共价键Covalent bond

量子力学表明，两个原子轨道重叠时电子在两核之间的概率密度增大，同时吸引两个原子核，使体系能量降低而成键。按重叠方式分为头对头重叠的 σ 键和肩并肩重叠的 π 键。成键电子对由一方单独提供时称为配位键。共价键强度用键能表示，几何特征用键长和键角表示。

Quantum mechanics shows that when two atomic orbitals overlap, electron density builds up between the nuclei and attracts both, lowering the energy and forming a bond. Head-on overlap gives σ bonds and side-on overlap gives π bonds. When one atom supplies both electrons the bond is a coordinate bond. Strength is expressed as bond energy, and geometry by bond lengths and angles.

1916 年美国化学家路易斯在《原子与分子》一文中提出原子间共用电子对形成化学键，并用电子点式表示。1919 年朗缪尔推广了这一思想并创造了 covalence（共价）一词。1927 年海特勒和伦敦首次用量子力学计算氢分子，证明了共价键的本质，奠定了价键理论。

In 1916 the American chemist Lewis, in 'The Atom and the Molecule', proposed that atoms bond by sharing electron pairs and drew them as electron dots. Langmuir popularised the idea in 1919 and coined the word covalence. In 1927 Heitler and London's quantum-mechanical treatment of the hydrogen molecule revealed the nature of the covalent bond and founded valence bond theory.

1930 年代鲍林发展了杂化轨道和共振论；同期洪德和密立根提出分子轨道理论，认为电子属于整个分子。两种理论各有优势：价键理论直观描述局域键和分子构型，分子轨道理论更好地解释离域、磁性和光谱。现代计算化学能精确计算各类共价分子的结构和性质。

In the 1930s Pauling developed hybridisation and resonance theory, while Hund and Mulliken proposed molecular orbital theory, in which electrons belong to the whole molecule. Each has its strengths: valence bond theory gives an intuitive picture of localised bonds and shapes, while MO theory better explains delocalisation, magnetism and spectra. Modern computational chemistry calculates the structures and properties of covalent molecules accurately.