(i) Transition metals have the ability to adsorb many other substances on their surface and activate them as a result of chemisorption. Transition metals also exhibit a variety of oxidation states, and can change oxidation states relatively easily. This makes transition metals and their compounds good catalysts. (ii) $E^{\circ}$ values are indicative of the stability of the oxidized form of the element. The lower the $E^{\circ}$ value, more stable the oxidized form of the element. $\mathrm{Mn}^{2+}$ with a half filled d-subshell $(d^5)$ is stable, so $\mathrm{Mn}$ is easily oxidized to $\mathrm{Mn}^{2+}$, making the $E^{\circ}$ value negative. $\mathrm{Cu}^{2+}$ with a partially filled d subshell $(d^9)$ is not stable, and is relatively easily reduced to element form. This makes its $E^{\circ}$ value positive. (iii) The electronic configurations of actinoids show irregularities because the energies of their $5 f, 6 d$, and $7 {s}$-orbitals are close to each other. Electrons can easily move between these subshells.