Four consecutive 3 d transition metals A , B , C and D have standard enthalpies of atomisation of 515, 397, 281 and 416 kJ mol ⁻¹ respectively. The number of unpaired electrons present in the ground state of element B is

Step-by-step solution

The standard enthalpies of atomisation for the 3 d series show a general trend with a notable dip at Chromium and a deep dip at Manganese. The given values exactly match the sequence from Vanadium to Iron: V = 515 kJ mol ⁻¹ Cr = 397 kJ mol ⁻¹ Mn = 281 kJ mol ⁻¹ Fe = 416 kJ mol ⁻¹ Since A , B , C and D are consecutive elements, element B corresponds to Chromium ( Cr ). The anomalous ground state electronic configuration of Cr is [ Ar ] 3 d ⁵ 4 s ¹ . It has 5 unpaired electrons in the 3 d subshell and 1 unpaired elec