An element with atomic number $21$ is a

Which pair of elements from the following has half-filled $d$-orbitals in their observed electronic configuration?

Which of the following hydroxides acts as the strongest base?

Determine whether the following statements are True $(T)$ or False $(F)$:
$(a)$ Transition elements contain $d$-orbitals.
$(b)$ Ions of transition elements contain incompletely filled $d$-orbitals.
$(c)$ Only elements with completely filled $3d$ orbitals are considered transition elements.
$(d)$ Only elements that have incompletely filled $d$-orbitals in their ground state as well as in their ions are called transition elements.

Which of the following metal ions will have the maximum number of unpaired electrons?

The transition metals are mostly