The following ionization energies were found for an unknown element: -
a) 1st ionization energy 9.32270 ev
b) 2nd ionization energy 18.2114 ev
c) 3rd ionization energy 153.89661 ev
d) 4th ionization energy 217.71865 ev
what is a common ion for the unknown element above

The common ion will be di-positive ion.

Explanation:

The ionization energy is defined as the amount of energy needed for removal of most loosely bound electron from an isolated atom in gaseous state.

The low ionization energy shows that the atom is able to give electron easily as after losing electron it may attain noble gas configuration or half filled stability.

Here the first and second ionization energy, both are low suggesting that the element is ready to give two electrons easily to form a di-positive ion however the third ionization energy is high which shows that it will not form tri-positive ion commonly.

it forms an anion that has a larger radius. anions have a negative charge and are larger than the original atom.