Name: electron affinity trends on the periodic table
Uploaded: 2020-09-03T13:38:28.000Z
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Electron Affinity

Когда электрон присоединяется к газообразному атому, наблюдается изменение энергии, называемое сродством к электрону. Сродство к электрону измеряет легкость связывания электрона атомом. Например, сродство к электрону хлора составляет 348, 6 килоджоулей на моль.Отрицательный знак означает, что это экзотермическое изменение. Однако аргон имеет положительное сродство к электрону, что указывает на то, что для образования аниона аргона требуется приток энергии. В общем, чем больше притяжение между атомом и добавленным электроном, тем более отрицательно его сродство к электрону.Сродство к электрону, подобно энергии ионизации, показывает тенденции в периодической таблице. При движении вниз по 1-й группе размер атома увеличивается, поскольку электроны занимают более высокие главные квантовые числа. Следовательно, входящие электроны испытывают меньшее ядерное притяжение, что приводит к меньшему отрицательному сродству к электрону.Однако бывают исключения. В галогенах хлор имеет более отрицательное значение сродства к электрону, чем фтор. Но почему?Фтор самый маленький атом из галогенов, и входящий электрон испытывает значительное отталкивание от уже имеющихся электронов. Однако в хлорид-анионе новый электрон добавляется в третью оболочку, занимающую больше места. Это уменьшает электрон-электронное отталкивание, делая прибывший электрон более притягательным.Как правило, при переходе через период сродство к электрону становится более отрицательным. Галогены имеют самое отрицательное сродство к электрону, поскольку входящий электрон помогает достичь конфигурации благородного газа. Для сравнения:благородные газы имеют полностью заполненную оболочку.Входящий электрон должен быть размещен на более высоком основном энергетическом уровне, что является энергетически невыгодным. Таким образом, сродство к электрону для этих элементов положительно. Во 2-й группе есть исключения.Электронная конфигурация показывает, что входящий электрон должен войти в подуровень с более высокой энергией. Таким образом, значения сродства к электрону либо положительны, либо менее экзотермичны. Интересно, что 15-я группа имеет меньшее отрицательное сродство к электрону, чем 14-я.Сравните фосфор и кремний. В отличие от кремния, фосфор имеет наполовину заполненный p-подуровень, и входящий электрон должен быть спарен с электроном, уже находящимся на p-орбитали. Это увеличило бы электрон-электронное отталкивание и, следовательно, является энергетически невыгодным процессом, что также отражается в меньшем отрицательном сродстве к электрону по сравнению с кремнием.

Электронное сродство

Сродство к электрону (ε) — это изменение энергии для добавления электрона в газообразный атом, образуя анион (отрицательный ион). 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/1050/1050_Equations-01.png"" style=""height: 40px;"" />
 
 Этот процесс может быть как эндотермическим, так и экзотермическим, в зависимости от элемента. Многие из этих элементов имеют отрицательные значения ε, что означает, что энергия высвобождается, когда газообразный атом принимает электрон. Однако для некоторых элементов требуется энергия для того, чтобы атом был отрицательно заряжен, и ценность их ε положительна. Как и в случае энергии ионизации, последующие значения связаны с образованием ионов с большим количеством заряда. Второй советник — это энергия, связанная с добавлением электрона в анион, чтобы сформировать 2-ион, и так далее. 
 
 Как можно было бы предсказать, по мере увеличения эффективного ядерного заряда атомов становится легче добавлять электрон через ряд атомов. По мере того как мы идем слева направо в течение некоторого времени, ε, как правило, становится более отрицательной. Исключения, найденные среди элементов группы 2 (2A), группы 15 (5A) и группы 18 (8A), можно понять на основе электронной структуры этих групп. Благородные газы, группа 18 (8A), имеют полностью заполненную оболочку, и входящий электрон должен быть добавлен на более высокий n уровень, что более трудно сделать. Группа 2 (2A) имеет заполненную оболочку НС, поэтому следующий добавленный электрон попадает в более высокую энергетическую np, поэтому, опять же, наблюдаемое значение ε не соответствует тренду. Наконец, группа 15 (5A) имеет полузаполненную np-подоболочку, и следующий электрон должен быть сопряжен с существующим np-электроном. Во всех этих случаях начальная относительно стабильная электронная конфигурации нарушает тенденцию в ε. 
 
 Можно было бы ожидать, что атом в верхней части каждой группы будет иметь самый отрицательный ε; их первые потенциалы ионизации предполагают, что эти атомы имеют самые большие эффективные ядерные заряды. Однако, по мере того как мы перемещаемся вниз по группе, мы видим, что второй элемент в группе чаще всего имеет самый отрицательный советник. Это может быть связано с малым размером оболочки n = 2 и возникающей в результате большой электронно-электронной отталкивания. Например, хлор, значение ε которого составляет –348 кДж/моль, имеет наибольшее значение для любого элемента периодической таблицы. ε фтора составляет –322 кДж/моль. При добавлении электрона к атому фтора для образования аниона фторида (F–), мы добавляем электрон в оболочку n = 2. Электрон притягивается к ядру, но есть и значительное отталкивание от других электронов, уже присутствующих в этой небольшой валентной оболочке. Атом хлора имеет одинаковую электронную конфигурацию в оболочке валентности, но поскольку входящий электрон попадает в оболочку n = 3, он занимает значительно большую область пространства и уменьшается электрон-электрон отталкивания. Входящий электрон не испытает столько отталкивания, и атом хлора принимает дополнительный электрон более охотно, что приводит к более отрицательному ε. 
 
 Этот текст адаптирован из <a href=""https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties"">OpenStax Химия 2е изд., раздел 6.5: Периодические изменения свойств элемента.</a>

Сродство к электрону (EA) — это изменение энергии при добавлении электрона к газообразному атому с образованием аниона (отрицательного иона).

Этот процесс может быть как ...

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electron affinity trends on the periodic table

Electron affinity is the energy change that happens when a gaseous atom gains an electron and becomes a negative ion, or anion. For many elements, this process releases energy, so the electron affinity value is negative. For some elements, energy must be added instead, so the value is positive.

Electron affinity also applies when more than one electron is added. The second electron affinity is the energy change for adding an electron to an anion to make a 2− ion. Later electron affinities follow the same idea for ions with even more charge.

Across a period, electron affinity usually becomes more negative from left to right. This pattern fits the increase in effective nuclear charge, which makes it easier for atoms to attract an added electron. Some groups do not follow the trend well, though. Group 2 has a filled ns subshell, group 15 has a half-filled np subshell, and group 18 has a full shell, so the next electron must enter a less favorable state.

These exceptions are linked to electron arrangement and stability. Noble gases must place the new electron in a higher n level. Group 2 elements add the electron to a higher-energy np orbital. Group 15 elements must pair the new electron with one already in the np subshell. In each case, the stable starting configuration changes the expected electron affinity.

Down a group, the most negative electron affinity is often found in the second element rather than the top one. The small size of the n = 2 shell can create stronger electron-electron repulsions. That is why chlorine has a more negative electron affinity than fluorine. Chlorine’s value is −348 kJ/mol, while fluorine’s is −322 kJ/mol. When fluorine gains an electron, the added electron enters the small n = 2 shell. In chlorine, the electron goes into the larger n = 3 shell, so repulsion is lower and the atom accepts the electron more readily.

Electron Affinity Trends on the Periodic Table

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periodic classification of the elements

How Valence Electrons Shape the Periodic Table

The periodic table organizes elements by increasing atomic number, and it also shows why elements in the same group behave alike. As the electron configurations are placed into the table, a repeating pattern appears in the outer shells of atoms. These outer electrons are called valence electrons, and they play the biggest role in chemical reactions. They are the highest-energy electrons in an atom, so they are easier to lose or share than core electrons. Valence electrons also help determine...

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Ionization Energy Trends in the Periodic Table

Ionization energy is the energy needed to remove an electron from a gaseous atom or ion in its ground state. The first ionization energy, or IE1, removes the most loosely bound electron and forms a 1+ cation. The second ionization energy, IE2, removes the next electron, and each later ionization energy removes another electron from the atom or ion. Ionization always requires energy, so these processes are endothermic. That means ionization energy values are always positive. In general, larger...

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Periodic Trends in Atomic Size

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Ionic radius describes the size of an ion. It changes when an atom gains or loses electrons. Cations are positive ions with fewer electrons than the parent atom, and they are smaller than the atoms they come from. Aluminum shows this pattern clearly. A neutral aluminum atom has a covalent radius of 118 pm, while Al3+ has an ionic radius of 68 pm. When electrons are removed from the outer valence shell, the remaining core electrons feel a stronger effective nuclear charge, or Zeff. That pull...

electron affinity

Electron affinity is the energy change that happens when a gaseous atom gains an electron and becomes a negative ion, or anion. For many elements, this process releases energy, so the electron affinity value is negative. For some elements, energy must be added instead, so the value is positive. Electron affinity also applies when more than one electron is added. The second electron affinity is the energy change for adding an electron to an anion to make a 2− ion. Later electron affinities...

alkali metals

Alkali Metals: Reactivity Trends

Alkali metals are Group 1 elements with one valence electron. They are soft, shiny metallic solids that are malleable, ductile, and good conductors of heat and electricity. Their melting points are unusually low for metals and drop as you move down the group. Density generally increases down the group, except for potassium. The trend in atomic size helps explain their behavior. Moving from lithium to cesium, each element has a higher principal quantum number, so the outer electron is farther...

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Halogens: Trends and Reactions

Halogens are Group 17 nonmetals that show clear trends in structure and reactivity. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Astatine is highly unstable and radioactive, so most of its properties are still unknown because it has a very short half-life. Tennessine is a synthetic element that is also predicted to belong to this group. These elements do not usually exist as single atoms. Instead, they form diatomic molecules, which means...

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Noble Gases: Sources, Uses, and Reactions

Noble gases are the group 18 elements helium, neon, argon, krypton, xenon, and radon. They were called “noble” because they were once thought to be nonreactive. That idea changed in 1962, when Dr. Neil Bartlett at the University of British Columbia showed that this assumption was not fully correct. These gases occur in very small amounts in the atmosphere. Some natural gas contains 1–2% helium by mass. Helium is separated from natural gas by removing the condensable components, which leaves...

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Learn how electron affinity changes across the periodic table, why some groups are exceptions, and why chlorine has a more negative value than fluorine.

Electron Affinity Trends on the Periodic Table | JoVE K12

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Electron Affinity Trends on the Periodic Table

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