Which has least first ionization enthalpy ? (i) H  (ii) He (iii) Xe (iv) Li  and also explain why ?

Li has least ionisation energy among the given elements. The reason is
H - 1s¹  - Closer to nucleus so need more energy
He- 1s² - Stable electronic configuration as its an inert gas
Li-1s²2s¹ - Easily removable electron. So less IE
Xe- ​ [Kr] 4d¹⁰5s²5p⁶ - Stable electronic configuration as its an inert gas​