电负性Electronegativity

同一周期从左到右，核电荷增加、原子半径减小，电负性增大；同一主族从上到下电负性减小。两原子电负性差决定键的极性：差值约大于 1.7 时以离子键为主，介于 0.4 与 1.7 之间为极性共价键，较小时为非极性共价键。电负性还用于判断氧化数、酸性强弱和诱导效应。

Across a period nuclear charge rises and radius shrinks, so electronegativity increases; down a main group it decreases. The difference between two atoms sets bond polarity: above about 1.7 the bond is mainly ionic, between 0.4 and 1.7 polar covalent, and below that essentially nonpolar. Electronegativity also guides oxidation numbers, acid strength and inductive effects.

贝采利乌斯在 19 世纪初已提出“电化学二元论”，按元素的电性排序。1932 年鲍林根据键能数据提出第一个定量电负性标度：A–B 键能超出 A–A 和 B–B 平均值的部分与两者电负性差的平方成正比。这一标度至今最常用。

In the early nineteenth century Berzelius ranked elements by electrical character in his electrochemical dualism. In 1932 Pauling proposed the first quantitative scale from bond energies: the extra strength of an A–B bond over the mean of A–A and B–B is proportional to the square of their electronegativity difference. His scale remains the most widely used.

1934 年密立根把电负性定义为电离能与电子亲和能的平均值；1958 年阿尔雷德-罗乔以有效核电荷对价电子的静电力定义；1989 年艾伦用价层电子平均能量定义光谱电负性。不同标度数值不同但趋势一致。电负性并非可直接测量的物理量，而是一个概念性工具。

In 1934 Mulliken defined electronegativity as the average of ionisation energy and electron affinity; in 1958 Allred and Rochow used the electrostatic force of the effective nuclear charge on valence electrons; in 1989 Allen used the average valence-electron energy to define a spectroscopic scale. The scales differ in numbers but agree on trends. Electronegativity is a conceptual tool rather than a directly measurable quantity.