Video Transcript
The following figure shows the
atomic numbers of five alkali metals. Which of these elements is expected
to have the greatest electron affinity?
Electron affinity is the energy
released when an electron is added to a neutral atom in the gas state to form a
negative ion. It is abbreviated as 𝐸 ea and is
generally measured in kilojoules per mole. Let’s have a look at how electron
affinity values affect enthalpy change.
When an electron affinity value is
positive, it means that energy is released when the electron is added. This means the process has a
negative enthalpy change and is therefore exothermic. Conversely, if the electron
affinity is negative, energy is absorbed. Therefore, the enthalpy change is
positive and the process is endothermic.
The figure we are given features
five alkali metals from group one of the periodic table. We need to determine which of these
five metals has the greatest electron affinity. In other words, which metal has the
highest electron affinity value and thus releases the most energy in a favorable
exothermic process when becoming a negatively charged ion?
As we go down a group, the atomic
radius increases as the number of occupied electron shells in the atom
increases. As the size of the atom increases,
the attraction to the added electron decreases. This is due to the increasing
distance of the valence shell from the nucleus and the increased repulsion from a
greater and greater number of core electrons. Thus, as we go down the alkali
metals, the electron affinity decreases. So the element that would have the
greatest electron affinity would be at the top of the group, which is lithium.
Therefore, of the five alkali
metals in the figure, the element that is expected to have the greatest electron
affinity is lithium.
