An ionic compound is formed by a bond between a metal and a nonmetal element. To form its formula, we first represent the metal atom and then the nonmetal atom in their ionic formulas, and then we change their charges, without the signal.
So, in the ionic compound X₂O, the ions must be:
and .
The element X loses one electron to be stable, so it's on group 1. Searching in the Periodic Table, this element is Sodium (Na).
The answer is Vestigal. It is a type of anatomical structure that no longer serves any function. Over time they are no longer needed in the body. Apart from the wisdom tooth, another example would be the tail bone which has no use for humans.