Ionic Bonding | 离子键

📚 Ionic Bonding | 离子键

Ionic bonding is an essential topic in Cambridge A-Level Chemistry. It explains how metals and non-metals combine by transferring electrons to form oppositely charged ions, which then attract each other in a giant lattice.

离子键是剑桥 A-Level 化学的重要主题。它解释了金属和非金属如何通过转移电子形成带相反电荷的离子，并在巨大晶格中相互吸引。

1. Definition of Ionic Bonding | 离子键的定义

Ionic bonding is the electrostatic force of attraction between oppositely charged ions formed by the complete transfer of one or more electrons from a metal atom to a non-metal atom.

离子键是由金属原子向非金属原子完全转移一个或多个电子后形成的带相反电荷的离子之间的静电吸引力。

In ionic bonding, atoms tend to achieve a stable noble-gas electron configuration. Metals lose valence electrons to form cations, while non-metals gain electrons to form anions.

在离子键中，原子倾向于获得稳定的惰性气体电子构型。金属失去价电子形成阳离子，非金属获得电子形成阴离子。

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2. Formation of Ions by Electron Transfer | 电子转移形成离子

Consider sodium chloride. A sodium atom has the electron configuration 2,8,1, and a chlorine atom has 2,8,7. Sodium transfers its one outer electron to chlorine.

以氯化钠为例。钠原子的电子构型为 2,8,1，氯原子为 2,8,7。钠将其一个外层电子转移给氯。

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

After transfer, Na⁺ has the electronic configuration 2,8 and Cl⁻ has 2,8,8. Both ions have full outer shells like the nearest noble gases.

转移后，Na⁺ 的电子构型为 2,8，Cl⁻ 为 2,8,8。两种离子都具有与最接近的惰性气体相同的满外层。

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3. Giant Ionic Lattice Structure | 巨型离子晶格结构

Ionic compounds do not exist as discrete molecules. They form a giant three-dimensional lattice in which each cation is surrounded by anions and each anion is surrounded by cations.

离子化合物不以孤立分子存在。它们形成巨大的三维晶格，其中每个阳离子被阴离子包围，每个阴离子被阳离子包围。

The lattice is held together by strong electrostatic attractions extending in all directions. This is why the formula NaCl represents the simplest whole-number ratio of ions rather than a single molecule.

晶格由向各个方向延伸的强静电吸引力维系。这就是为什么 NaCl 化学式表示离子的最简整数比，而不是单个分子。

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4. Coordination Numbers in Common Lattices | 常见晶格中的配位数

The coordination number is the number of oppositely charged ions immediately surrounding a given ion in the lattice. It depends on the relative sizes of the ions.

配位数是指晶格中紧邻某一离子的带相反电荷离子的数目。它取决于离子的相对大小。

Compound	Lattice type	Coordination numbers
NaCl	Rock salt	Na⁺ 6, Cl⁻ 6
CsCl	Caesium chloride	Cs⁺ 8, Cl⁻ 8
CaF₂	Fluorite	Ca²⁺ 8, F⁻ 4

For example, in NaCl each Na⁺ ion is surrounded by six Cl⁻ ions, and each Cl⁻ ion is surrounded by six Na⁺ ions, giving a 6:6 coordination.

例如，在 NaCl 中每个 Na⁺ 离子被六个 Cl⁻ 离子包围，每个 Cl⁻ 离子被六个 Na⁺ 离子包围，因此配位数为 6:6。

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5. Lattice Energy | 晶格能

Lattice energy is the enthalpy change when one mole of an ionic solid is formed from its gaseous ions under standard conditions. It is always exothermic, so the value is negative.

晶格能是在标准条件下，由气态离子形成一摩尔离子固体时的焓变。它总是放热的，因此数值为负。

Na⁺(g) + Cl⁻(g) → NaCl(s) ΔH = -787 kJ mol⁻¹

A more negative lattice energy means stronger ionic bonding and a more stable lattice. Lattice energy can be calculated indirectly using a Born-Haber cycle.

晶格能越负，意味着离子键越强，晶格越稳定。晶格能可以通过玻恩-哈伯循环间接计算。

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6. Factors Affecting Lattice Energy | 影响晶格能的因素

Lattice energy depends on two main factors: the product of the ionic charges and the sum of the ionic radii.

晶格能主要取决于两个因素：离子电荷的乘积和离子半径之和。

Lattice energy ∝ (q⁺ × q⁻) ÷ (r⁺ + r⁻)

A higher ionic charge gives a much stronger attraction, so MgO has a far more exothermic lattice energy than NaCl. Smaller ions can pack more closely, giving stronger attraction.

离子电荷越高，吸引力越强，因此 MgO 的晶格能比 NaCl 更放热。离子越小，排列越紧密，吸引力也越强。

Between compounds with similar charges, the one with smaller ions has the more exothermic lattice energy. For example, LiF has a more exothermic lattice energy than KI.

在电荷相似的化合物之间，离子半径较小的化合物晶格能更放热。例如，LiF 的晶格能比 KI 更放热。

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7. Melting and Boiling Points | 熔点与沸点

Ionic compounds have high melting and boiling points because strong electrostatic forces between oppositely charged ions require a large amount of energy to overcome.

离子化合物具有较高的熔点和沸点，因为带相反电荷离子之间的强静电力需要大量能量才能克服。

Compounds with higher lattice energies generally have higher melting points. For example, MgO has a much higher melting point than NaCl because Mg²⁺ and O²⁻ have double charges and smaller radii.

晶格能越大的化合物通常熔点越高。例如，MgO 的熔点远高于 NaCl，因为 Mg²⁺ 和 O²⁻ 带有双倍电荷且半径更小。

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