Number of possible isomers for the complex [Co(en)₂Cl₂]Cl will be (en = ethylenediamine)

Gadolinium belongs to 4f series. Its atomic number is 64. Which of the following is the correct electronic configuration of gadolinium?

Which of the following reaction(s) can be used for the preparation of alkyl halides?

The oxidation of benzene by V₂O₅ in the presence of air produces