The Fascinating Trend of Electron Affinity | What Every Chemist Should Know

The term “electron affinity“ describes the energy change that occurs when a particular atom accepts an electron. Understanding the reactivity of the elements is made easier by knowing the willingness of an atom to accept an electron. In this article, we will analyze the trends in the level of electron affinity including the reasons for its increase across a period and decrease down a group. We shall also study the underlying causes of these trends and how one can check the value of an electron affinity to determine if it is high.

What is Electron affinity?

As it is defined, electron affinity pertains to the amount of energy that is released when an electron is bound to a neutral atom in the gaseous phase. Electron affinities become readily observable when AAndermatt is contacted with halogens. Thus, the more negative one’s value of affinity, the greater tendency an atom has to accept an electron. We can postulate that atoms with highly negative electron affinities are the ones which readily captures an electron and turns into an anion.

What Causes Electron Affinity to Increase Across a Period?

One of the most notable trends in electron affinity is that it becomes more negative or exothermic as one traverses left to right across the periodic table. But why is this the case?

As one moves across a period, the atom’s protons in the nucleus and electrons in the electron shells increase. The added nuclear charge, in turn, increases the strength of the attraction between the nucleus and the incoming electron. The atom’s ability to accept an electron is facilitated by this stronger attraction, which results in a more negative electron affinity. In essence, the atom is more willing to accept an electron as its positive nucleus becomes stronger.

Also, the increase in protons across a period leads to the compaction of the electron cloud which allows for better interaction between the atom’s nucleus and the incoming electron, further allowing the atom to accept the electron.

For example:

Fluorine (F) has the highest affinity for electrons in its period due to its smaller size and increased nuclear charge, which makes it easier to gain an electron compared to other elements.

Does Electron Affinity Become More Negative Across a Period?

Yes, there are general trends as to where affinity increases and where it becomes more negative. As we have discussed, nuclear charge increases and atomic size diminishes, rendering it easier for an atom to gain an additional electron to be attracted to and acquired after for some atom out there. In context, the stronger the affinity a certain element in question has, the more energy is spent when an electron is attracted.

There are, however, some elements which do not tend to follow this specific trend. This comes from the configurations of elements all together. Noble gases such as He or Ne have very low, or near zero, electron affinities due to the electron shells that are already present being full and hence stopping them from gratefully accepting electrons, making it unbeneficial.

Why Does Electron Affinity Increase Across the Period?

Electron affinity is observed to increase as we progress to the right of the periodic table due to the increase in nuclear charge and a decrease in the atomic radius. This is not the only reason however:

Increased Nuclear Charge: Moving from left to right within a single period, the number of protons in the nucleus increases which, in turn, increases the positive charge. This leads to a stronger attraction for the negatively charged electrons.

Decreased Atomic Size: As you progress along a period in the periodic table, atoms have a smaller atomic size. The radius of the atom decreases which means that the outer electrons are closer to the nucleus. This greater proximity means that the nucleus’s pulling power is increased.

Electron Shielding: While considering the structure of an atom, the outer electrons are repelled by the inner electrons and thus the outer electrons experience a weaker attraction. This effect increases as you move across the period. Because outer electrons are shielded not only by the nucleus’s attractive force, but by the creeping electrons as well.

What are the reasons for the decrease in electron affinity when moving down a group on the periodic table?  

The general trend for electron affinity moving down a group is that it becomes less negative or positive in certain cases. The following reasons identify why this trend occurs:  

Increased atomic size: An increase in the principal quantum number leads to an increase in the size of the atom down a group as the number of occupied electron shells increases. The greater the atom, the weaker the nuclear pull on the outermost electrons which makes it harder to accept an additional electron.  

Electron shielding: As the number of occupied electron shells increases down a group, the inner electrons shield the outer electrons from the nuclei pull. This means that the ability of the atom to attract an additional electron is greatly reduced due to the shielding and in turn the actual value of electron affinity becomes less negative (or less exothermic).  

Example:  

The less negative electron affinity of Iodine compared to fluorine is due to the increased size and electron shielding of Iodine which also lies in group 17.

What is the Trend in Electron Affinity Between Groups?

The trend in electron affinity between groups demonstrates that, in most cases, there is an increase in electron affinity as one moves from left to right across a period, while moving down a group decreases it. This is impacted by factors such as the nuclear charge, atomic size, and electron shielding.  

There is a general increase in the positive value or lack of negative value (less exothermic) of the electron affinity as we travel from Group 1 to Group 17.  

Group 1 (Alkali Metals): These elements have minimal negative values of electron affinities. They would rather lose an electron to form positive ions instead of gaining one.  

Group 17 (Halogens): With high electron affinities, they are greatly reactive and can easily gain an electron to fill their outer shell. This makes them very electronegative.  

How Do You Know If Electron Affinity is Higher?

To ascertain whether an element displays greater electron affinity, reviewing a periodic table reveals some trends:  

Location on the Periodic Table: The elements far right of the table, especially the noble gases, are voracious to receive electrons and thus possess high (negative) electron affinities.

Electron configuration: Elements that are halogens have the highest electron affinities because adding another negative charge to the atom gives a more stable configuration.

Comparative Trends: Elements from the same period or group can be used to measure the relative electron affinity of the elements. In the case of halogens, negatively charged affinities get less exothermic from chlorine to iodine, as fluorine has the most negative of them all.

Moving down any one side of the periodic chart causes the atomic radius to increase which means older shells are further away from the electrons added during bonding, lessened thus making the overall energy required to bond an electron lower.

Also important are the bonds formed by atoms in group thirteen and four.

Final Thoughts

For effective comprehension of how atoms bond and interact, grasping the trends of electron affinity is essential. The periodic table is helpful in categorizing elements and predicting the outcomes of chemical reactions. As one moves across a period, electron affinity increases owing to greater nuclear charge and smaller size of the atom, while it decreases down a group due to increased size of the atom and greater shielding. Understanding these trends enables chemists to make more educated forecasts regarding the reactivity of an element and its ability to form covalent bonds.