A black-coloured oxide (A) of a 3d transition metal, when heated in a stream of dihydrogen gas, yields a reddish-brown metal (B) and a colourless liquid. The spin-only magnetic moment of the metal ion in the oxide (A) is:

Step-by-step solution

The reddish-brown metal (B) obtained by the reduction of the oxide with dihydrogen is copper ( Cu ). The colourless liquid is water ( H ₂ O ). The black-coloured oxide (A) of copper is copper(II) oxide ( CuO ). In CuO , the oxidation state of copper is +2 . The electronic configuration of neutral Cu is [ Ar ] 3 d ¹⁰ 4 s ^1 . The electronic configuration of Cu ²⁺ is [ Ar ] 3 d ^9 . The 3 d ^9 configuration has 1 unpaired electron ( n = 1 ). The spin-only magnetic moment ( ) is given by: = n(n+2) BM = 1(1+2) = 3 1.73