`=>` The `color{red}(d)`-block of the periodic table contains the elements of the groups `3-12` in which the `color{red}(d)`-orbitals are progressively filled in each of the four long periods.

`=>` The elements constituting the `color{red}(f)`-block are those in which the `color{red}(4 f)` and `color{red}(5 f)` orbitals are progressively filled in the latter two long periods; these elements are formal members of group `3` from which they have been taken out to form a separate `color{red}(f)`-block of the periodic table.

`=>` The names transition metals and inner transition metals are often used to refer to the elements of `color{red}(d)`-and `color{red}(f)`-blocks respectively.

`=>` There are mainly three series of the transition metals, `color{red}(3d)` series (`color{red}(Sc)` to `color{red}(Zn)`), `color{red}(4d)` series (`color{red}(Y)` to `color{red}(Cd)`) and `color{red}(5d)` series (`color{red}(La)` to `color{red}(Hg)`, omitting `color{red}(Ce)` to `color{red}(Lu)`). The fourth `color{red}(6d)` series which begins with `color{red}(Ac)` is still incomplete.

`=>` The two series of the inner transition metals, (`color{red}(4f)` and `color{red}(5f)`) are known as lanthanoids and actinoids respectively.

`=>` A transition element is defined as the one which has incompletely filled `color{red}(d)`-orbitals in its ground state or in any one of its oxidation states.

● Zinc, cadmium and mercury of group `12` have full `color{red}(d^(10))` configuration in their ground state as well as in their common oxidation states and hence, are not regarded as transition metals.

● However, being the end members of the three transition series, their chemistry is studied along with the chemistry of the transition metals.

`=>` The presence of partly filled `color{red}(d)` or `color{red}(f)` orbitals in their atoms sets the study of the transition elements and their compounds apart from that of the main group elements.

● However, the usual theory of valence as applicable to the main group elements can also be applied successfully to the transition elements.

`=>` Various precious metals such as silver, gold and platinum and industrially important metals like iron, copper and titanium form part of the transition metals.

`=>` The `color{red}(d)`–block occupies the large middle section flanked by `color{red}(s)-` and `color{red}(p)-` blocks in the periodic table.

`=>` The very name ‘transition’ given to the elements of `color{red}(d)`-block is only because of their position between `color{red}(s-)` and `color{red}(p-)` block elements.

`=>` The `color{red}(d-)`orbitals of the penultimate energy level in their atoms receive electrons giving rise to the three rows of the transition metals, i.e., `color{red}(3d)`, `color{red}(4d)` and `color{red}(5d)`. The fourth row of `color{red}(6d)` is still incomplete. These series of the transition elements are shown in Table 8.1.

`=>` In general the electronic configuration of these elements is `color{red}((n-1)d^(1–10) ns^(1–2))`. The (`color{red}(n–1)`) stands for the inner `color{red}(d)`-orbitals which may have one to ten electrons and the outermost `color{red}(ns)`-orbital may have one or two electrons.

`=>` However, this generalisation has several exceptions because of very little energy difference between `color{red}((n-1)d)` and `color{red}(ns)` orbitals.

● Furthermore, half and completely filled sets of orbitals are relatively more stable.

● A result of this factor is reflected in the electronic configurations of `color{red}(Cr)` and `color{red}(Cu)` in the `color{red}(3d)` series.

● Consider the case of `color{red}(Cr)`, for example, which has `color{red}(3d^5 4s^1)` instead of `color{red}(3d^4 4s^2)`; the energy gap between the two sets (`color{red}(3d)` and `color{red}(4s)`) of orbitals is small enough to prevent electron entering the `color{red}(3d)` orbitals.

● Similarly in case of `color{red}(Cu)`, the configuration is `color{red}(3d^(10) 4s^1)` and not `color{red}(3d^9 4s^2)`. The outer electronic configurations of the transition elements are given in Table 8.1.

`=>` The electronic configurations of `color{red}(Zn)`, `color{red}(Cd)` and `color{red}(Hg)` are represented by the general formula `color{red}((n-1)d^(10) ns^2)`.

● The orbitals in these elements are completely filled in the ground state as well as in their common oxidation states.

● Therefore, they are not regarded as transition elements.

`=>` The `color{red}(d)`-orbitals of the transition elements project to the periphery of an atom more than the other orbitals (i.e., `color{red}(s)` and `color{red}(p)`), hence, they are more influenced by the surroundings as well as affecting the atoms or molecules surrounding them.

● In some respects, ions of a given `color{red}(d^n)` configuration (`color{red}(n = 1 - 9)`) have similar magnetic and electronic properties.

● With partly filled `color{red}(d)`-orbitals these elements exhibit certain characteristic properties such as display of a variety of oxidation states, formation of coloured ions and entering into complex formation with a variety of ligands.

● The transition metals and their compounds also exhibit catalytic property and paramagnetic behaviour.

`=>` There are greater horizontal similarities in the properties of the transition elements in contrast to the main group elements.

● However, some group similarities also exist.