Periodic Trend: Electron Affinity (Simplified) - Online Tutor, Practice Problems & Exam Prep

Electron affinity (e_a) measures the energy released when an electron is added to a gaseous atom or ion. Generally, electron affinity increases across a period and decreases down a group. Noble gases exhibit electron affinities of less than or equal to 0 due to their stable electron configurations. Other exceptions include nitrogen, zinc, and alkaline earth metals, which also resist accepting electrons. Understanding these trends is crucial for predicting an element's reactivity and stability in chemical reactions.

Periodic Trend: Electron Affinity (Simplified) Concept 1

Electron affinity, which is abbreviated as e_a, represents the energy released from the addition of an electron to a gaseous atom or ion in kilojoules. So here, let's say we have carbon gas. We're dealing with a gaseous atom. We need to add an electron to it. Adding an electron means that it will be a reactant. The carbon absorbs that electron and gains a negative charge because electrons are negative. Now, in order to connect them together, energy has to be released. This energy represents the electron affinity. Here, it has to be a product because it's being released. In chemistry, of course, there are exceptions that arise sometimes, and this is one of them. With electron affinity, the exception is we have electron affinities that are less than or equal to 0, and that happens when the element will not readily accept an electron. But why would an element want to accept an electron? Well, they may have a uniquely stable electron configuration or arrangement, i.e. the noble gases. We've talked about noble gases being perfect. If you're perfect in terms of your number of electrons, you don't have a need to accept another electron. So the noble gases are a great example of elements that will not readily accept an electron. Now the general trend is that electron affinity increases as we move from left to right across a period and going up a group. What does it mean that an electron can easily accept an electron or not? We're going to say here that if the lower your electron affinity, then the electron will not easily be accepted. So the smaller your electron affinity is, the harder it is for that element or ion to accept an electron. And we're going to say here that the greater your electron affinity is, then the more readily that ion or atom will accept an electron.

Periodic Trend: Electron Affinity (Simplified) Concept 2

So electron affinity is not as clean-cut as other periodic trends. There are exceptions that pop up all over the place. Even more so with rules 5, 6, and 7, so much so that you really don't need to pay attention to them. Now realize here that as we're heading towards the top right corner, electron affinity more or less will be increasing. Also remember that when it comes to the noble gases, they're electronically perfect. They don't want additional electrons, so their electron affinities are less than or equal to 0. We also see this trend pop up with nitrogen, zinc, manganese, and in group 2A, beryllium, magnesium, and calcium. Because of their electron arrangements, they're also electronically stable, not perfect, just stable, and they don't readily accept an electron. So again, the general trend is as we're heading towards the top right corner, electron affinity increases. Noble gases don't ever want to accept an electron, and then these other ones that I've circled also fit this idea of being electronically stable. So they also don't readily accept an electron. Keep this in mind when you're looking at electron affinity.

Periodic Trend: Electron Affinity (Simplified) Example 1

So here, let's take a look at this example question, and I've left the periodic table here just to help us. It says, which of the following halogens will release the most energy with the addition of an electron? So remember, the general trend is as we're heading up towards a group, electron affinity more or less will be increasing. Now, if we take a look here, we have sulfur, neon, nitrogen, astatine, and bromine. Alright. So, first of all, a halogen is in group 7A. These are not even halogens; they're not in group 7A. The answer is going to be either d or e. Realize here that I don't even show a t. It's in one of the roles that we ignore because it's very unpredictable in terms of electron affinity, which means that it wouldn't be a viable option. So e will be our answer here through the process of elimination. Now, of course, there are exceptions to electron affinity. It's all over the place. We can see that the general trend should be as we go up a group, your electron affinity increases. But you can see that actually chlorine has a slightly higher electron affinity than fluorine. But here we don't have to worry about that because I wasn't asking about either one of those halogens. It was between bromine and astatine instead. But just remember the general trend is as we head up a group, electron affinity more or less will be increasing.

Determine which atom in the following set has the largest electron affinity: N, O, C, B, Ne

• A. N
• B. O
• C. C
• D. B
• E. Ne

Rank the following elements in order of increasing electron affinity: Cs, Hg, F, S

• A. Hg < S < F < Cs
• B. Hg < F < S < Cs
• C. Cs < S < F < Hg
• D. Cs < S < Hg < F
• E. Cs < Hg < S < F

Which one of the following atoms has the least tendency to gain another electron?

• A. Neon
• B. Oxygen
• C. Chlorine
• D. Sodium
• E. Hydrogen

Arrange the following elements from greatest to least tendency to accept an electron: F, Li, C, O, Be

• A. F > Li > C > O > Be
• B. F > O > C > Li > Be
• C. F > O > C > Be > Li
• D. O > F > C > Li > Be

Here’s what students ask on this topic:

What is electron affinity and how is it measured?

Electron affinity (ea) is the energy released when an electron is added to a gaseous atom or ion. It is measured in kilojoules per mole (kJ/mol). For example, when a carbon atom in the gas phase gains an electron, it releases energy, which is the electron affinity. The general trend is that electron affinity increases across a period (left to right) and decreases down a group (top to bottom) in the periodic table. This trend helps predict an element's reactivity and stability in chemical reactions.

Why do noble gases have electron affinities less than or equal to 0?

Noble gases have electron affinities less than or equal to 0 because they possess a stable electron configuration. Their outer electron shells are fully occupied, making them electronically perfect. As a result, they do not have a tendency to accept additional electrons. This stability means that adding an electron to a noble gas does not release energy, and in some cases, it may even require energy, resulting in an electron affinity of 0 or a negative value.

How does electron affinity change across a period and down a group?

Electron affinity generally increases as you move from left to right across a period in the periodic table. This is because atoms become more eager to gain electrons to achieve a stable electron configuration. Conversely, electron affinity decreases as you move down a group. This is due to the increasing atomic radius, which makes it harder for the nucleus to attract additional electrons. Therefore, elements at the top right of the periodic table typically have higher electron affinities.

What are some exceptions to the general trend of electron affinity?

While the general trend is that electron affinity increases across a period and decreases down a group, there are notable exceptions. Noble gases, for instance, have electron affinities less than or equal to 0 due to their stable electron configurations. Other exceptions include nitrogen, zinc, and alkaline earth metals like beryllium, magnesium, and calcium. These elements have electron arrangements that make them electronically stable, so they do not readily accept additional electrons.

Why do elements like nitrogen, zinc, and alkaline earth metals resist accepting electrons?

Elements like nitrogen, zinc, and alkaline earth metals (beryllium, magnesium, and calcium) resist accepting electrons because they have relatively stable electron configurations. For example, nitrogen has a half-filled p-orbital, which is relatively stable. Zinc has a filled d-orbital, and alkaline earth metals have filled s-orbitals. These stable configurations mean that adding an electron would disrupt their stability, making them less likely to accept additional electrons.