Video Transcript
Which of the following atoms has
the greatest electron affinity? (A) K, (B) Na, (C) Rb, or (D)
Cs.
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 represented by the symbol 𝐸
ea, and we usually measure it in kilojoules per mole.
Let’s look at how electron affinity
relates to enthalpy. If the electron affinity is
positive, then energy is released. The enthalpy change is negative,
and the overall process is exothermic. Conversely, if the electron
affinity is negative, then energy is absorbed. The enthalpy change is positive,
and it’s endothermic.
Now that we understand this
process, let’s look at trends of electron affinities for the elements given. They’re in group one of the
periodic table. Note that we are only looking at
first electron affinities, meaning the energy released by the first electron that we
add to a neutral atom. The atoms of the elements increase
in size as we move down the group because they contain more occupied electron
shells. For period one elements, the
attraction to an additional electron decreases as the atoms grow larger. If a species is less likely to
accept an electron, we say that it has a lower electron affinity. The electron affinity will be less
positive.
Therefore, of the atoms given, the
one with the greatest electron affinity will be closer to the top of period one. This atom is sodium. Therefore, the atom with the
greatest electron affinity is (B), Na.
