Chapter 4: The d & f Block Elements

(i) Cr²⁺ is strongly reducing in nature. It has a d⁴ configuration. While acting as a reducing agent, it gets oxidised to Cr³⁺ (electronic configuration, d³). This d³ configuration can be written as configuration, which is a more stable configuration. In the case of Mn³⁺ (d⁴), it acts as an oxidising agent and gets reduced to Mn²⁺ (d⁵). This has an exactly half-filled d-orbital and is highly stable.

(ii) Co(II) is stable in aqueous solutions. However, in the presence of strong field complexing reagents, it is oxidised to Co (III). Although the 3rd ionisation energy for Co is high, the higher amount of crystal field stabilisation energy (CFSE) released in the presence of strong-field ligands overcomes this ionisation energy.

(iii) The ions in the d¹ configuration tend to lose one more electron to get into a stable d⁰ configuration. Also, the hydration or lattice energy is more than sufficient to remove the only electron present in the d-orbital of these ions. Therefore, they act as reducing agents.

It is found that sometimes a relatively less stable oxidation state undergoes an oxidation-reduction reaction in which it is simultaneously oxidised and reduced. This is called disproportionation.

For example,

(a)

Cr(V) is oxidised to Cr(VI) and reduced to Cr(III).

(b)

Mn (VI) is oxidized to Mn (VII) and reduced to Mn (IV).

In the first series of transition metals, Cu exhibits +1 oxidation state very frequently. We know Cu has an electronic configuration of [Ar] 3d ¹⁰, that is, the completely filled d-orbital makes it highly stable.

Cr³⁺ is the most stable in aqueous solutions owing to a

(i) In the case of a lower oxide of a transition metal, the metal atom has a low oxidation state. This means that some of the valence electrons of the metal atom are not involved in bonding. As a result, it can donate electrons and behave as a base. On the other hand, in the case of a higher oxide of a transition metal, the metal atom has a high oxidation state. This means that the valence electrons are involved in bonding, so they are unavailable. There is also a highly effective nuclear charge. As a result, it can accept electrons and behave as an acid.

For example,

(ii) Oxygen and fluorine act as strong oxidising agents because of their high electronegativities and small sizes. Hence, they bring out the highest oxidation states from the transition metals. In other words, a transition metal exhibits higher oxidation states in oxides and fluorides.

For example, in OsF₆ and V₂O₅, the oxidation states of Os and V are +6 and +5, respectively.

(iii) Oxygen is a strong oxidising agent due to its high electronegativity and small size. So, oxo-anions of metal have the highest oxidation state. For example, in, the oxidation state of Mn is +7.

(i) Potassium dichromate ( K₂Cr₂O₇ ) is prepared from chromite ore (FeCr₂O₄) in the following steps.

Step (1): Preparation of sodium chromate

Step (2): Conversion of sodium chromate into sodium dichromate

Step(3): Conversion of sodium dichromate to potassium dichromate

Potassium chloride is less soluble than sodium chloride and is obtained in the form of orange-coloured crystals and can be removed by filtration.

The dichromate ion

However, by changing the pH, they can be interconverted.

(ii) Potassium permanganate ( KMnO₄ ) can be prepared from pyrolusite (MnO₂). The ore is fused with KOH in the presence of either atmospheric oxygen or an oxidising agent, such as KNO₃ or KClO₄, to give K₂MnO₄.

The green mass can be extracted with water and then oxidized either electrolytically or by passing chlorine/ozone into the solution. Electrolytic oxidation

At the anode, manganate ions are oxidised to permanganate ions.

Oxidation by chlorine

Oxidation by ozone

An alloy is a solid solution of two or more elements in a metallic matrix. It can be a partial solid solution or a complete solid solution. Alloys generally are created as they have varied properties compared to their constituent elements. An important alloy of lanthanoids is Mischmetal. It contains lanthanoids (94−95%), iron (5%), and traces of Al, Ca, Si, C, S.

Uses

(a) It is used in tracer bullets and shells.

(b) Mischmetal is used in cigarettes and gas lighters.

(c) It is used in flame-throwing tanks.

Inner transition metals: Elements in which the last electron enters the f-orbital.

The elements in which the 4f and the 5f orbitals are progressively filled are called f-block elements.

From the given atomic numbers, the inner transition elements are the ones with atomic numbers 59, 95, and 102.

Lanthanoids project 3 oxidation states (+2, +3, +4). From these, the +3 state is the most common. Lanthanoids display a limited number of oxidation states because the energy difference between 4f, 5d, and 6s orbitals’ is quite large. While the energy difference between 5f, 6d, and 7s orbitals’ is very less. Hence, actinoids display a large number of oxidation states.

For example, neptunium displays +3, +4, +5, and +7, while uranium and plutonium display +3, +4, +5, and +6 oxidation states. The most common oxidation state in the case of actinoids is also +3.

In the actinoid series, the last element is lawrencium, Lr. The atomic number of the element is 103, and its electronic configuration is [Rn] 5f¹⁴ 6d¹ 7s². The most common oxidation state displayed by it is +3; because after losing 3 electrons, it attains a stable f¹⁴ configuration.