Name: electron affinity trends on the periodic table
Uploaded: 2020-09-03T13:38:28.000Z
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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.2.2

Electron Affinity

Wenn ein Elektron zu einem gasförmigen Atom hinzugefügt wird, beobachtet man eine Energieänderung, Elektronaffinität genannt. Elektronenaffinität misst die Leichtigkeit, ein Elektron durch ein Atom zu gewinnen. Zum Beispiel beträgt die Elektronenaffinität von Chlor 348, 6 kJ/mol.Das negative Vorzeichen zeigt an, dass es sich um eine exotherme Umsetzung handelt. Argon hat jedoch eine positive Elektronenaffinität, die darauf hinweist, dass die Bildung eines Argon-Anions die Bereitstellung von Energie erfordert. Im Allgemeinen gilt:Je größer die Anziehungskraft zwischen einem Atom und einem hinzugefügtem Elektron, desto negativer ist die Elektronenaffinität.Elektronenaffinitäten, ähnlich den Ionisationsenergien, zeigen Trends im Periodensystem auf. Nach unten in Gruppe 1, nimmt die atomare Größe zu, wenn die Elektronen höhere Hauptquantenzahlen besetzen. Eingehende Elektronen, erleben daher weniger Kernanziehung, was zu weniger negativen Elektronenaffinitäten führt.Es gibt jedoch Ausnahmen. Bei den Halogenen hat Chlor einen negativeren Elektronenaffinitätswert als Fluor. Aber warum?Fluor ist das kleinste Atom unter den Halogenen und ein eintreffendes Elektron erlebt eine deutliche Abstoßung der bereits vorhandenen Elektronen. Im Chlorid-Anion hingegen wird das neue Elektron in der dritten Hülle eingefügt und nimmt mehr Platz ein. Dadurch werden die Elektron-Elektron-Abstoßungen verringert, wodurch es für ein Elektron attraktiver wird, aufgenommen zu werden.Im Allgemeinen gilt das in einer Periode, die Elektronenaffinitäten negativer werden. Halogene haben die negativsten Elektronenaffinitäten, da das einströmende Elektron zum Erreichen der Edelgaskonfiguration beiträgt. Im Vergleich dazu haben Edelgase eine vollständig gefüllte Hülle.Das ankommende Elektron muss in der höheren Hauptenergieebene untergebracht werden, was energetisch gesehen ungünstig ist. Daher sind die Elektronenaffinitäten für diese Elemente positiv. Gruppe 2 zeigt Ausnahmen.Die Elektronenkonfiguration zeigt an, dass das ankommende Elektron in eine Unterschale mit höherer Energie eintreten muss. Daher sind die Elektronenaffinitätswerte entweder positiv oder weniger exotherm. Interessanterweise hat Gruppe 15 weniger negative Elektronenaffinitäten als Gruppe 14.Vergleichen Sie Phosphor und Silizium. Anders als Silizium, hat Phosphor eine halbgefüllte p Unterschale und das aufgenommene Elektron muss sich mit einem Elektron paaren, das sich bereits im p-Orbital befindet. Dies würde die Elektron-Elektron-Abstoßungen erhöhen und ist daher ein energetisch ungünstiger Prozess, der sich auch in der geringeren negativen Elektronenaffinität im Vergleich zu Silizium reflektiert.

elektronenaffinität

Die Elektronenaffinität (EA) ist die auftretende Energieänderung beim Hinzufügen eines Elektrons zu einem gasförmigen Atom, wobei ein Anion (negatives Ion) gebildet wird. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1050/1050_Equations-01.png" style="height: 40px;"> Dieser Prozess kann je nach Element entweder endotherm oder exotherm sein. Viele der Elemente haben negative EA-Werte, was bedeutet, dass Energie freigesetzt wird, wenn das gasförmige Atom ein Elektron aufnimmt. Bei einigen Elementen ist jedoch Energie erforderlich, damit das Atom negative Ladung aufnehmen kann und der Wert ihres EA ist positiv. Genau wie bei der Ionisierungsenergie entsprechen die nachfolgenden EA-Werte der Bildung von Ionen mit höherer Ladung. Der zweite EA ist die Energie, die mit der Addition eines Elektrons an ein Anion zur Bildung eines zweifach negativ geladenen 2– Ions usw. verbunden ist. Wie anzunehmen, wird es einfacher, einem Atom ein Elektron hinzuzufügen, wenn die effektive Kernladung der Atome zunimmt. Wenn wir uns entlang einer Periode hinweg von links nach rechts bewegen, neigen die EAs dazu, negativer zu werden. Die Ausnahmen zwischen den Elementen der Gruppe 2 (2A), Gruppe 15 (5A) und Gruppe 18 (8A) können anhand der elektronischen Struktur dieser Gruppen erklärt werden. Die Edelgase der Gruppe 18 (8A) haben eine vollständig gefüllte Hülle und das einfallende Elektron muss auf einem höheren n-Niveau hinzugefügt werden, energetisch ungünstig ist. Gruppe 2 (2A) hat eine gefüllte ns-Unterschale, und daher geht das nächste hinzugefügte Elektron in das np mit höherer Energie, sodass der beobachtete EA-Wert sich nicht verhält, wie es die vorherige Annahme vorhersagen würde. Schließlich hat Gruppe 15 (5A) eine halbgefüllte NP-Unterschale, und das nächste Elektron muss mit einem vorhandenen NP-Elektron gepaart werden. In all diesen Fällen ist die relative Stabilität der Elektronenkonfiguration verantwortlich für das abweichende Verhalten der EA-Zunahme. Man könnte erwarten, dass das Atom an der Spitze jeder Gruppe den negativsten EA aufweist; Ihre ersten Ionisierungspotentiale lassen darauf schließen, dass diese Atome die größten effektiven Kernladungen haben. Wenn wir uns jedoch in einer Gruppe nach unten bewegen, sehen wir, dass das zweite Element in der Gruppe am häufigsten den negativsten EA aufweist. Dies ist auf die geringe Größe der n = 2 Schale und die daraus resultierenden großen Elektron-Elektron-Abstoßungen zurückzuführen. Beispielsweise hat Chlor mit einem EA-Wert von –348 kJ/mol den höchsten Wert aller Elemente im Periodensystem. Der EA von Fluor beträgt –322 kJ/mol. Wenn wir einem Fluoratom ein Elektron hinzufügen, um ein Fluoridanion (F–) zu bilden, fügen wir der n = 2 Schale ein Elektron hinzu. Das Elektron wird vom Kern angezogen, es kommt jedoch auch zu einer erheblichen Abstoßung durch die anderen Elektronen, die bereits in dieser kleinen Valenzschale vorhanden sind. Das Chloratom hat in der Valenzschale die gleiche Elektronenkonfiguration, da das eintretende Elektron jedoch in die n = 3-Schale gelangt, hält es sich in einem erheblich größeren Raumbereich auf und die Elektron-Elektron-Abstoßungen sind geringer verglichen zu F-. Das eintretende Elektron erfährt keine so starke Abstoßung und das Chloratom nimmt das zusätzliche Elektron leichter auf was zu einem negativeren EA führt. Dieser Text wurde adaptiert von <a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties">OpenStax Chemistry 2e, Section 6.5: Periodic Variations in Element Properties.</a>

Elektronenaffinität

Die Elektronenaffinität (EA) ist die Energieänderung, die durch das Hinzufügen eines Elektrons zu einem gasförmigen Atom entsteht, um ein Anion (negatives Ion) zu bilden.

Dieser Prozess kann entweder ...

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Periodic Properties of the Elements

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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

electron-affinity-trends-on-the-periodic-table

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...

ionization energy

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...

atomic radii and effective nuclear charge

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Periodic Trends in Atomic Size

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ionic radii

How Ionic Charge Changes Ion Size

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...

halogens

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...

noble gases

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...

key terms and concepts

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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The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



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Electron Affinity Trends on the Periodic Table

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