Question

The electron affinity of sodium is lower than that of lithium, while the electron affinity of chlorine is higher than that of fluorine. Suggest an explanation for this observation.

Short answer

Sodium has a lower electron affinity than lithium due to the increased atomic size and weaker electron-nucleus attraction. Chlorine has a higher electron affinity than fluorine because the added electron experiences less electron-electron repulsion in chlorine's larger atomic size.

Step-by-step solution

Understanding Electron Affinity

Electron affinity is the change in energy (usually released) when an electron is added to a neutral atom in the gas phase to form a negative ion. When comparing electron affinity across the periodic table, it generally increases across a period, from left to right, and decreases down a group, from top to bottom.

Analyzing Sodium and Lithium

Sodium (Na) is positioned below lithium (Li) in the same group of the periodic table. As we move down a group, the size of the atoms increases, and the added electron would be further from the nucleus in sodium compared to lithium. This increased distance leads to a weaker attraction of the added electron in sodium, which explains the lower electron affinity of sodium compared to lithium.

Analyzing Chlorine and Fluorine

Fluorine (F) is positioned above chlorine (Cl) in the same group of the periodic table. Although electron affinity generally increases from top to bottom in a group, an exception occurs here due to fluorine’s small atomic size. This small size causes higher electron-electron repulsion when an electron is added, which is less efficient energy-wise compared to chlorine. This repulsion is less in chlorine due to its larger size, resulting in chlorine having a higher electron affinity than fluorine.

Key concepts

Understanding Electron Affinity

Electron affinity is a measure of how much energy is released when an electron is added to a neutral atom. It's an important concept in chemistry because it helps us predict how atoms will interact with each other. Generally, the more energy released, the more likely an atom is to accept an additional electron, which can impact the atom's reactivity and the formation of molecules.

Imagine each atom as a house and electrons as potential new residents. Some houses (atoms) are more welcoming, releasing lots of energy (like giving a warm welcome) when a new resident (electron) arrives. Other houses are less inviting, releasing less energy and being less likely to attract new residents. In more technical terms, a high electron affinity value indicates that an atom releases a lot of energy when it gains an electron, becoming more stable.

Periodic Trends in Electron Affinity

As we journey across the periodic table, electron affinity doesn't always follow a straightforward path—it often changes in a zigzag pattern. But, as a general trend, electron affinity usually increases as we move from left to right across a period. This happens because the atoms get more protons, pulling electrons in more tightly due to the increased positive charge in the nucleus.

Exceptions to the Rule

Just like any good story, there are twists and turns. Some elements break the trend due to their unique atomic structure. An excellent example is the comparison between chlorine and fluorine. Chlorine has a higher electron affinity than fluorine, which is surprising because fluorine is to its left. This exception is explained by the atomic size and electron-electron repulsions that we'll explore further.

Atomic Size and Its Effect on Electron Affinity

Think of atomic size as the space a house occupies. Bigger houses (larger atomic size) mean that the new residents (electrons) can be further from the nucleus (the central living area). As atoms become larger, the outermost electrons feel less pull from the nucleus because of the increased distance. This concept helps us understand why sodium, a 'bigger house' compared to lithium, doesn't release as much energy when it takes in a new electron. Its larger size means that the additional electron is less attracted to the nucleus, leading to a lower electron affinity.

As we move down a group on the periodic table, atoms generally increase in size because they have more electron shells. This is why elements toward the bottom of a group tend to have lower electron affinities compared to their upstairs neighbors.

Electron-Electron Repulsion in Small Atoms

Now, for electron-electron repulsion, imagine a full house with a lot of residents (electrons). If the house is small, it's going to feel cramped, and the residents are going to get in each other's way, creating tension (repulsion). In a small atom like fluorine, adding another electron is like inviting one more person to an already crowded space. This tension or electron-electron repulsion within the compact space makes it harder for the atom to welcome that new electron, resulting in less energy released—reflecting a lower electron affinity compared to a larger atom like chlorine, which has more room and less repulsion among its electrons.

This repulsion factor is a key reason why some elements, despite being higher up on the periodic table, have lower electron affinities than you might expect. This adds a layer of complexity to predicting trends but provides fascinating insights into the behavior of elements.