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What are Transition Elements?
Transition elements are those elements which partially fill d and F subshells. According to IUPAC definition, a transition metal is an element with a partially-filled d sub-shell atom, or which can give rise to cations that have an incomplete d sub-shell.”
Primarily, transition element refers to the d block elements. However, a few 2B elements such as cadmium, zinc, and mercury are often regarded as the defined elements due to their similar properties. Moreover, the elements of f block are also known as “inner transition elements”.
These elements occupy the middle part of the periodic table and situate between the left-hand side groups and right-hand side groups.
Placement of Transition Elements
The transition metals remain between s and p block elements. Primarily they are classified into three segments.
First transition series (Sc to Cu)
Second transition series (Y to Ag)
Third transition series (La and the elements from Hf to Au)
(Image to be added soon)
Fig: Periodic table
Sixth and seventh series contain f-block elements that are named as lanthanoids and actinoids. The fourteen elements starting from Cerium to Lutetium are known as lanthanoids.
On the other hand, the fourteen elements starting from nuclear number 90 (Thorium) to 103 (Lawrencium) are actinoids. The elements of actinides are radioactive, and those that are above Z=92, are generally humanmade in accelerators or nuclear reactors.
Some important transition elements are copper, iron and silver. Also, titanium and iron are the most abundant among all.
The first member of the fourth series of transition is Actinium (Ac), which contains elements from Rf to Rg as well. II- B has Zn, Cd, and Hg and III-A possesses Sc, Y, La, and Ac that are non-typical transition elements and the remaining ones are typical transition elements.
However, note that mercury, zinc, cadmium are not listed as transition metals because of their full d orbital.
Transition Elements Properties
The second and third-row elements of periodic table gradually show changes in properties if we move from left to right across the table. The outer shells of these elements have negligible shielding effects that increase effective nuclear charge as more protons add to the nucleus.
Therefore, it makes an effect on the atomic properties such as increased first ionisation energy, smaller atomic radius, enhanced electronegativity and other non-metallic characters.
These characteristic persist up to Calcium (Z=20), and then there is a shift. The succeeding ten elements are known as first transition elements, which contain almost same chemical and physical properties. Since additional electrons are added to the penultimate 3d shell, giving a shield between outer 4s shell and the nucleus.
The properties of transition elements are different than the main block elements (s-block). Following are a few characteristics.
Possess high charge/radius ratio.
Contain high density and hard.
Boiling and melting points are high.
Form paramagnetic compounds.
Display variable oxidation states.
Compounds and ions are usually coloured.
Form compounds having catalytic activity.
Produce stable complexes.