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Introduction
Transition elements metals are any of various chemical elements that possess valence electrons—i.e., electrons which participate in the formation of chemical bonds—in two shells instead of only one. Whereas the term transition has no particular chemical significance, it is only a convenient name which is used to distinguish the similarity of the atomic structures and resulting properties of the elements so designated. They are present at the middle portions of the long periods of the periodic table of elements between the groups on the left-hand side and the groups on the right. Basically, they form Groups 3 (IIIb) through 12 (IIb).
What are Transition Elements?
The IUPAC definition defines a transition element as “an element whose atom possesses a partially filled d subshell, or that can give rise to cations with an incomplete d subshell”. English chemist Charles Rugeley Bury was the first to use the word transition in his context in the year 1921, when he referred to a transition series of elements at the change of an inner layer of electrons (for example n = 3 in the 4th row of the periodic table) from a stable group of 8 to one of 18, or from 18 to 32. These elements are now referred to as the d-block. Compounds of various transition elements are different for being widely coloured. As visible light passes through a transition metal compound gets dissolved in water, the d-orbitals absorbs light of various energies. The visible light of a given energy level that is not absorbed produces a distinctly coloured solution.
Electronic Configuration of Transition Elements
The electronic configuration of the d-block elements (i.e. noble gas) can be represented as (n – 1)d1–10ns0–2. Here the “noble gas” is the configuration of the last noble gas preceding the atom in question, and n represents the highest principal quantum number of an occupied orbital in that atom. As we can see Ti(Z = 22) lies in period 4 so n = 4, the first 18 electrons possess the same configuration of Ar at the end of period 3, and the overall configuration is (Ar)3d24s2. The period 6 and 7 transition metals also add core (n – 2)f14 electrons. The single exception is lawrencium, which possesses a 7p occupancy because of the relativistic effects that become important at such high Z (though the p-orbitals can also contribute to chemical bonding in lighter transition elements).
The Madelung rule suggests that the inner d orbital is filled after the valence-shell s orbital. The general electronic structure of transition metal atoms can be represented as ns2(n – 1)d1-10. This rule holds true only for some of the transition elements, and only then in the neutral ground states.
The d subshell is present next-to-last subshell and is written as (n–1)d sub-shell. The number of s electrons in the outermost s sub-shell is basically one or two except palladium (Pd), that have no electron in that s subshell in its ground state. The s subshell in the valence shell is denoted as the ns sub-shell, e.g. 4s. In the periodic table, the transition metals are situated in the eighth group (4 to 11).
General Properties of Transition Elements
The important characteristics of transition metals are:
The transition elements are generally metallic in nature, i.e. all are metals.
These metals possess varying oxidation states.
Transition metal atoms or ions usually form the complexes having neutral, negative and positive ligands.
All the compounds of transition metals are mainly coloured.
The compounds of these metals are generally paramagnetic in nature.
The compounds of transition metals function as good catalysts, i.e., they show catalytic activities.
These metals form different alloys with other metals of the series.
These metals are able to form interstitial compounds with C, N, B and H.
The occurrence of partially filled d-orbitaLs in the electronic configuration of atomic and ionic species of these elements are responsible for the characteristic properties of transition elements. They are known as transition elements due to their position in the periodic table. These elements are situated at the middle of periodic table between s and p-blocks. A transition element might also be defined as an element whose atom or at least one of its simple ions comprises partially filled d-orbitals, such as iron, copper, chromium, nickel etc.
The elements of group 12 such as Zinc, Cadmium, and Mercury are usually not regarded as transition elements since their atoms and all ions formed possess completely filled d-orbitals i.e., these do not possess partially filled d-orbitals in atomic state or common oxidation state (Zn2+, Cd2+, Hg2+).
Conclusion
The transition elements are the largest group of elements that are present on the periodic table, and they are situated in the middle of the table. However, the two rows of elements below, the main body of the periodic table (lanthanides and actinides) are subsets of these metals. These elements are known to be “transition metals” since their atoms’ electrons transition to filling the d subshell or d sublevel orbital. As a result, transition metals are also referred to as d-block elements. Some of the examples of the transition elements are listed below:
