Name: shielding and orbital penetration
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 3 min 21 s
Channel: JoVE

K12 Standards

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Chemistry and Physics

IPC.7

IPC.7.E

The Energies of Atomic Orbitals

Атомные орбитали имеют разную энергию, что объясняется кулоновскими взаимодействиями, эффектом экранирования и проникновением орбит. Закон Кулона указывает на то, что сила притяжения или отталкивания между двумя заряженными частицами имеет обратную квадратичную зависимость от расстояния между ними. Размеры атомных орбиталей увеличиваются с увеличением числа оболочек, и электроны отталкиваются от пространства, занимаемого орбиталями нижних оболочек.Таким образом, закон Кулона предполагает, что по мере увеличения числа оболочек электроны испытывают меньшее притяжение к ядру, что соответствует более высоким орбитальным энергиям. Кроме того, электроны, которые находятся примерно на том же расстоянии от ядра или ближе к нему, создают экранирующий эффект, который еще больше снижает притяжение к ядру. Чем больше экранирование, тем меньше ощущается притяжение к ядру.Это одна из причин различий в орбитальных энергиях внутри электронных оболочек. Например, 3s-и 3p-электроны значительно экранируют 3d-электроны. Эффективный заряд ядра, воспринимаемый электроном, рассчитывается путем вычитания константы экранирования S, которая зависит от количества экранирующих электронов и занимаемых ими подуровней, из атомного номера.Например, два 1s-электрона в литии, имеющем атомный номер три, экранируют его 2s-электрон. Константа экранирования для этого электрона определяется по полуэмпирическим правилам и равна 1, 7. Следовательно, эффективный заряд ядра, воспринимаемый 2s-электроном, равен 1, 3.Форма орбиталей также определяет их энергию. Если электроны на внешней орбитали могут перемещаться далеко в области, занятые внутренними электронами, чтобы быть ближе к ядру, они будут намного меньше экранироваться там. Таким образом, энергия этой внешней орбитали ниже.Это можно визуализировать с помощью функции радиального распределения, описывающей вероятность обнаружения электрона на заданном расстоянии от ядра. Графики функции радиального распределения для подуровней 1s, 2s и 2p показывают, что 2s-электроны имеют довольно низкую вероятность нахождения рядом с ядром, тогда как 2p-электроны в основном остаются за пределами или на внешнем крае 1s-области. Поэтому говорят, что орбиталь 2s обладает большей проникающей способностью.В третьей оболочке 3s-электроны проникают больше всего, а 3d-электроны проникают меньше всего. Как правило, орбитальная энергия атома возрастает с увеличением номера оболочки, а для под-уровней от s к f. Однако эффект проникновения становится настолько значительным в четвертой и пятой оболочках, что орбитали 4s и 5s часто имеют более низкие относительные энергии, чем орбитали 3d и 4d соответственно.

Энергии атомных орбиталей

В атоме отрицательно заряженные электроны притягиваются к положительно заряженным ядру. В многоэлектронном атоме также наблюдаются электрон-электронные отталкивания. Притягивающие и отталкивающие силы зависят от расстояния между частицами, а также знака и величины зарядок отдельных частиц. Когда заряды частиц противоположны, они притягивают друг друга. Если оба частицы заряжена одинаково, они отталкивают друг друга. 
 
 По мере увеличения величины зарядов увеличивается сила. Однако, когда разделение зарядов больше, силы уменьшаются. Таким образом, сила притяжения между электроном и его ядром прямо пропорциональна расстоянию между ними. Если электрон ближе к ядру, он более тесно связан с ядром; поэтому электроны в разных оболочках (на разных расстояниях) имеют разные энергии. 
 
 Для атомов с несколькими энергетическими уровнями внутренние электроны частично экранируют внешние электроны от вытягивания ядра из-за электрон-электронных отталкиваний. Электроны ядра экранируют электроны в наружных оболочках, в то время как электроны в одной и той же оболочке валентности не блокируют притяжения, испытываемые друг другом, так же эффективно. Это можно объяснить концепцией эффективного ядерного заряда, Zeff. Это тяга, оказываемая ядром на конкретный электрон с учетом любых электрон-электронных отталкиваний. Для водорода существует только один электрон, поэтому ядерный заряд (Z) и эффективный ядерный заряд (Zeff) равны. Для всех остальных атомов внутренние электроны частично экранируют внешние электроны от вытягивания ядра, таким образом: 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/985/985_Equations-01.png"" style=""height: 40px;"" />
 
 Орбитальное проникновение описывает способность электрона быть ближе к ядру. Электроны на S-орбитали могут приблизиться к ядру и обладать более проникающей способностью.Плотность вероятности для сферической s -орбитали отлична от нуля в ядре. & nbsp; Различные подоболочки имеют разную пространственную ориентацию. Из-за того, что орбиталь имеет форму гантели, p -электрон проникает гораздо меньше. Его волновая функция имеет узел, проходящий через ядро, где вероятность найти электрон равна нулю. Таким образом, s -орбитальный электрон более прочно связан с ядром и имеет меньшую энергию, чем p -электрон. d -электрон имеет даже меньшее проникновение и более высокую энергию, чем орбитальный электрон p . 
 Для различных оболочек и подоболочек тенденцию проникающей способности электрона можно изобразить следующим образом 
 
 <img alt=""Eq2"" src=""https://www.jove.com/files/core/CDNSource/lessons/985/985_Equations-02.png"" style=""height: 40px;"" />
 
 Эффект экранирования и проникновения большой, и электрон 4s может иметь меньшую энергию, чем электрон 3d. 
 
 Этот текст адаптирован из <a href=""https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations"">Openstax, Химия 2е изд., раздел 6.4: Электронная структура атомов (конфигурации электронов).</a>

В атоме отрицательно заряженные электроны притягиваются к положительно заряженному ядру. В многоэлектронном атоме электрон-электронные отталкивания...

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Electronic Structure of Atoms

Электронное строение атомов

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

shielding and orbital penetration

Shielding and orbital penetration help explain why electrons in an atom have different energies. Electrons are attracted to the positive nucleus, but they also repel one another in atoms with more than one electron. The strength of these forces depends on the charge and the distance between the particles. Opposite charges attract, like charges repel, and the force gets weaker as the particles move farther apart.

An electron that stays closer to the nucleus is held more tightly. That means electrons in different shells can have different energies because they sit at different distances from the nucleus. In atoms with several energy levels, inner electrons partly block the nucleus from outer electrons. This blocking is called shielding, and it happens because of electron-electron repulsions.

The pull felt by one electron after shielding is the effective nuclear charge, or Zeff. It describes the attraction from the nucleus after repulsions are taken into account. For hydrogen, which has only one electron, Z and Zeff are the same. In atoms with more than one electron, core electrons shield outer electrons, while electrons in the same valence shell do not block the nuclear pull very well.

Orbital penetration describes how close an electron can get to the nucleus. s orbitals penetrate best because their spherical probability density is non-zero at the nucleus. p orbitals penetrate less because their dumbbell shape has a node through the nucleus, where the chance of finding an electron is zero. d orbitals penetrate even less than p orbitals.

Because of shielding and penetration, s electrons are usually bound more tightly and have lower energy than p electrons. d electrons have higher energy than p electrons. These effects can be strong enough that a 4s electron may have lower energy than a 3d electron.

Shielding and Orbital Penetration

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the energies of atomic orbitals

Shielding and orbital penetration help explain why electrons in an atom have different energies. Electrons are attracted to the positive nucleus, but they also repel one another in atoms with more than one electron. The strength of these forces depends on the charge and the distance between the particles. Opposite charges attract, like charges repel, and the force gets weaker as the particles move farther apart. An electron that stays closer to the nucleus is held more tightly. That means...

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electron configuration of multielectron atoms

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Valence electrons and noble gas shorthand help show how electrons are arranged in multielectron atoms. Sodium, the alkali metal with atomic number 11, has one more electron than neon. That extra electron goes into the lowest-energy subshell available, the 3s orbital, so sodium has the configuration 1s2 2s2 2p6 3s1. Electrons in the outermost shell, where n is highest, are called valence electrons. Electrons in the inner shells are called core electrons. Because core electrons often match a...

key terms and concepts

Learn how shielding, effective nuclear charge, and orbital penetration affect electron energy levels, and why s, p, and d orbitals differ in energy.

Shielding and Orbital Penetration | JoVE K12

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Shielding and Orbital Penetration

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