Periodic Trend: Electron Affinity: Videos & Practice Problems

Electron Affinity Trends

Electron affinity is a key concept in understanding how atoms interact with electrons. It refers to the energy change that occurs when an electron is added to a neutral atom, and it is typically expressed in kilojoules per mole (kJ/mol). As we examine the periodic table, we observe that electron affinity generally becomes more exothermic, or more negative, as we move towards the top right corner. This trend indicates that elements in this region, such as fluorine, have a stronger tendency to attract electrons compared to those in the lower left, like cesium.

For instance, hydrogen has an electron affinity of -72.8 kJ/mol, while fluorine's is significantly more negative at -328 kJ/mol. This means fluorine has a greater desire to gain an electron than hydrogen. Conversely, cesium, with an electron affinity of -45.5 kJ/mol, shows a much weaker attraction for additional electrons.

Interestingly, some elements, particularly those in the bottom row of the periodic table, do not have reported electron affinity values. This is primarily due to their large size and instability, which complicates their ability to attract electrons. Additionally, certain elements like beryllium, nitrogen, and the noble gases exhibit electron affinities that are equal to or greater than 0. This indicates that these elements do not favor the addition of an electron, as they are already stable in their current state.

Other elements, such as manganese, zinc, cadmium, and mercury, also show electron affinities that are equal to or greater than 0, though the reasons for this behavior can vary. Notably, hydrogen fluoride (HF) is an anomaly, as its electron affinity does not fit neatly into the established trends, lacking a clear justification for its value being greater than or equal to 0.

Understanding these trends in electron affinity helps to explain the reactivity and stability of different elements, providing insight into their chemical behavior and interactions.

Electron Affinity Example

When considering which halogen has the most exothermic electron affinity, it's essential to focus on the elements in Group 7A of the periodic table, known as halogens. Among the options provided—sulfur, neon, nitrogen, astatine, and bromine—only astatine and bromine are halogens. To determine which of these has a more exothermic electron affinity, we need to understand that a more exothermic electron affinity corresponds to a more negative value.

As we move towards the top right corner of the periodic table, the electron affinity generally increases, indicating that elements become more exothermic in their electron affinity. In this context, bromine, being higher up in Group 7A compared to astatine, exhibits a more exothermic electron affinity. Therefore, bromine is the halogen with the most exothermic electron affinity, reflecting its ability to release more energy when gaining an electron.

Electron Affinity Exceptions

Electron affinity refers to the energy change that occurs when an electron is added to a neutral atom. However, certain elements exhibit exceptions to this trend due to their stable or symmetrical electron configurations. Elements with completely filled or half-filled orbitals tend to be less likely to accept additional electrons, as this would disrupt their stability.

For instance, Beryllium (Be) has a completely filled 2s orbital, resulting in an electron affinity of greater than or equal to 0, indicating a reluctance to gain an electron. Similarly, Nitrogen (N) possesses half-filled 2p orbitals, which are particularly stable; thus, it also resists accepting an electron. Manganese (Mn) showcases a similar behavior with its half-filled 3d orbitals, further illustrating this trend.

Moreover, Zinc (Zn) and elements below it in the periodic table have fully filled d orbitals, contributing to their stability and reluctance to gain electrons. These exceptions highlight the importance of electron configuration in determining an element's electron affinity.

In general, as one moves towards the top right corner of the periodic table, electron affinity tends to become more exothermic, meaning that the process of gaining an electron releases more energy. Understanding these exceptions is crucial for predicting the behavior of elements in chemical reactions.

Which of the following represents the third electron affinity of Si?

• A.
• B.
• C.
• D.

Answer: (C)

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

• A. N
• B. O
• C. C
• D. B
• E. Ne

Answer: (B)

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. Hg < Cs < S < F
• E. Cs < Hg < S < F

Answer: (D)

Here's what students ask on this topic:

What is electron affinity and how is it measured?

Electron affinity (EA) is the energy change that occurs when an electron is added to a neutral gaseous atom or ion, typically releasing energy, making the process exothermic. It is usually measured in kilojoules per mole (kJ/mol). When an atom gains an electron, it becomes more negatively charged, and the energy released during this process is called electron affinity. A more negative electron affinity value indicates a stronger tendency of the atom to accept an electron. For example, fluorine has a highly negative electron affinity, meaning it strongly attracts electrons. However, some elements have electron affinities that are zero or positive, indicating they do not readily accept electrons due to their stable electron configurations.

How does electron affinity change across the periodic table?

Electron affinity generally increases (becomes more negative) as you move from left to right across a period and from bottom to top within a group on the periodic table. This means elements in the top right corner, such as fluorine, have the most exothermic electron affinities and strongly attract electrons. Conversely, elements toward the bottom left, like cesium, have less negative electron affinities and are less likely to gain electrons. This trend occurs because atoms on the right side have nearly full valence shells and a stronger effective nuclear charge, which attracts additional electrons more strongly. However, there are exceptions due to electron configuration stability.

Why do some elements have zero or positive electron affinity values?

Some elements have electron affinity values that are zero or positive because adding an electron would disrupt their stable electron configurations. For example, beryllium has a completely filled s orbital, nitrogen has half-filled p orbitals, and zinc has a fully filled d subshell. These configurations are particularly stable, so these elements do not readily accept additional electrons. Noble gases also have zero or positive electron affinities because their valence shells are already full, making them very stable and unlikely to gain electrons. This stability explains why their electron affinity values are not negative.

What are the exceptions to the general trend of electron affinity in the periodic table?

The main exceptions to the general trend of increasing electron affinity across the periodic table are elements with stable or symmetrical electron configurations. For instance, beryllium has a filled s orbital, nitrogen has half-filled p orbitals, manganese has half-filled d orbitals, and zinc has fully filled d orbitals. These stable configurations make these elements less likely to accept an electron, resulting in electron affinity values that are zero or positive. Additionally, noble gases have full valence shells, so they also do not tend to gain electrons. These exceptions highlight the importance of electron configuration stability in determining electron affinity.

How does electron affinity influence chemical bonding and reactivity?

Electron affinity influences how atoms form chemical bonds and their reactivity. Atoms with high (more negative) electron affinity values tend to attract electrons strongly, making them more likely to gain electrons during chemical reactions. This property is crucial in forming ionic bonds, where one atom donates electrons and another accepts them. For example, halogens like fluorine have high electron affinities and readily gain electrons to form negative ions. Conversely, elements with low or positive electron affinities are less likely to gain electrons and may prefer to lose electrons or share them in covalent bonds. Understanding electron affinity helps predict how elements interact and combine in chemical reactions.