JEE Main Chemistry Question (2026) — Solution

Step-by-step solution

The borax bead test is given by transition metal ions that have unpaired electrons and form coloured compounds. Zn^ 2+ has a completely filled 3d^ 10 configuration, making it colourless, so it does not give a positive borax bead test. The remaining ions and their electronic configurations are: Fe^ 2+ : [Ar] 3d^ 6 Cr^ 2+ : [Ar] 3d^ 4 Fe^ 3+ : [Ar] 3d^ 5 The ionisation enthalpy of an ion refers to the energy required to remove an electron from it. Fe^ 3+ has a highly stable half-filled d-subshell configuration (3d^ 5 ). Removing an electron from this stable configuration requires significantly more energy compared to removing an electron from Fe^ 2+ (3d^ 6 ) or Cr^ 2+ (3d^ 4 ). Therefore, Fe^ 3+ shows a positive borax bead test and has the highest ionisation enthalpy among the given ions. Answer: Fe^ 3+