CBSE Class 12 Chemistry Notes: d and f Block Elements – Transition Elements, Complexes & Properties

Complexes and their Properties:

The transition elements have an unparalleled tendency to form coordination compounds with the Lewis bases, which are called as ligands.

s and p block elements form very few complexes. The reason transition elements are so good at forming complex is that they have small, highly charged ions and have vacant low energy orbitals to accept lone pairs of electrons donated by ligands.

Size of Atoms and Ions

The covalent radii of the elements decrease from left to right across a row in the transition series.

This is because of the poor screening by the d electrons due to which, the nuclear charge attracts all of the electrons more strongly, hence a contraction in size occurs.

The elements in the first group in the d-block show the excepted increase (due to the addition of extra shell) in size Sc-> Y -> La. However in the subsequent groups there is an increase between first and second members, but hardly any increase between second and third elements. This is due to lanthanide contraction (discussed in f-block elements).

Colour

Many compounds of transition elements are coloured in contrasts to those of s and p block elements.

In compound state due to the surrounding groups (ligands), the d-orbitals of transition elements are not degenerate but split into two groups of different energy. Thus it is possible to promote electrons from one group to another group. This corresponds to fairly small amount of energy difference and so light is absorbed in visible region. Some compounds of transition metals are white, for example ZnSO₄ and TiO₂. In these compounds it is not possible to promote the electrons within the d-level.

Illustration 1. Why Zn²⁺ salts are white while Ni²⁺ salts are blue?

Solution: Zn²⁺ has completely filled d-orbitals (3d¹⁰) while Ni²⁺ has incompletely filled dorbitals (3d⁸).

Illustration 2. Why Zn²⁺ salts are white while Cu²⁺ salts are blue?

Solution: Reason same as above.

Illustration 3. Giving reasons indicate which one of the following would be coloured? Cu⁺, VO²⁺, Sc³⁺, Ni²⁺ (At. Nos Cu = 29, V = 23, Sc = 21, Ni = 28)

Solution: Ni²⁺ due to incompletely filled d-orbitals.

Transition Elements complexes and Their Properties-II:

Magnetic Properties

On the basis of behaviour in a magnetic field, substance are classified as paramagnetic, diamagnetic and ferromagnetic. Those substance which are attracted by the applied magnetic field are called paramagnetic where as those which are repelled by the magnetic field are called diamagnetic. Substances which are very strongly attracted by the applied field are called ferromagnetic.

Paramagnetism is a property due to the presence of unpaired electrons. Thus most of the transition metals are paramagnetic. As the number of unpaired electrons increases, the paramagnetic character also increases.

The magnetic moment is calculated from the following formula

Where n is the number of unpaired electrons and B. M stands for Bohr magneton.

Illustration 1. Why does Mn(II) show maximum papamagentic character amongst the bivalent ions of the first transition series?

Solution: Mn2+ has maximum number of unpaired electrons i.e. 3d⁵.

Illustration 2. A substance is found to have a magnetic moment of 3.9 B.M. How many unpaired electrons does it contain?

Solution: Using the formula,

Catalytic Properties

Many transition metals and their compounds have catalytic properties. For e.g. V₂O₅, Fe, FeCl₃, Ni, Pd etc.

This property of transition elements is due to their variable oxidation states. In some cases the transition metals with their variable valency may form variable unstable intermediate compounds. In other cases the transition metal provides a suitable reaction surface.

NON STOICHIOMETRY

Another feature of the transition elements is that they sometimes form non stoichiometry compounds. These are compounds of indefinite structure and proportions. For example .

It is mostly due to the variable valency of transition elements. Sometimes, non stoichiometry is caused by defects in the solid structures.

ALLOY FORMATION

Alloys are homogenous solid solutions of two or more metals obtained by melting the components and then cooling the melt. These are formed by metals whose atomic radii differ by not more than 15% so that the atoms of one metal can easily take up the positions in the crystal lattice of the other. Since transition metals have similar atomic radii, they form alloys very readily.

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