X could be S, Se, Te, Po or Lv.
Explanation:
From the formula 
, it can be determined the charge of the ion X:
Li is a metal from the group 1 (alkaline metal) in the periodic table,so the only charge possible for it is +1. As the formula contains two Li, the total positive charge gives +2. This impplies that X must holds a charge of -2.
Inspection of the periodic table shows that atoms that commonly gain two electrons forming -2 charged anions are the ones in group 16.
The second part of the text states that X can accommodate 12 electrons expanding its octect. This effect can be observed in atoms that have empty d-orbitals available to accomodate those extra electrons. Thus, atoms from period 3 and above show octect expansion.
The elements that are in group 16 in period 3 or above are S, Se, Te, Po and Lv.
