Name: electron affinity trends on the periodic table
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 3 min 7 s
Channel: JoVE

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Chemistry

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Electron Affinity

Wanneer een elektron wordt toegevoegd aan een gasvormig atoom, wordt een verandering in energie waargenomen die elektronenaffiniteit wordt genoemd. Elektronenaffiniteit meet het gemak van het verkrijgen van een elektron door een atoom. De elektronenaffiniteit van chloor is bijvoorbeeld 348, 6 kilojoules per mol.Het minteken geeft aan dat het een exotherme verandering is. Argon heeft echter een positieve elektronenaffiniteit, wat aangeeft dat voor de vorming van een argonanion energie moet worden geleverd. In het algemeen geldt:hoe groter de aantrekkingskracht tussen een atoom en een toegevoegd elektron, hoe negatiever de elektronenaffiniteit.Elektronenaffiniteiten, vergelijkbaar met ionisatie-energieën, vertonen trends in het periodiek systeem. Als je in groep 1 naar beneden gaat, neemt de atomaire grootte toe naarmate de elektronen hogere hoofdkwantumgetallen bezetten. Binnenkomende elektronen ondervinden daarom minder nucleaire aantrekkingskracht, wat leidt tot minder negatieve elektronenaffiniteiten.Er zijn echter uitzonderingen. In halogenen heeft chloor een negatievere elektronenaffiniteitswaarde dan fluor. Maar waarom?Fluor is het kleinste atoom van de halogenen en een binnenkomend elektron ervaart een aanzienlijke afstoting van de reeds aanwezige elektronen. In het chloride-anion wordt het nieuwe elektron echter toegevoegd aan de derde schil, waardoor het meer ruimte inneemt. Dit vermindert de elektronen-elektronenafstotingen, waardoor het aantrekkelijker wordt voor een elektron om te worden aangetrokken.Over het algemeen worden elektronenaffiniteiten, verplaasend binnen een periode, negatiever. Halogenen hebben de meest negatieve elektronenaffiniteiten, omdat het binnenkomende elektron helpt bij het bereiken van edelgasconfiguraties. Ter vergelijking:edelgassen hebben een volledig gevulde schil.Het binnenkomende elektron moet worden ondergebracht in het hogere hoofdenergieniveau, dat energetisch ongunstig is. De elektronenaffiniteiten voor deze elementen zijn dus positief. Groep 2 toont uitzonderingen.De elektronenconfiguratie geeft aan dat het binnenkomende elektron een hogere energie-subschil moet binnengaan. De waarden van de elektronenaffiniteit zijn dus of positief, of minder exotherm. Interessant is dat groep 15 minder negatieve elektronenaffiniteiten heeft dan groep 14.Vergelijk fosfor en silicium. In tegenstelling tot silicium heeft fosfor een halfgevulde p-subschil en moet het binnenkomende elektron worden gepaard met een elektron dat zich al in de p-orbitaal bevindt. Dit zou de elektronen-elektronenafstoting vergroten en is daarom een energetisch ongunstig proces, wat ook tot uiting komt in de minder negatieve elektronenaffiniteit in vergelijking met silicium.

elektronenaffiniteit

De elektronenaffiniteit (EA) is de energieverandering voor het toevoegen van een elektron aan een gasvormig atoom om een anion (negatief geladen ion) te vormen. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1050/1050_Equations-01.png" style="height: 40px;"> Dit proces kan endotherm of exotherm zijn, afhankelijk van het element. Veel van deze elementen hebben negatieve EA-waarden, wat betekent dat er energie vrijkomt wanneer het gasvormige atoom een elektron accepteert. Voor sommige elementen is echter energie nodig om het atoom negatief geladen te laten worden, en de waarde van hun EA is positief. Net als bij ionisatie-energie worden daaropvolgende EA-waarden geassocieerd met de vorming van ionen met meer lading. De tweede EA is de energie die gepaard gaat met het toevoegen van een elektron aan een anion om een 2–ion te vormen, enzovoort. Zoals je zou kunnen voorspellen, wordt het gemakkelijker om een elektron aan een reeks atomen toe te voegen naarmate de effectieve nucleaire lading van de atomen toeneemt. Naarmate we in een bepaalde periode van links naar rechts gaan, hebben EA’s de neiging negatiever te worden. De uitzonderingen die worden aangetroffen tussen de elementen van groep 2 (2A), groep 15 (5A) en groep 18 (8A) kunnen worden begrepen op basis van de elektronische structuur van deze groepen. De edelgassen, groep 18 (8A), hebben een volledig gevulde schil en het binnenkomende elektron moet worden toegevoegd aan een hoger n-niveau, wat moeilijker is. Groep 2 (2A) heeft een gevulde ns-subshell, en dus gaat het volgende toegevoegde elektron naar de np met hogere energie, dus nogmaals, de waargenomen EA-waarde is niet zoals de trend zou voorspellen. Ten slotte heeft groep 15 (5A) een halfgevulde np-subschil en moet het volgende elektron worden gepaard met een bestaand np-elektron. In al deze gevallen verstoort de initiële relatieve stabiliteit van de elektronenconfiguratie de trend in EA. Je zou verwachten dat het atoom aan de top van elke groep de meest negatieve EA heeft; hun eerste ionisatiepotentieel suggereert dat deze atomen de grootste effectieve nucleaire lading hebben. Naarmate we echter een groep lager gaan, zien we dat het tweede element in de groep meestal de meest negatieve EA heeft. Dit kan worden toegeschreven aan de kleine omvang van de n = 2-schil en de resulterende grote elektron-elektron-afstoting. Chloor heeft bijvoorbeeld, met een EA-waarde van –348 kJ/mol, de hoogste waarde van alle elementen in het periodiek systeem. De EA van fluor is –322 kJ/mol. Wanneer we een elektron aan een fluoratoom toevoegen om een fluoride-anion (F–) te vormen, voegen we een elektron toe aan de n = 2-schil. Het elektron wordt aangetrokken door de atoomkern, maar er is ook aanzienlijke afstoting van de andere elektronen die al in deze kleine valentieschil aanwezig zijn. Het chlooratoom heeft dezelfde elektronenconfiguratie in de valentieschil, maar omdat het binnenkomende elektron de n = 3-schil binnengaat, neemt het een aanzienlijk groter ruimtegebied in beslag en wordt de elektron-elektron-afstoting verminderd. Het binnenkomende elektron ondervindt niet zoveel afstoting en het chlooratoom accepteert gemakkelijker een extra elektron, wat resulteert in een negatievere EA. Deze tekst is aangepast vanuit <a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties">OpenStax Chemie 2e, Sectie 6.5: Periodieke variaties in de eigenschappen van elementen.</a>

Elektronenaffiniteit

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



This process can be either ...

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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

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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...

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...

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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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Electron Affinity Trends on the Periodic Table

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