Among Fe²⁺, Fe³⁺, Cr²⁺ and Zn²⁺ , the ion that shows positive borax bead test and with highest ionisation enthalpy is:

Step-by-step solution

The borax bead test is given by transition metal ions that have unpaired electrons and form coloured compounds. Zn²⁺ has a completely filled 3d¹⁰ configuration, making it colourless, so it does not give a positive borax bead test. The remaining ions and their electronic configurations are: Fe²⁺ : [Ar] 3d⁶ Cr²⁺ : [Ar] 3d⁴ Fe³⁺ : [Ar] 3d⁵ The ionisation enthalpy of an ion refers to the energy required to remove an electron from it. Fe³⁺ has a highly stable half-filled d -subshell configuration ( 3d⁵ ). Removing an el