Ionisation energy of He+  is  19.6  ×10−18  J  atom−1. The energy of the first stationary state (n = 1) of Li2+  is

IE=−E1E1  for  He+=−19.6  ×10−18  J  atom−1E1He+E1Li2+ = ZHe+2ZLi2+2−19.6  ×10−18E1Li2+=49Or      E1Li2+=−19.6  ×9 × 10−184=−4.41 × 10−17  J  atom−1