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

Kiedy elektron jest dodawany do atomu gazowego, obserwuje się zmianę energii zwaną powinowactwem elektronowym. Powinowactwo elektronowe mierzy łatwość uzyskania elektronu przez atom .

Na przykład powinowactwo elektronowe chloru wynosi −348,6 kJ/mol. Znak ujemny wskazuje, że jest to zmiana egzotermiczna.

Argon ma jednak dodatnie powinowactwo elektronowe, co wskazuje, że powstanie anionu argonu wymaga dostarczenia energii. 

Ogólnie rzecz biorąc, im większe przyciąganie między atomem a dodanym elektronem, tym bardziej ujemne powinowactwo elektronowe. Powinowactwo elektronowe, podobne do energii jonizacji, pokazuje trendy w układzie okresowym pierwiastków. 

Przesuwając się w dół grupy 1, rozmiar atomu wzrasta, gdy elektrony zajmują wyższe główne liczby kwantowe. Przychodzące elektrony doświadczają zatem mniejszego przyciągania jądrowego, co prowadzi do mniejszego ujemnego powinowactwa elektronowego. 

Są jednak wyjątki. W halogenach chlor ma bardziej ujemną wartość powinowactwa elektronowego niż fluor. Ale dlaczego?

Fluor jest najmniejszym atomem halogenów, a nadlatujący elektron doświadcza znacznego odpychania od już obecnych elektronów.

Jednak w anionie chlorkowym nowy elektron jest dodawany do trzeciej powłoki, zajmującej więcej miejsca. Zmniejsza to odpychanie elektron-elektron, czyniąc go bardziej atrakcyjnym dla uzyskania elektronu.

Ogólnie rzecz biorąc, poruszając się w okresie, powinowactwa elektronowe stają się bardziej ujemne. Halogeny mają największe ujemne powinowactwo elektronowe, ponieważ przychodzący elektron pomaga osiągnąć konfiguracje gazów szlachetnych.

Dla porównania, gazy szlachetne mają całkowicie wypełnioną powłokę. Przychodzący elektron musi być zaakomodowany na wyższym poziomie energii głównej, który jest energetycznie niekorzystny. Zatem powinowactwo elektronowe do tych pierwiastków jest dodatnie.

Grupa 2 pokazuje wyjątki. Konfiguracja elektronowa wskazuje, że przychodzący elektron musi wejść do podpowłoki o wyższej energii. Zatem wartości powinowactwa elektronowego są albo dodatnie, albo mniej egzotermiczne. 

Co ciekawe, grupa 15 ma mniej ujemnego powinowactwa elektronowego niż grupa 14. Porównaj fosfor i krzem. W przeciwieństwie do krzemu, fosfor ma w połowie wypełnioną podpowłokę p, a przychodzący elektron musi być sparowany z elektronem już znajdującym się na orbicie p. 

Zwiększyłoby to odpychanie elektronów i dlatego jest to proces niekorzystny energetycznie, co znajduje również odzwierciedlenie w mniej ujemnym powinowactwie elektronowym w porównaniu z krzemem.

powinowactwo elektronowe

Powinowactwo elektronowe (PE) to zmiana energii spowodowana dodaniem elektronu do atomu gazowego w celu utworzenia anionu (jonu ujemnego).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1050/1050_Equations-01.png" style="height: 40px;">

Proces ten może być endotermiczny lub egzotermiczny, w zależności od pierwiastka. Wiele z tych pierwiastków ma ujemne wartości PE, co oznacza, że energia jest uwalniana, gdy atom gazowy przyjmuje elektron. Jednak w przypadku niektórych pierwiastków potrzebna jest energia, aby atom został naładowany ujemnie, a wartość ich PE była dodatnia. Podobnie jak w przypadku energii jonizacji, kolejne wartości PE są związane z powstawaniem jonów o większym ładunku. Drugi PE to energia związana z dodaniem elektronu do anionu w celu utworzenia jonu 2– i tak dalej.

Jak można przewidzieć, dodanie elektronu do szeregu atomów staje się łatwiejsze w miarę wzrostu efektywnego ładunku jądrowego atomów. W miarę jak przechodzimy od lewej do prawej strony przez okresy, wartości PE stają się coraz bardziej negatywne. Wyjątki występujące wśród elementów z grupy 2 (2A), grupy 15 (5A) i grupy 18 (8A) można zrozumieć na podstawie struktury elektronowej tych grup. Gazy szlachetne z grupy 18 (8A) mają całkowicie wypełnioną powłokę i docierający elektron musi zostać dodany do wyższego poziomu n, co jest trudniejsze. Grupa 2 (2A) ma wypełnioną podpowłokę ns, więc następny dodany elektron przechodzi do wyższej energii np, więc ponownie zaobserwowana wartość EA nie jest zgodna z przewidywaniami trendu. Wreszcie grupa 15 (5A) ma w połowie zapełnioną podpowłokę np, a następny elektron musi być sparowany z istniejącym elektronem np. We wszystkich tych przypadkach początkowa względna stabilność konfiguracji elektronów zakłóca trend w PE.

Można by oczekiwać, że atom na szczycie każdej grupy będzie miał najbardziej ujemny PE; ich pierwsze potencjały jonizacyjne sugerują, że atomy te mają największe efektywne ładunki jądrowe. Jednak w miarę przesuwania się w dół grupy widzimy, że drugi element w grupie ma najczęściej najbardziej ujemny PE. Można to przypisać niewielkim rozmiarom powłoki n = 2 i wynikającym z tego dużym odpychaniom elektron-elektron. Na przykład chlor o wartości PE wynoszącej –348 kJ/mol ma najwyższą wartość spośród wszystkich pierwiastków układu okresowego. PE fluoru wynosi –322 kJ/mol. Kiedy dodajemy elektron do atomu fluoru, tworząc anion fluorkowy (F–), dodajemy elektron do powłoki n = 2. Elektron jest przyciągany do jądra, ale występuje również znaczne odpychanie od innych elektronów już obecnych na tej małej powłoce walencyjnej. Atom chloru ma tę samą konfigurację elektronową na powłoce walencyjnej, ale ponieważ wchodzący elektron przechodzi do powłoki n = 3, zajmuje znacznie większy obszar przestrzeni, a odpychanie elektron-elektron jest zmniejszone. Wchodzący elektron nie podlega tak dużemu odpychaniu, a atom chloru łatwiej przyjmuje dodatkowy elektron, co skutkuje bardziej ujemnym PE.

Powinowactwo elektronowe

Powinowactwo elektronowe (EA) to zmiana energii potrzebna do dodania elektronu do atomu gazowego w celu utworzenia anionu (jonu ujemnego).

Ten proces może być jednym z ...

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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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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...

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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.

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Electron Affinity Trends on the Periodic Table

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