Transition Metal - Classification

Classification

In the d-block the atoms of the elements have between 1 and 10 d electrons.

With a few minor exceptions, the electronic structure of transition metal atoms can be written as ns2(n-1)dm, where the inner d orbital has more energy than the valence-shell s orbital. In divalent and trivalent ions of the transition metals, the situation is reversed such that the s electrons have higher energy. Consequently, an ion such as Fe2+ has no s electrons: it has the electronic configuration 3d6 as compared with the configuration of the atom, 4s23d6.

The elements of groups 4–11 are now generally recognized as transition metals, as are Sc and Y in Group 3. For the elements La-Lu and Ac-Lr and also for Group 12, different sets of definitions are used by different authors.

1. Many chemistry textbooks and printed periodic tables classify La and Ac as Group 3 elements and transition metals, since their atomic ground-state configurations are s2d1 like Sc and Y. The elements Ce-Lu are considered as the “lanthanide” series (or “lanthanoid” according to IUPAC) and Th-Lr as the “actinide” series. The two series together are classified as f-block elements, or (in older sources) as “inner transition elements”.
2. Some inorganic chemistry textbooks include La with the lanthanides and Ac with the actinides. This classification is based on similaritites in chemical behaviour, and defines 15 elements in each of the two series even though they correspond to the filling of an f subshell which can only contain 14 electrons.
3. A third classification defines the f-block elements as La-Yb and Ac-No, while placing Lu and Lr in Group 3. This is based on the aufbau principle (or Madelung rule) for filling electron subshells, in which 4f is filled before 5d (and 5f before 6d), so that the f subshell is actually full at Yb (and No) while Lu (and Lr) has an s2f14d1 configuration. However La and Ac are exceptions to the Aufbau principle with electron configuration s2d1 (not s2f1 as the aufbau principle predicts) so it is not clear from atomic electron configurations whether La or Lu (Ac or Lr) should be considered a transition metal.

Zinc, cadmium, and mercury are sometimes not classified as transition metals as they have the electronic configuration d10s2, with no incomplete d shell. In the oxidation state +2 the ions have the electronic configuration d10. These elements can exist many other oxidation states, including the +1 oxidation state, as in the diatomic ion Hg2+
2. The group 12 elements Zn, Cd and Hg may be classed as post-transition metals in this case, because of the formation of a covalent bond between the two atoms of the dimer. However, it is often convenient to include these elements in a discussion of the transition elements. For example, when discussing the crystal field stabilization energy of first-row transition elements, it is convenient to also include the elements calcium and zinc, as both Ca2+ and Zn2+ have a value of zero against which the value for other transition metal ions may be compared. Another example occurs in the Irving-Williams series of stability constants of complexes.

However the recent synthesis of mercury(IV) fluoride (HgF₄) has led to debate as to whether these elements should now always be considered a transition metal.