$A$ transition element $X$ has a configuration $[Ar]3d^4$ in its $+3$ oxidation state. Its atomic number is

Which of the following compounds is expected to be coloured?

Niobium $(Nb)$ and ruthenium $(Ru)$ have " $x$ " and " $y$ " number of electrons in their respective $4d$ orbitals. The value of $x+y$ is $...........$

The aqueous solution containing which one of the following ions will be colourless?

In the periodic table,metals usually used as catalysts belong to:

Which of the following ions is diamagnetic?