The minimum amount of energy required to remove the most loosely bound electron form an isolated atom in the gaseous state is known as ionization energy or first ionization energy or inonization enthalpy (IE_(1)) of the element. The energy required to remove the second electron from the monovalent cation is called second ionization enthalpy (IE_(2)). Simllarly, we have third, fourth .................... fonization enthalpies. The values of ionization energy depends on a number of factors such as (i) size of the atom (ii) screening effect (iii) nuclear charge (iv) half filled and fuly filled orbitals (v) shape of orbital. In a group, the ionization energy decreases from top to bottom. In a period, the valup of ionisation energy increases from left to right with breaks where atoms have somewhat stable configurations. In a period, the ionization energy is lowest for the