Bonding and Structure: Ionic, Covalent and Metallic | 化学键合与结构：离子键、共价键与金属键

📚 Bonding and Structure: Ionic, Covalent and Metallic | 化学键合与结构：离子键、共价键与金属键

Chemical bonding is the glue that holds atoms together in molecules, lattices and metallic structures. In Edexcel A-Level Chemistry, Topic 2 demands a deep, quantitative understanding of ionic, covalent and metallic bonding, electronegativity, molecular shape and intermolecular forces. Mastering this topic underpins later work in inorganic chemistry, organic chemistry and physical chemistry.

化学键是把原子结合成分子、晶格和金属结构的“胶水”。在爱德思 A-Level 化学中，主题 2 要求深入、定量地理解离子键、共价键和金属键、电负性、分子形状以及分子间作用力。掌握本主题将为后续的无机化学、有机化学和物理化学内容奠定基础。

1. Ionic Bonding | 离子键

Ionic bonding arises from the electrostatic attraction between oppositely charged ions. Metals in Groups 1 and 2 lose electrons to form cations such as Na⁺, Mg²⁺ and Al³⁺, while non-metals in Groups 15, 16 and 17 gain electrons to form anions such as N³⁻, O²⁻ and Cl⁻. In Edexcel, you must be able to describe the formation of an ionic lattice and calculate the empirical formula from the charge balance.

离子键来自带相反电荷的离子之间的静电吸引。第 1、2 族金属失去电子形成阳离子，如 Na⁺、Mg²⁺、Al³⁺；第 15、16、17 族非金属得到电子形成阴离子，如 N³⁻、O²⁻、Cl⁻。在爱德思考试中，你必须能描述离子晶格的形成，并根据电荷平衡计算离子的经验式。

When sodium reacts with chlorine, each sodium atom loses one 3s electron and each chlorine atom gains one electron to fill its 3p subshell. The resulting Na⁺ and Cl⁻ ions are held in a giant, regular lattice by strong electrostatic forces acting in all directions.

钠与氯反应时，每个钠原子失去一个 3s 电子，每个氯原子得到一个电子填充其 3p 亚层。生成的 Na⁺ 和 Cl⁻ 离子通过向各个方向作用的强静电引力，形成巨大、规则的离子晶格。

Na → Na⁺ + e⁻   |   Cl + e⁻ → Cl⁻

• Ionic compounds have high melting and boiling points because the lattice is held by strong electrostatic attractions.
• 离子化合物具有高熔点和高沸点，因为晶格由强烈的静电引力维持。
• They are brittle: when a force displaces ions, like-charged ions align and repel.
• 它们很脆：当外力使离子移位时，同号离子相互靠近并排斥。
• Solid ionic compounds do not conduct electricity because ions are fixed in the lattice; molten or aqueous ionic compounds do conduct because the ions are free to move.
• 固态离子化合物不导电，因为离子被固定在晶格中；熔融态或水溶液中的离子化合物能够导电，因为离子可以自由移动。

2. Covalent Bonding | 共价键

A covalent bond is a shared pair of electrons that electrostatically attracts two positive nuclei. Edexcel expects you to draw dot-and-cross diagrams for simple molecules such as H₂, Cl₂, HCl, H₂O, NH₃, CH₄, CO₂ and N₂, showing only outer-shell electrons unless the question states otherwise.

共价键是一对共用电子，它通过静电作用吸引两个带正电的原子核。爱德思考试要求你能画出 H₂、Cl₂、HCl、H₂O、NH₃、CH₄、CO₂ 和 N₂ 等简单分子的点叉图，除非题目另有说明，通常只画出最外层电子。

A dative covalent bond, also called a coordinate bond, is a covalent bond in which both electrons in the shared pair come from the same atom. The ammonium ion, NH₄⁺, is the classic example: the nitrogen atom donates a lone pair to an H⁺ ion.

配位共价键，又称配位键，是一种共价键，其中共用电子对的两个电子都来自同一个原子。铵根离子 NH₄⁺ 是最典型的例子：氮原子向 H⁺ 提供一对孤对电子。

Multiple covalent bonds arise when atoms share more than one electron pair. Oxygen molecules contain a double bond, O=O, and nitrogen molecules contain a triple bond, N≡N. The shorter, stronger multiple bonds are shown in dot-and-cross diagrams as two or three shared pairs.

当原子共用不止一对电子时，就形成多重共价键。氧分子含有双键 O=O，氮分子含有三键 N≡N。多重键更短、更强，在点叉图中表示为两对或三对共用电子。

3. Metallic Bonding | 金属键

Metallic bonding is the electrostatic attraction between a regular lattice of positive metal ions and a sea of delocalised outer-shell electrons. This bonding model explains the characteristic properties of metals: high electrical and thermal conductivity, malleability, ductility and generally high melting points.

金属键是规则排列的正金属离子晶格与离域的最外层电子海之间的静电吸引。这一键合模型可以解释金属的典型性质：高导电性、高导热性、可锻性、延展性以及通常较高的熔点。

The delocalised electrons are no longer attached to any particular metal ion, so they can move through the entire lattice and carry charge or transfer kinetic energy efficiently. When a metal is hammered or drawn into a wire, the layers of ions slide past one another without breaking the metallic bonding because the electron sea continually surrounds them.

离域电子不再依附于某个特定的金属离子，因此它们能够在整个晶格中自由移动，高效地传导电荷或传递动能。当金属被锤打或拉成金属丝时，离子层彼此滑动，但金属键不会断裂，因为电子海始终包围着它们。

4. Electronegativity and Bond Polarity | 电负性与键的极性

Electronegativity is the ability of an atom in a covalent bond to attract the bonding pair of electrons. On the Pauling scale, fluorine is the most electronegative element with a value of 4.0, and caesium is among the least electronegative with a value close to 0.7. Edexcel candidates should use electronegativity differences to classify bonding.

电负性是共价键中原子吸引成键电子对的能力。在鲍林标度中，氟是电负性最强的元素，数值为 4.0；铯是电负性最弱的元素之一，约为 0.7。爱德思考生应使用电负性差值来区分键型。

If the electronegativity difference is zero, the bond is pure covalent, as in Cl₂. If the difference is small, the bond is polar covalent, with the electron pair pulled towards the more electronegative atom. A large difference produces an ionic bond, but the boundary is not sharp and bonding often lies along a continuum.

如果电负性差值为零，则为非极性共价键，如 Cl₂。如果差值较小，则为极性共价键，电子对会被拉向电负性较大的原子。差值很大时形成离子键，但边界并不绝对，键型往往是一个连续谱。

In hydrogen chloride, chlorine is more electronegative than hydrogen, so the bonding pair is shifted towards chlorine. This creates a permanent dipole with a partial negative charge on chlorine and a partial positive charge on hydrogen.

在氯化氢中，氯的电负性大于氢，因此成键电子对偏向氯。这会产生永久偶极，氯端带部分负电荷，氢端带部分正电荷。

H—Cl:    δ⁺H — Clδ⁻

5. Shapes of Molecules: VSEPR Theory | 分子形状：VSEPR 理论

Valence Shell Electron Pair Repulsion theory states that electron pairs around a central atom arrange themselves as far apart as possible to minimise repulsion. Lone pairs repel more strongly than bonding pairs, so the presence of lone pairs reduces bond angles below the ideal values.

价层电子对互斥理论指出，中心原子周围的电子对会尽可能远离，以最小化排斥作用。孤对电子的排斥力强于成键电子对，因此孤对电子的存在会使键角小于理想值。