Video Transcript
The table below shows the first and
second electron affinities for atoms of nitrogen and phosphorus. Why is the change in energy for
both electron affinities lower for atoms of phosphorus than for atoms of
nitrogen? (A) An atom of phosphorus has fewer
electrons in its outer valence shell than an atom of nitrogen, and so there is less
repulsion from the addition of electrons. (B) The larger size of an atom of
phosphorus results in less electron repulsion when the first and second electrons
are added, compared with the smaller atom of nitrogen. (C) The added electrons are placed
closer to the nucleus in an atom of phosphorus, and so they experience strong
attraction from the nucleus. Or (D) an atom of phosphorus
obtains a half-filled p subshell of three electrons through the addition of an
electron, which an atom of nitrogen does not.
In this question, we must identify
why the first and second electron affinities of phosphorus and nitrogen differ. Let’s clear some space on screen to
discuss these differences.
This question discusses electron
affinity, which is the amount of energy released when an electron is added to a
neutral atom in the gas state to form a negative ion. It is typically measured in
kilojoules per mole. It can be abbreviated as 𝐸 ea. When the electron affinity is
positive, energy is released. This means the enthalpy change is
negative and the process of adding an electron is exothermic.
Conversely, when electron affinity
is negative, energy must be absorbed and the process is endothermic with a positive
enthalpy change. For nitrogen, both the first and
second electron affinities are negative and thus are endothermic processes,
requiring energy to be absorbed from the surroundings in order for electrons to be
added to the atom, while for phosphorus, the first electron affinity is exothermic
and the second is less endothermic than that of nitrogen.
If we locate nitrogen and
phosphorus on the periodic table, we will find that they both reside in group 15,
with nitrogen in period two and phosphorus in period three. This means phosphorus has more
occupied electron shells than nitrogen. So nitrogen will have a smaller
atomic radius. This means the electrons are closer
together. Thus, there is more repulsion to
the added electron and more energy is required for this process to occur.
The opposite is true for
phosphorus, where the electrons and negative charge are more spread out in a larger
atomic radius. There is less repulsion for the
added electron and the process requires less energy. This is true for both the first and
second electrons added.
Using this information, let’s have
another look at the answer choices.
We can see that only one answer
choice mentions the large size of phosphorus resulting in less repulsion for the
added electrons. Answer choice (B) is correct. So, the reason that the change in
energy for both electron affinities is lower for atoms of phosphorus than for atoms
of nitrogen is answer choice (B). The larger size of an atom of
phosphorus results in less electron repulsion when the first and second electrons
are added, compared with a smaller atom of nitrogen.
