Which of the letters corresponds to
the element with the highest second ionization energy?
We’re given an abbreviated periodic
table with most of the elements missing and some of them filled in. The second ionization energy is the
energy that’s needed to remove an outer electron from a plus one ion. So, this would be the energy you
need to remove an electron from a plus one ion so that you end up with a plus two
ion. Remember that electrons are
attracted to the nucleus of an atom, so removing an outer electron means that we
need to put in energy to overcome this attraction to the nucleus.
Successive ionization energies, so
the second ionization energy or higher, is always going to be larger than the first
ionization energy. This is because in the second
ionization energies and higher ionization energies, we’re removing an electron from
a species that’s positively charged, which means that there’s more protons than
there are electrons. Since there’s more protons than
electrons, the outer electrons are being attracted to the nucleus more than they
would be in a neutral atom.
Higher ionization energies follow
the same general trend as the first ionization energy. And that it decreases going down a
group and increases across a period moving left to right. But there are some interesting
exceptions in the second and higher ionization energies. Let’s look at the first three
ionization energies for both lithium and beryllium, which are right next to each
other on the periodic table. This question is about the second
ionization energy, not the third. But looking at all three will give
us a good idea of the trend that’s occurring here.
Looking at the first three
ionization energies for lithium, we’ll see that between the first two ionization
energies, there’s an enormous jump. This is a factor of 14 increase
between the two. Between the second and the third,
there is an increase, but it’s not as dramatic as between the first and the
second. Between the second and the third,
the ionization energy only increases by about a factor of two.
If we look at the first three
ionization energies of beryllium, there’s an enormous jump between the second and
the third ionization energies. The second ionization energy of
beryllium is actually lower than the second ionization energy of lithium. So, what’s going on here?
Well, lithium has just one outer
electron and beryllium has two. So, for the first ionization energy
we remove lithium’s one outer electron. And the remaining ionization
energies are removing the core electrons from lithium. So, the second ionization energy
for lithium is so much higher than the first because we’re removing a core electron,
not an outer electron.
Beryllium has two outer
electrons. So, there’s two electrons that we
can remove before we start removing core electrons. This is reflected by the large jump
in the third ionization energy that we see in beryllium just like we see in
lithium. Lithium just has fewer outer
electrons, so this jump happened sooner.
In this question, we want to
identify which element will have the highest second ionization energy. So, the answer to this question
will probably be similar to what we see in lithium, where the second ionization
energy is really high because we’re removing a core electron. However, lithium isn’t an answer
choice. So, the element that we’re looking
for must be A because, like with lithium, A only has one outer electron. So, its second ionization energy
will be really high because we’re trying to remove a core electron.