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.
