Identify the element having highest ionisation enthalpy ( IE ₁ ) from following.

Step-by-step solution

The first ionization enthalpy represents the energy needed to remove the most loosely bound electron from a neutral gaseous atom. Lanthanides exhibit a general increasing trend due to the lanthanide contraction , which raises the effective nuclear charge with atomic number. However, deviations occur when stable electronic configurations (half-filled or fully-filled f-orbitals) are present. Relevant electronic configurations include La: [ Xe ] 5d^1 6s^2 , Ce: [ Xe ] 4f^1 5d^1 6s^2 , Gd: [ Xe ] 4f^7 5d^1 6s^2 (half-f