Transition Elements

Transition elements are the d-block elements, and the modern name for the block is outer transition elements, because their valence electrons fill the subshell one shell in from the outside, the $(n-1)d$ level, while the outermost $ns$ shell already holds its one or two electrons. Their general outermost configuration is $(n-1)d^{1-10}ns^{1-2}$. The first row, the 3d series, runs from scandium to zinc and covers atomic numbers 21 to 30. Group IIIB marks the start with Sc, Y and La, and the block closes at group IIB with Zn, Cd and Hg. Beyond the d-block sit the f-block elements, the inner transition series, with the lanthanides filling the 4f level and the actinides the 5f. Counted together, the transition elements number 58.

The definition has an edge, and the exam tests it. A transition element is properly one that has a partly filled d subshell, either in the element itself or in one of its common ions. Zinc, cadmium and mercury fail that test, because their configuration ends in $d^{10}$ in the element and stays $d^{10}$ in their only common ion, so they are non-typical and lack most of the characteristic properties, even though they are drawn inside the block and do form a few complexes with ammonia, halide ions and amines. The coinage metals copper, silver and gold of group IB pass, because although the free metals are $d^{10}$, their ions break the shell open: $Cu^{2+}$ is $3d^9$, $Ag^{2+}$ is $4d^9$ and $Au^{3+}$ is $5d^8$. Group IIIB is the second non-typical group, because Sc, Y and La lose their single d electron when they form the $+3$ ion and leave none behind.