Name: electron spin and orbital pairing
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 3 min 6 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.E

The Pauli Exclusion Principle

原子轨道是一个原子的电子最有可能 被发现的区域。但是每个轨道能容纳多少电子呢？泡利不相容原理回答了这个问题，因为它意味着一个原子中没有两个电子 可以有一组相同的四个量子数。每个轨道对应于固定的主量子数、角动量或方位角和磁量子数 的值。例如，在 1s 轨道上的电子的 主量子数总是为 1，而方位角和磁量子数总是为零。因此，电子必须有不同的自旋量子数 的值，或自旋，才能驻留在同一个原子轨道上。回想一下，自旋量子数 只有两个可能的值：1/2 和 1/2。因此，只有两个电子可以占据同一轨道。因此，每个 s 亚壳层有一个轨道，只能容纳两个电子，而每个 p 亚壳层有三个轨道，可以容纳六个电子。每个 d 和 f 亚壳层的最大容量 分别为 10 和 14 个电子。电子在原子的原子轨道上的分布，是用它的文字或图表形式的 电子构型来表示的。考虑一个基态的氢原子，其中一个电子占据最低能量轨道：1s。书写的电子构型 用相应的壳层数目、子壳层的字母，以及一个 用于指定子壳层中电子数量的上标数字，来表示每个被占据的子壳层。电子构型的轨道图样式 将被占据的子壳层中的每个轨道 表示为一个盒子或一条线，每个电子表示为一个箭头。一个向上的箭头表示 1/2 的旋转，或自旋向上，一个向下的箭头表示 1/2 的旋转，或自旋向下。因此，氢的轨道图 有一个向上的箭头。氦的电子构型是 1s^2。这两个电子有三个相同的量子数，因为它们属于同一个壳层和亚壳层。根据泡利不相容原理，它们的自旋量子数是不同的。自旋相反的电子如果占据同一轨道 就称为"成对电子"对于原子序数为 3 的锂，1s 轨道上的两个电子是成对的，2s 轨道上的电子是不成对的。传统上，不成对的电子 被表示为自旋向上。

保利排除原则（pep）亦称：泡利不相容原理

电子在原子轨道上的排列称为其电子构型。我们用一个包含三个信息的符号来描述一个电子结构：
 
 <ol>
 <li>主量子壳的数量 n </ li>
 <li>指定轨道类型的字母（子壳， l ）和</ li>
 <li>上标数字，指定该特定子壳中的电子数。</ li>
 </ ol>
 
 例如，符号2 p 4 表示 p 子壳（ l = 1 ）的主量子数（ n ）为2。符号3 d＆nbsp; 8 表示 d n = 3的主体外壳的em>子外壳（ l = 2）。
 
 尽管这三个量子数可以很好地描述电子轨道，但一些实验表明它们不足以解释所有观察到的结果。在1920年代证明，当以极高的分辨率检查氢谱线时，某些谱线实际上不是单个峰，而是成对的紧密间隔的谱线。这就是所谓的光谱精细结构，这意味着即使电子位于同一轨道上，电子的能量也会存在其他小的差异。这些发现导致塞缪尔·古德斯米特（Samuel Goudsmit）和乔治·乌伦贝克（George Uhlenbeck）提出电子具有第四量子数。他们称其为自旋量子数或 m s 。
 
 在施加的磁场中，电子具有两种可能的方向，它们具有不同的能量，一个方向向上旋转，与磁场对齐；另一个方向向下旋转，与磁场对齐。
 
 第四个量子数，自旋量子数（ m s ）描述了电子的这两种不同的自旋态。自旋量子数具有两个可能的值，即-1/2（向下旋转）和+1/2（向上旋转）。
 
 电子自旋描述了本征电子的“旋转”。或“旋转”。每个电子都可以充当具有角动量的微小磁体或微小旋转物体，或具有电流的回路，即使无法从空间坐标的角度观察到这种旋转或电流。
 
 整个电子自旋的大小只能有一个值，而电子只能“自旋”。处于两个量化状态之一。一种被称为＆alpha;状态，旋转的 z 分量位于 z轴的正方向。这对应于自旋量子数 m s = + 1 / 2 。另一个称为＆beta;。状态，自旋的z分量为负，并且 m s =＆minus; 1 / 2 。
 
 任何电子，无论其位于哪个原子轨道上，都只能具有自旋量子数的这两个值之一。如果施加外部磁场，具有不同自旋的电子的能量将不同。
 
 原子中的电子完全由四个量子数描述： n ， l ， m l 和 m s 。前三个量子数定义了轨道，并且是相互依赖的，而第四个量子数则独立于其他量子数，因为它描述了一种称为自旋的内在电子性质。奥地利物理学家沃尔夫冈·保利（Wolfgang Pauli）（诺贝尔物理学奖：1945年）制定了一条总则，给出了我们了解原子中电子的总行为所需要的最后一条信息。保利排斥原理可以表述为：同一原子中的两个电子在所有四个量子数中都不能具有完全相同的集合。这意味着两个电子可以共享相同的轨道（同一组量子数 n ， l 和 m l ），除非它们的自旋量子数具有不同的值。由于自旋量子数（ m s ）只能具有两个值+1/2和-1/2，因此最多可以有两个电子占据相同的轨道（并且如果两个电子位于同一轨道上，则它们必须具有相反的自旋）。因此，任何原子轨道只能由零个，一个或两个电子组成。
 
 电子构型的轨道图样式将被占据子壳内的每个轨道表示为框或线，将每个电子表示为箭头。氢的轨道图（电子构型为1 s 1 ）为：
 
 <img alt =“ Figure1” src =“ https://www.jove.com/files/core/CDNSource/lessons/984/984Image_1.png” style =“ height：40px;” /> 
 
 向上箭头表示正向减半旋转或向下旋转，而向下箭头表示负向半减旋转或向下旋转。因此，氢的轨道图具有一个向上的箭头。
 
 氦的电子构型为1 s 2 。 这两个电子具有三个相同的量子数，因为它们属于相同的壳和子壳。 根据保利排除原理，它们的自旋量子数是不同的。 具有相反自旋的电子如果占据相同的轨道，则称为成对电子。
 
 <img alt =“ Figure2” src =“ https://www.jove.com/files/core/CDNSource/lessons/984/984Image_2.png” style =“ height：40px;” /> 
 
 此文本改编自<a 6-3-development-of-quantum-theory =“” books =“” chemistry-2e =“” href =“” https：=“” openstax.org = “” pages =“”> Openstax，化学2e，第6.3节：量子理论的发展。</a>

保利排除原则（PEP）亦称：泡利不相容原理

原子轨道中电子的排布称为电子构型。我们用一种符号来描述电子构型，该符号……

Education

教育

JoVE Core

Chemistry

化学

Electronic Structure of Atoms

原子的电子结构

KEY TERMS AND CONCEPTS

关键术语与概念

electron spin and orbital pairing

Electron spin and orbital pairing explain why only two electrons can fit in one atomic orbital. Electron configuration describes how electrons are arranged in an atom’s orbitals. It is written with three parts: the principal quantum number, n, the orbital type, or subshell, and a superscript that shows how many electrons are in that subshell. For example, 2p4 means four electrons in a p subshell with n = 2, and 3d8 means eight electrons in a d subshell with n = 3.

The three quantum numbers describe orbitals well, but they do not explain every experimental result. In the 1920s, very high-resolution hydrogen spectra showed fine structure, where some lines split into two close peaks instead of one. This suggested that electrons in the same orbital can still have small energy differences. Samuel Goudsmit and George Uhlenbeck proposed a fourth quantum number to explain this behavior. They called it the spin quantum number, ms.

An electron can have two spin states in an external magnetic field. One state is spin up, which aligns with the magnetic field, and the other is spin down, which points against it. The spin quantum number has only two values, +1/2 and -1/2. These are also called the alpha and beta states. Electron spin is an intrinsic property, so each electron behaves like a tiny magnet, even though this motion cannot be observed in normal space.

A complete electron description uses four quantum numbers: n, l, ml, and ms. The first three quantum numbers define the orbital and depend on one another. The spin quantum number is independent because it describes the electron’s spin. Austrian physicist Wolfgang Pauli used this idea to state the Pauli exclusion principle. It says that no two electrons in the same atom can have the same set of all four quantum numbers.

This principle means that two electrons may share the same orbital only if their spin quantum numbers are different. Because ms has only two possible values, no more than two electrons can occupy one orbital. An atomic orbital can therefore hold zero, one, or two electrons. In orbital diagrams, each orbital is shown as a box or line and each electron as an arrow. A hydrogen atom with configuration 1s1 has one upward arrow, which means one spin-up electron.

Helium has the configuration 1s2. Its two electrons share the same shell and subshell, so they have three identical quantum numbers. They differ in spin, which follows the Pauli exclusion principle. Electrons with opposite spins in the same orbital are called paired.

Electron Spin and Orbital Pairing

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Learn how electron spin, quantum numbers, and orbital pairing explain why each atomic orbital holds at most two electrons in high school chemistry.

Electron Spin and Orbital Pairing | JoVE K12

chemistry

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9-12

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Structure And Properties Of Matter

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electron-configuration-and-chemical-properties

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IPC.7.A

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IPC.7.B

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IPC.7.C

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Electron Spin and Orbital Pairing

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