The following table shows rounded successive ionization energies of element X. What is the correct identity of X? (A) Germanium, (B) tellurium, (C) strontium, (D) bromine, or (E) krypton.
In this question, we must determine the identity of element X from its successive
ionization energies. Successive ionization energies are the amounts of energy needed to consecutively
remove electrons from one gaseous-state element. These equations show the first and second ionization energies of sulfur. A neutral sulfur atom loses one electron during the first ionization energy
event. And a sulfur one plus cation loses another electron during the second ionization
Let’s use sulfur as an example and consider its first eight ionization energies. We can see that the first six ionization energies increase steadily. We can also see that there is a striking difference between the sixth and seventh
ionization energies for sulfur. We can understand this significant difference if we look at a Bohr model of a sulfur
atom. The first six electrons removed are all in the outermost energy level of the
atom. These increase gradually as the atom becomes a more and more positive cation with the
removal of consecutive electrons. Thus, more and more energy is required to overcome the attraction of the electrons to
the positive nucleus.
However, the first six electrons removed were on the valence shell. Sulfur is in group 16 of the periodic table and has six electrons in its outermost
shell. The seventh removed electron of sulfur is much closer to the nucleus than the first
six, as it is in an altogether different electron shell. Much more energy is needed to remove the seventh electron because it is much closer
to the nucleus. The pattern of successive ionization energies can reveal the group number of the main
group elements, as we saw with sulfur. So, let’s have a look at the pattern of successive ionization energies of element
The first six ionization energies increase steadily. Then, there is a striking increase between the sixth and seventh ionization
energies. This means element X must have six valence electrons and be located in group 16 of
the periodic table. If we locate our answer choices on the periodic table, tellurium is the only element
given that is located in group 16 and contains six valence electrons. Therefore, the correct identity of element X is answer choice (B) tellurium.