Question Video: Identifying the Explanation for the Anomalous Electron Affinity of Fluorine | Nagwa Question Video: Identifying the Explanation for the Anomalous Electron Affinity of Fluorine | Nagwa

Question Video: Identifying the Explanation for the Anomalous Electron Affinity of Fluorine Chemistry • Second Year of Secondary School

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The table shows how electron affinity generally decreases when moving down elements in group 17, the halogens. However, the data shows fluorine to be anomalous. Which of the following provides the correct explanation for the lower electron affinity of fluorine? [A] An atom of chlorine obtains a full outer shell through the addition of an electron, which an atom of fluorine does not. [B] An atom of fluorine is smaller than an atom of chlorine, so the extra electron experiences significantly more electron repulsion in fluorine, reducing its electron affinity. [C] There is a stronger attraction between the nucleus and the incoming electron in an atom of chlorine, giving it a greater electron affinity. [D] Chlorine is actually the anomalous element, as its electron affinity is greater than it should be because of screening from the 3d electrons.

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Video Transcript

The table shows how electron affinity generally decreases when moving down elements in group 17, the halogens. However, the data shows fluorine to be anomalous. Which of the following provides the correct explanation for the lower electron affinity of fluorine? (A) An atom of chlorine obtains a full outer shell through the addition of an electron, which an atom of fluorine does not. (B) An atom of fluorine is smaller than an atom of chlorine, so the extra electron experiences significantly more electron repulsion in fluorine, reducing its electron affinity. (C) There is a stronger attraction between the nucleus and the incoming electron in an atom of chlorine, giving it a greater electron affinity. Or (D) chlorine is actually the anomalous element, as its electron affinity is greater than it should be because of screening from the 3d electrons.

Let’s clear some space on screen so we can discuss electron affinity. The electron affinity can be defined as the energy released when an electron is added to a neutral atom in the gas state to form a negative ion and is generally measured per mole of atoms. We can think of it as a measure of how energetically favorable it is to add an electron to the valence shell. The atomic radius of a fluorine atom is extremely small, whereas the atomic radius of a chlorine atom is much larger. As we go down the group of the halogens, the atomic radius continues to increase.

Because of the fluorine atom’s very small atomic radius, the negative charge of its nine electrons are very close to the nucleus, meaning the electron being added is experiencing a greater repulsion than it would if it was being added to chlorine. In other words, fluorine is less attracted to the extra electron being added. So less energy will be released when the negative ion is formed compared to chlorine. Because of this, the electron affinity of fluorine is lower than that of chlorine. This is why fluorine is an anomaly in the trend of electron affinity of the halogens.

With this information in mind, let’s have a look at our answer choices. We can see that answer choice (B) fits the reasoning we discussed. Therefore, the correct explanation for the lower electron affinity of fluorine is answer choice (B). An atom of fluorine is smaller than an atom of chlorine, so the extra electron experiences significantly more electron repulsion in fluorine, reducing its electron affinity.

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