化学 1916

原子与分子

吉尔伯特·N·路易斯

化学键，就是两个原子共同持有的一对电子。

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In depth · the introduction

如果两个原子都想要一层填满的外壳，却不是靠彼此抢夺电子，而是靠共享一对电子，各自如愿呢？

核心想法

原子对自己最外层的电子，比对任何别的东西都更挑剔。到 1916 年，化学家已经知道，许多原子在外层有八个电子时最为安稳——这就是「八隅」——而旧的解释是：一个原子把电子交给另一个，两边便都凑成整齐的外壳。这对食盐是行得通的，钠把一个电子给了氯。可它解释不了化学的大半江山：气体、油脂、生命赖以构成的碳化合物——在那里，没有哪个原子肯把电子白白送人。

吉尔伯特·路易斯给出了一个更好的主意。两个原子可以各自凑足八个，办法是共享一对同时属于双方的电子。这对共享的电子，就是化学键。共享一对，是单键；共享两对、三对，便是双键、三键。为了记账，路易斯把每个外层电子画成原子符号周围的一个点——这些点式的画法，至今仍在每一堂化学课上教着。

它是如何诞生的

路易斯把这个想法追溯到他 1902 年画的一张教学草图：把原子看成一个小立方体，八个角上各可以有一个电子——填满的外壳，就是填满的立方体。他把这想法搁了许多年。1916 年终于发表时，他并不孤单：德国的瓦尔特·科塞尔同年厘清了电子转移那一面的故事；而路易斯的八隅之规，又承接自更早的理查德·阿贝格。

随后，欧文·朗缪尔——一位才华横溢、不知疲倦的演说家——接过了路易斯的方案，给「八隅（octet）」命了名，并如此有力地把它带遍化学世界，以至于人们开始把它称作朗缪尔理论——这刺痛了最先想到它的路易斯。而尽管影响如此之大，路易斯被提名诺贝尔奖数十次，却始终未能获奖，这是诺奖史上最著名的疏漏之一。

它为何重要

共享对把一堆无从解释的事实，变成了一条清晰的规则，也给了化学家一种方法：把任何分子画出来，并预测它会如何行事。一个化学学生学着在纸上做的几乎一切——推断分子的形状、用箭头推着电子对去追踪一个反应、判断一个结构是否合理——都直接源自路易斯的那些点。很难找到比它更深地织进化学日常的想法了。

一个可以想象的画面

想象两个人，各自都需要一副完整的手套，可每人只有相配一双中的一只。谁都不肯把手套送人。于是他们约定，共同握着一双手套——这一双，对两人都算「完整」。这双共握的手套，就是键。要是还嫌冷，他们可以再共握第二、第三双。而正如一只手戴不下超过一定数目的手套，一个小原子也容不下超过八个外层电子——硬要塞更多，这个排布根本就不会形成。

它的位置

路易斯出现的时刻，恰在周期表（mendeleev-1869）已把元素分门别类之后，又正当物理学家在把原子拆开——汤姆森的电子、卢瑟福的原子核、玻尔的壳层。他用了他们的电子，却仍带着化学家的眼光，问的不是原子是什么，而是它如何成键。更深的「为什么」——是什么让一对共享电子黏得住——则随后到来，当量子力学经鲍林之手抵达化学（pauling-1931）。路易斯的点，正是周期表通向量子化学键的那座桥。

The original document

Original source text

G. N. Lewis · Journal of the American Chemical Society 38 (1916) 762–785 · communicated from the University of California

One picture for every bond

Lewis opens by rejecting the assumption that atoms in a molecule are always held together by the outright transfer of electrons from one to another. Transfer accounts for the salts — the strongly polar compounds — but not for the vast class of non-polar substances, nor for the fact that most stable molecules contain an even number of electrons. He sets out to cover both kinds of compound with a single model of the atom.

He states his account as a short series of postulates, paraphrased here: every atom has an inner kernel (the nucleus plus the inner electrons) that survives ordinary chemical change; around it sits an outer shell that can hold from zero to eight electrons; the shell tends to hold an even number, and especially eight, normally arranged at the eight corners of a cube; and two atoms can complete their shells at once by sharing electrons that belong to both.

…the total difference between the maximum negative and positive valences or polar numbers of an element is frequently eight, and is in no case more than eight.

From this regularity — drawn from Abegg's earlier rule — Lewis builds the theory of the cubical atom: eight valence electrons sit at the corners of a cube, and two atoms that share an edge share the two electrons on it. A shared edge is a single bond; the shared pair counts toward the octet of both atoms at once.

The decisive proposal is that a chemical bond is a pair of electrons held jointly by two atoms. Sharing — not transfer — is the ordinary non-polar bond, and one, two or three shared pairs give the single, double and triple bonds of organic chemistry. To record it on paper, Lewis marks each valence electron as a dot around the symbol of its element: the dot diagrams still in use today.

[ … ]

Lewis notes that the pairing of electrons — what later authors called the rule of two — may run deeper than the rule of eight itself; he could not say why electrons pair, a question answered only by quantum mechanics a decade later. The paper closes by applying the scheme to a range of molecules, polar and non-polar alike.

G. N. Lewis · University of California, Berkeley · 1916