pulls bonding electrons towards itself. The lone pair on the neighbouring molecule carries a partial negative charge (δ⁻). The attraction between these opposite partial charges is the hydrogen bond.

由于 N、O、F 原子把成键电子拉向自身，氢原子带部分正电荷（δ⁺）。邻近分子上的孤对电子带部分负电荷（δ⁻）。这些相反部分电荷之间的吸引就是氢键。

2. Conditions for Hydrogen Bond Formation | 氢键形成的条件

For a molecule to form hydrogen bonds, two conditions must be met. First, it must contain a hydrogen atom directly bonded to nitrogen, oxygen or fluorine. Second, it must provide access to a lone pair on N, O or F, either on the same molecule or on another molecule.

分子要形成氢键，必须满足两个条件。第一，必须含有与氮、氧或氟直接成键的氢原子。第二，必须提供 N、O 或 F 上的孤对电子，可在同一分子上，也可在另一个分子上。

Water (H₂O) has O-H bonds and lone pairs on oxygen, so it can form hydrogen bonds. Ammonia (NH₃) has N-H bonds and one lone pair on nitrogen. Hydrogen fluoride (HF) has H-F bonds and lone pairs on fluorine. In contrast, methane (CH₄) cannot form hydrogen bonds because carbon is not electronegative enough, and HCl does not form hydrogen bonds because chlorine is not N/O/F.

水（H₂O）有 O-H 键和氧上的孤对电子，因此可以形成氢键。氨（NH₃）有 N-H 键和氮上的一对孤对电子。氟化氢（HF）有 H-F 键和氟上的孤对电子。相反，甲烷（CH₄）不能形成氢键，因为碳的电负性不够强；HCl 也不能形成氢键，因为氯不是 N、O、F。

3. Comparing Intermolecular Forces | 氢键与其他分子间作用力的比较

Hydrogen bonding is the strongest type of intermolecular force in A-Level chemistry, but it is still much weaker than covalent bonding. The table below compares typical strengths.

氢键是 A-Level 化学中最强的分子间作用力，但仍远弱于共价键。下表比较了典型强度。