An unknown transition metal salt X is subjected to the borax bead test. It forms a transparent blue bead when heated in an oxidizing flame and allowed to cool. When the same bead is heated in a reducing flame and cooled, it turns opaque red. The spin-only magnetic moment of the metal cation responsible for the blue colour in the oxidizing flame is closest to:

Step-by-step solution

The characteristic colour changes in the borax bead test identify the metal present in salt X . A blue bead in an oxidizing flame (when cold) that turns opaque red in a reducing flame (when cold) is characteristic of copper. In the oxidizing flame, copper forms copper(II) metaborate, Cu ( BO ₂)₂ . The metal cation responsible for the blue colour is Cu ²⁺ . The atomic number of copper is 29 . Its electronic configuration is [ Ar ] 3d¹⁰ 4s^1 . For the Cu ²⁺ ion, the electronic configuration is [ Ar ] 3d^9 . In the 3d