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

Matter and its Interactions

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Electron Affinity

When an electron is added to a gaseous atom, a change in energy is observed called electron affinity. Electron affinity measures the ease of gaining an electron by an atom. 

For example, the electron affinity of chlorine is &#8722;348.6 kJ/mol. The negative sign indicates that it is an exothermic change.

Argon, however, has a positive electron affinity, indicating that formation of an argon anion requires energy to be supplied. 

In general, the greater the attraction between an atom and an added electron, the more negative the electron affinity. Electron affinities, similar to ionization energies, show trends in the periodic table. 

Moving down group 1, the atomic size increases as the electrons occupy higher principal quantum numbers. Incoming electrons, therefore, experience less nuclear attraction leading to less negative electron affinities. 

However, there are exceptions. In halogens, chlorine has a more negative electron affinity value than fluorine. But why?

Fluorine is the smallest atom of the halogens and an incoming electron experiences a significant repulsion from the electrons already present.

In the chloride anion, however, the new electron is added into the third shell, occupying more space. This reduces the electron-electron repulsions, making it more attractive for an electron to be gained.

Generally, moving across a period, electron affinities become more negative. Halogens have the most negative electron affinities, as the incoming electron helps to achieve noble gas configurations.

In comparison, noble gases have a completely filled shell. The incoming electron has to be accommodated in the higher principal energy level, which is energetically unfavorable. Thus, electron affinities for these elements are positive.

Group 2 shows exceptions. The electron configuration indicates that the incoming electron needs to enter a higher-energy subshell. Thus, electron affinity values are either positive or less exothermic. 

Interestingly, group 15 has less negative electron affinities than group 14. Compare phosphorus and silicon. Unlike silicon, phosphorus has a half-filled p-subshell and the incoming electron needs to be paired with an electron already residing in the p-orbital. 

This would increase the electron-electron repulsions and is therefore an energetically unfavourable process, which is also reflected in the less negative electron affinity compared to silicon.

electron affinity

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

<img alt="Electron affinity process; formula: X(g) + e&#8315; &#8594; X&#8315;(g); diagram for chemical reaction analysis." src="/files/core/CDNSource/lessons/1050/1050_Equations-01.png" style="height: 40px;" data-altupdatedat="1754904545000" data-alt="Electron affinity equation X(g) + e&#8315; &#8594; X&#8315;(g), EA&#8321;, chemical reaction formula" />

This process can be either endothermic or exothermic, depending on the element. Many of these elements have negative values of EA, which means that energy is released when the gaseous atom accepts an electron. However, for some elements, energy is required for the atom to become negatively charged, and the value of their EA is positive. Just as with ionization energy, subsequent EA values are associated with forming ions with more charge. The second EA is the energy associated with adding an electron to an anion to form a 2&#8211; ion, and so on.

As one might predict, it becomes easier to add an electron across a series of atoms as the effective nuclear charge of the atoms increases. As we go from left to right across a period, EAs tend to become more negative. The exceptions found among the elements of group 2 (2A), group 15 (5A), and group 18 (8A) can be understood based on the electronic structure of these groups. The noble gases, group 18 (8A), have a completely filled shell, and the incoming electron must be added to a higher n level, which is more difficult to do. Group 2 (2A) has a filled ns subshell, and so the next electron added goes into the higher energy np, so, again, the observed EA value is not as the trend would predict. Finally, group 15 (5A) has a half-filled np subshell, and the next electron must be paired with an existing np electron. In all of these cases, the initial relative stability of the electron configuration disrupts the trend in EA.

One might expect the atom at the top of each group to have the most negative EA; their first ionization potentials suggest that these atoms have the largest effective nuclear charges. However, as we move down a group, we see that the second element in the group most often has the most negative EA. This can be attributed to the small size of the n = 2 shell and the resulting large electron-electron repulsions. For example, chlorine, with an EA value of &#8211;348 kJ/mol, has the highest value of any element in the periodic table. The EA of fluorine is &#8211;322 kJ/mol. When we add an electron to a fluorine atom to form a fluoride anion (F&#8211;), we add an electron to the n = 2 shell. The electron is attracted to the nucleus, but there is also significant repulsion from the other electrons already present in this small valence shell. The chlorine atom has the same electron configuration in the valence shell, but because the entering electron is going into the n = 3 shell, it occupies a considerably larger region of space and the electron-electron repulsions are reduced. The entering electron does not experience as much repulsion, and the chlorine atom accepts an additional electron more readily, resulting in a more negative EA.

This text is adapted from <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>

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

KEY TERMS AND CONCEPTS

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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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 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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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...

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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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