Introduction to transition metal
Most of the d block elements in the periodic table are transition metal elements. The following figure shows the d-block elements in periodic table.
Not all the d-block elements are transition metals. Transition metals are only those d-block elements which contain unfilled d-orbital even after losing electron to form ion.
Transition metals in periodic table
The electronic configuration of the d-block elements in 4th period are:
Here we can see that the continuation of filling one electron to the successive elements breaks in case of chromium and copper. This is because of the tendency to achieve filled (Cu) or half filled (Cr) inner shell d-orbitals. Though this explanation is not true for all other elements in periodic table. So sometimes we just need to remember the exceptions without any explanation.
Now we need to identify, which elements are transition metals among all these d-block elements. The electronic configuration of scandium is [Ar]3d14s2. Scandium loses 3 electrons to achieve Sc3+. The electronic configuration of Sc3+ is same as argon, that means no unfilled d-orbital left. Thus scandium is not a transition metal element. On the same way, the electron configuration of zinc after losing two electrons is [Ar]3d10. As the d-orbitals of zinc ion is filled with electrons, zinc is not a transition metal element. These two elements are in group 3 and 12 respectively.
On contrast the electron configuration of copper is [Ar]3d104s1. By losing 1 or 2 electrons copper becomes Cu+ ion with electron configuration [Ar]3d10 and Cu2+ ion with electron configuration [Ar]3d9 respectively. As copper contains electrons in d-orbitals after losing electrons, copper is a transition metal element. Thus generally the elements in group 4-11 are transition metal elements.
Properties of transition metal elements
Due to have partially filled d-orbitals, transition metals show characteristic properties which may differ them from other metals in periodic table.
Transition metals posses metal like characteristic. It has high density, high boiling and high melting point. Metallic bond in transition metals are formed by the delocalization of unfilled d-orbitals. The attraction between two atoms involved in metallic bond is increased with the increase of electrons in d-orbitals. Because of the free movement of the electrons within these d-orbitals, the transition metals are good conductor of electricity.
Transition metals have more than one oxidation states. They can lose the electrons from the s- or d-orbitals. The different oxidation states of transition metals are given below:
As for example oxidation states of manganese starts from +2 to +7. In KMnO4 manganese has +7 oxidation state and in MnO2 it has +4.
Because of having one or more unpaired electrons, transition metals are paramagnetic in nature. Paramagnetic properties of transition metal increases with the increase of unpaired electrons. In period it increases from left to right until it has maximum 5 or 6 unpaired electrons (such as in period 4, chromium), then it decreases going further down to the right till it has 1 unpaired electron (such as in period 4, silver). Transition metal compounds can also show paramagnetic nature if it contains unpaired electrons.
Transitional metals form colored compounds. As for example: the color of the aqueous solution of Co(NO3)2, K2Cr2O7, K2CrO4, NiCl2, CuSO4 and KMnO4 are shown below:
The reason behind this color is, when white light passes through these solutions the electrons can move between the d-orbitals by absorbing particular light wavelengths. The light wavelengths which are not absorbed, are showed as the color of the solutions.
The transition metals, itself or its compounds have catalytic properties. They can perform both as homogeneous and heterogeneous catalyst. As for example, catalytic hydrogenation in presence of nickel has given below:
For this case transition metal like nickel form lose bond with the reacting molecules on their surface using their d or s orbitals to form the product.