Ionic, Covalent and Metallic Bonding | 离子键、共价键与金属键

📚 Ionic, Covalent and Metallic Bonding | 离子键、共价键与金属键

Chemical bonding is the fundamental force that holds atoms together in elements and compounds. Understanding how and why atoms bond allows chemists to predict the structure, shape and physical properties of substances.

化学键是将原子结合在一起形成元素和化合物的基本作用力。理解原子如何以及为何成键，能够帮助化学家预测物质的结构、形状和物理性质。

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1. Ionic Bonding | 离子键

An ionic bond is the electrostatic attraction between oppositely charged ions. It is formed when a metal atom transfers one or more electrons to a non-metal atom, producing cations and anions.

离子键是带相反电荷离子之间的静电引力。当金属原子将一个或多个电子转移给非金属原子时，会产生阳离子和阴离子，从而形成离子键。

Electron transfer leaves both ions with full outer shells. For example, a sodium atom loses one electron to form Na⁺, while a chlorine atom gains one electron to form Cl⁻.

电子转移使两种离子都具有满外层电子。例如，钠原子失去一个电子形成Na⁺，氯原子得到一个电子形成Cl⁻。

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

The resulting ionic lattice is a giant three-dimensional structure. The strength of the attraction depends on the size of the ionic charges and the distance between the ions. For example, MgO has a higher melting point than NaCl because the ions carry larger charges and are smaller.

所形成的离子晶格是一种巨大的三维结构。吸引力的大小取决于离子电荷的大小和离子之间的距离。例如，MgO的熔点高于NaCl，因为MgO中离子所带电荷更大、离子半径更小。

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2. Covalent Bonding and Dative Bonds | 共价键与配位键

A covalent bond is formed by sharing a pair of electrons between non-metal atoms. Each atom usually contributes one electron to the shared pair, giving both atoms a stable outer shell.

共价键由非金属原子之间共享一对电子形成。通常每个原子为共用电子对提供一个电子，从而使两个原子都获得稳定的外层电子结构。

The shared pair of electrons is localised between the two nuclei. This creates a strong electrostatic attraction between the positively charged nuclei and the negatively charged shared pair.

共用电子对局域在两个原子核之间。这使得带正电的原子核与带负电的共用电子对之间产生强烈的静电吸引力。

A dative covalent (coordinate) bond is a covalent bond in which both shared electrons come from one atom. It is drawn as an arrow pointing from the lone pair to the atom that accepts the electrons. Examples include NH₄⁺, H₃O⁺ and Al₂Cl₆.

配位共价键是指两个共享电子均来自同一个原子的共价键。通常用箭头从孤对电子指向接受电子的原子来表示。NH₄⁺、H₃O⁺和Al₂Cl₆中都存在配位键。

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3. Metallic Bonding | 金属键

Metallic bonding is the electrostatic attraction between positive metal ions and delocalised electrons within a giant metallic lattice. The outer electrons of each metal atom are released into a shared ‘sea’ of electrons.

金属键是巨大金属晶格中正金属离子与离域电子之间的静电引力。每个金属原子的外层电子被释放到共享的“电子海”中。

This model explains several key properties. Metals conduct electricity because delocalised electrons can move freely throughout the lattice. They conduct heat because these free electrons can transfer kinetic energy efficiently.

该模型解释了许多重要性质。金属能导电，因为离域电子可以在晶格中自由移动；金属能导热，因为这些自由电子可以高效地传递动能。

Metals are malleable and ductile because layers of positive ions can slide over each other without breaking the metallic bonds. The strength of metallic bonding varies with the charge of the cation and the number of delocalised electrons.

金属具有延展性，因为正离子层可以相互滑动而不破坏金属键。金属键的强度随阳离子电荷和离域电子数量的不同而变化。

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

Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. It increases across a period and decreases down a group. Fluorine is the most electronegative element, with a Pauling value of 4.0.

电负性是原子在共价键中吸引成键电子对的能力。同一周期从左到右电负性增大，同一族自上而下电负性减小。氟是电负性最强的元素，鲍林标度为4.0。

When two atoms in a bond have different electronegativities, the bonding electrons are unevenly shared. The more electronegative atom becomes slightly negative (δ⁻), and the less electronegative atom becomes slightly positive (δ⁺).

当成键的两个原子电负性不同时，成键电子分布不均。电负性较强的原子略带负电荷（δ⁻），电负性较弱的原子略带正电荷（δ⁺）。

This polarity is seen in bonds such as O−H and H−Cl. However, a molecule as a whole is only polar if its shape is not symmetrical enough for the individual bond dipoles to cancel.

O−H键和H−Cl键都具有这样的极性。但是，整个分子是否具有极性，还取决于分子的形状是否对称到足以使各个键偶极相互抵消。

For example, CO₂ has two strongly polar C=O bonds, but the molecule is linear, so the dipoles cancel and CO₂ is non-polar. Water is bent, so the dipoles add together instead of cancelling, giving water a permanent molecular dipole.

例如，CO₂含有两个强极性的C=O键，但分子呈直线形，偶极相互抵消，因此CO₂是非极性分子。水分子呈弯曲形，键偶极方向叠加而不是抵消，因此水具有永久分子偶极。

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5. Shapes of Molecules: VSEPR Theory | 分子形状：价层电子对互斥理论

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