An acidified solution of potassium chromate was layered with an equal volume of amyl alcohol. When it was shaken after the addition of 1 mL of 3% H₂O₂, a blue alcohol layer was obtained. The blue colour is due to the formation of a chromium (VI) compound 'X'. What is the number of oxygen atoms bonded to chromium through only single bond in a molecule of X?

Among the species given below, the total number of diamagnetic species is _____________.

$$H$$ atom, $$N{O_2}$$ monomer, $${O_2}^ - $$ (superoxide), dimeric sulphur in vapor phase, $$M{n_3}{O_4},\,\,$$ $${\left( {N{H_4}} \right)_2}\left[ {FeC{l_4}} \right],$$ $${\left( {N{H_4}} \right)_2}\left[ {NiC{l_4}} \right],\,$$ $$\,{K_2}Mn{O_4},\,$$$${K_2}Cr{O_4}$$

Among PbS, CuS, HgS, MnS, Ag₂S, NiS, CoS, Bi₂S₃ and SnS₂ the total number of black coloured sulphides is