Video Transcript
The electron configuration for the
d valence electrons of three first-row transition metals are shown. Which transition metal’s atom would
you predict to have the lowest electron affinity?
Electron affinity is the measure of
the energy released when an electron is added to a neutral atom in the gas state to
form a negative ion. It is represented by 𝐸 ea and is
usually measured in kilojoules per mole of atoms. A positive electron affinity value
means that energy is released. A negative electron affinity value
means that for the electron to be added, energy must be absorbed. We must identify the lowest
electron affinity value of the three transition metals vanadium, manganese, and
iron.
The transition metal with the
lowest electron affinity value will require the most amount of energy to add another
electron to its valence shell. The process will have a positive
enthalpy change, meaning it is an endothermic process.
Let’s have a look at the general
trend of electron affinity on the periodic table. In general, from left to right
across a period on the periodic table, electron affinity values increase. This is because as we go left to
right across a period, the number of electron shells stays the same. But the number of protons in the
nucleus increases. The electrostatic attraction
between the increasingly positive nucleus and the valence electrons increases. This causes the atomic radius to
decrease.
In general, a newly added electron
experiences more attraction to the nucleus when added to a smaller atom. We can see that the three given
metals are found in period four. If we reference the trend alone, we
could assume that vanadium would have the lowest electron affinity and iron would
have the highest. However, we find that the overall
trend alone does not explain the differences in electron affinity for these three
metals. We must consider their electronic
configurations.
Manganese has a highly favorable
configuration with a half-filled d subshell. Forming a manganese anion by adding
a sixth electron to the d subshell is so difficult that this process does not
release energy but requires energy to be absorbed. Manganese has a negative electron
affinity, and the addition of an electron to manganese is a highly endothermic
process.
Therefore, the transition metal’s
atom we would predict to have the lowest electron affinity would be manganese.
