Name: electron affinity trends on the periodic table
Uploaded: 2020-09-03T13:38:28.000Z
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K12 Standards

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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

Lorsqu'un électron est ajouté à un atome gazeux, un changement d'énergie est observé appelé affinité électronique. Affinité électronique mesure la facilité de gagner un électron par un atome. Par exemple, l'affinité électronique du chlore est de 348, 6 kilojoules par mole.Le signe négatif indique qu'il s'agit d'un changement exothermique. L'argon, cependant, a une affinité électronique positive, indiquant que la formation d'un anion d'argon nécessite de l'énergie pour être fournie. En général, plus l'attraction entre un atome et un électron ajouté est grande, plus l'affinité électronique est négative.Les affinités électroniques, semblables aux énergies d'ionisation, montrent des tendances dans le tableau périodique. En descendant le groupe 1, la taille atomique augmente à mesure que les électrons occupent des nombres quantiques principaux plus élevés. Les électrons entrants subissent donc moins d'attraction nucléaire, ce qui entraîne des affinités électroniques moins négatives.Cependant, il y a des exceptions. Dans les halogènes, le chlore a une valeur d'affinité électronique plus négative que le fluor. Mais pourquoi?Le fluor est le plus petit atome des halogènes et un électron entrant subit une répulsion importante de la part des électrons déjà présents. Dans l'anion chlorure, cependant, le nouvel électron est ajouté dans la troisième couche, occupant plus d'espace. Cela réduit la répulsion électron-électron, ce qui rend plus attrayant le gain d'un électron.Généralement, en se déplaçant sur une période, les affinités électroniques deviennent plus négatives. Les halogènes ont les affinités d'électrons les plus négatives, car l'électron entrant aide à réaliser des configurations de gaz rares. En comparaison, les gaz rares ont une shell complètement remplie.L'électron entrant doit être logé dans le niveau d'énergie principal plus élevé, ce qui est énergétiquement défavorable. Ainsi, les affinités électroniques pour ces éléments sont positives. Le groupe 2 montre les exceptions.La configuration électronique indique que l'électron entrant doit entrer dans une sous-shell d'énergie plus élevée. Ainsi, les valeurs d'affinité électronique sont soit positives, soit moins exothermiques. Il est intéressant de noter que le groupe 15 a moins d'affinités électroniques négatives que le groupe 14.Comparez le phosphore et le silicium. Contrairement au silicium, le phosphore a une sous-couche p à moitié remplie et l'électron entrant doit être jumelé à un électron résidant déjà dans l'orbitale p. Cela augmenterait les répulsions électron-électron et est donc un processus énergétiquement défavorable, qui se reflète également dans l'affinité électronique moins négative par rapport au silicium.

affinité électronique

L'affinité électronique (Ae) est la variation d'énergie qui permet d'ajouter un électron à un atome gazeux pour former un anion (ion négatif). 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/1050/1050_Equations-01.png" style="height: 40px;" />
 
 Ce processus peut être endothermique ou exothermique, selon l'élément. Beaucoup de ces éléments ont des valeurs de Ae négatives, ce qui signifie que de l'énergie est libérée lorsque l'atome gazeux accepte un électron. Cependant, pour certains éléments, il faut de l'énergie pour que l'atome devienne chargé négativement, et la valeur de leur Ae est positive. Tout comme pour l'énergie d'ionisation, les valeurs de Ae suivantes sont associées à la formation d'ions ayant plus de charge. La deuxième Ae est l'énergie associée à l'ajout d'un électron à un anion pour former un ion 2–, et ainsi de suite. 
 
 Comme on pourrait le prévoir, il devient plus facile d'ajouter un électron à une série d'atomes à mesure que la charge nucléaire effective des atomes augmente. Lorsque nous allons de gauche à droite sur une période donnée, les Ae ont tendance à devenir plus négatives. Les exceptions qui se trouvent parmi les éléments du groupe 2 (2A), du groupe 15 (5A) et du groupe 18 (8A) peuvent être expliquées d'après la structure électronique de ces groupes. Les gaz nobles, groupe 18 (8A), ont une couche entièrement remplie, et l'électron entrant doit être ajouté à un niveau n plus élevé, ce qui est plus difficile à faire. Le groupe 2 (2A) a une sous-couche ns remplie, et donc le prochain électron qui est ajouté va sur le niveau d'énergie np plus élevée, donc encore une fois, la valeur de Ae qui est observée ne correspond pas à ce que la tendance pourrait prédire. Enfin, le groupe 15 (5A) a une sous-couche np à moitié remplie et le prochain électron doit être apparié à un électron np existant. Dans tous ces cas, la stabilité relative initiale de la configuration électronique perturbe la tendance pour l'Ae. 
 
 On pourrait s'attendre à ce que l'atome au sommet de chaque groupe ait l’Ae le plus négatif ; leurs premiers potentiels d'ionisation suggèrent que ces atomes ont les plus grandes charges nucléaires effectives. Cependant, à mesure que nous descendons d'un groupe, nous constatons que le deuxième élément du groupe a le plus souvent l'Ae le plus négatif. Ceci peut être attribué à la petite taille de la couche n = 2 et aux grandes répulsions électron-électron qui en résultent. Par exemple, le chlore, avec une valeur de Ae de –348 kJ/mol, possède la valeur la plus élevée de n'importe quel élément dans le tableau périodique. L'Ae du fluor est de –322 kJ/mol. Lorsque nous ajoutons un électron à un atome de fluor pour former un anion fluorure (F–), nous ajoutons un électron à la couche n = 2. L'électron est attiré par le noyau, mais il y a aussi une répulsion importante de la part des autres électrons déjà présents dans cette petite couche de valence. L'atome de chlore a la même configuration électronique pour la couche de valence, mais comme l'électron entrant va dans la couche n = 3, il occupe une région de l'espace beaucoup plus grande et les répulsions électron-électron sont réduites. L'électron entrant ne subit pas autant de répulsion, et l'atome de chlore accepte plus facilement un électron supplémentaire, ce qui se traduit par un Ae plus négatif. 
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties">OpenStax Chemistry 2e, Section 6.5 : Variations périodiques dans les propriétés des éléments.</a>

Affinité électronique

L’affinité électronique (EA) est le changement d’énergie nécessaire pour ajouter un électron à un atome gazeux pour former un anion (ion négatif).

Ce processus peut être soit ...

Education

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Periodic Properties of the Elements

Propriétés périodiques des éléments

KEY TERMS AND CONCEPTS

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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

Periodic trends in atomic size help explain why atoms get larger or smaller across the periodic table. Elements in the same group show similar chemical behavior because they have the same number and arrangement of valence electrons. Across a period, each new element adds one proton to the nucleus and one electron to the valence shell. Down a group, the number of valence electrons stays the same, but the principal quantum number increases by one. Atomic radius is not easy to define with perfect...

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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Study Of Matter

Periodic trends of the elements

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Periodic Table Trends: Atomic Radii, Ionization, and Electronegativity

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Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

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Electron Affinity Trends on the Periodic Table

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