According to his analogy, the inner planets are rocky and mostly heavier metals,

The appropriate response would be Tellurium. For the reason that it can receive a - 2 charge, and when it grabs two electrons, it fills the 5p shell with six electrons, so it will have the electronic arrangement of [Xe]. It cannot be Barium since if it loses two electrons, it will get the electronic configuration of [Xe] when it loses the 6s2 electrons, but it will be a +2 cation, not an anion.