Chapter 4: The d & f Block Elements

(i) Transition metals show paramagnetic behaviour. Paramagnetism arises due to the presence of unpaired electrons, with each electron having a magnetic moment associated with its spin angular momentum and orbital angular momentum. However, in the first transition series, the orbital angular momentum is quenched. Therefore, the resulting paramagnetism is only because of the unpaired electron.

(ii) Transition elements have a highly effective nuclear charge and a large number of valence electrons. Therefore, they form very strong metallic bonds. As a result, the enthalpy of atomisation of transition metals is high.

(iii) Most of the complexes of transition metals are coloured. This is because of the absorption of radiation from the visible light region to promote an electron from one of the d−orbitals to another. In the presence of ligands, the d-orbitals split up into two sets of orbitals having different energies. Therefore, the transition of electrons can take place from one set to another. The energy required for these transitions is quite small and falls in the visible region of radiation. The ions of transition metals absorb the radiation of a particular wavelength, and the rest is reflected, imparting colour to the solution.

(iv) The catalytic activity of the transition elements can be explained by two basic facts.

(a) Transition metals provide a suitable surface for the reactions to occur.

(b) Owing to their ability to show variable oxidation states and form complexes, transition metals form unstable intermediate compounds. Thus, they provide a new path with lower activation energy, Ea, for the reaction.

Transition metals are large in size, and they also contain lots of interstitial sites.

These interstitial sites can be used to trap atoms of other elements (that have small atomic sizes), such as H, C, N. The compounds resulting are called interstitial compounds.

In the case of transition elements, the oxidation state can vary from +1 to the highest oxidation state by removing all its valence electrons. Also, in transition elements, the oxidation states differ by 1 (Fe²⁺ and Fe³⁺; Cu⁺ and Cu²⁺). In non-transition elements, the oxidation states differ by 2, for example, +2 and +4 or +3 and +5, etc.

From iron chromite ore, Potassium dichromate is prepared as 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.

(i)

(ii)

(iii)

Potassium permanganate 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 oxidised 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

(i) Acidified KMnO₄ solution oxidises Fe (II) ions to Fe (III) ions, i.e., ferrous ions to ferric ions.

(ii). Acidified potassium permanganate oxidises SO₂ to sulphuric acid.

(iii) Acidified potassium permanganate oxidises oxalic acid to carbon dioxide

(i) The reduction potentials for the given pairs increase in the following order.

Mn²⁺ / Mn < Cr²⁺ / Cr < Fe²⁺ /Fe

So, the oxidation of Fe to Fe²⁺ is not as easy as the oxidation of Cr to Cr²⁺ and the oxidation of Mn to Mn²⁺. Thus, these metals can be arranged in the increasing order of their ability to get oxidised as Fe < Cr < Mn

(ii) The

Ions with electrons in the d-orbital will be the only ones that will be coloured, and the ions with empty d-orbital will be colourless.

All other ions, except Sc³⁺, will be coloured in an aqueous solution because of d−d transitions. Not Sc^3+, as it has an empty d-orbital.

 From the table above,

Mn shows the maximum number of oxidation states, changing between +2 to +7.

The number of oxidation states increases as we move on from Sc to Mn.

On moving from Mn to Zn, the number of oxidation states decreases due to a decrease in the number of available unpaired electrons.

The relative stability of the +2 oxidation state increases on moving from top to bottom. This is because, on moving to the bottom from the top, it becomes more and more difficult to remove the third electron from the d-orbital.

(i) Electronic configuration

The general electronic configuration for lanthanoids is [Xe]⁵⁴ 4f^0-14 5d^0-1 6s^2, and that for actinoids is [Rn]⁸⁶ 5f^1-14 6d^0-1 7s². Unlike 4f orbitals, 5f orbitals are not deeply buried and participate in bonding to a greater extent.

(ii) Atomic and ionic sizes

Similar to lanthanoids, actinoids also exhibit actinoid contraction (overall decrease in atomic and ionic radii). The contraction is greater due to the poor shielding effect of 5f orbitals.

(iii) Oxidation states

The principal oxidation state of lanthanoids is (+3). However, sometimes we also encounter oxidation states of + 2 and + 4. This is because of the extra stability of fully-filled and half-filled orbitals. Actinoids exhibit a greater range of oxidation states. This is because the 5f, 6d, and 7s levels are of comparable energies. Again, (+3) is the principal oxidation state for actinoids. Actinoids such as lanthanoids have more compounds in the +3 state than in the +4 state.

(iv) Chemical reactivity

In the lanthanide series, the earlier members of the series are more reactive. They have reactivity that is comparable to Ca. With an increase in the atomic number, the lanthanides start behaving similarly to Al. Actinoids, on the other hand, are highly reactive metals, especially when they are finely divided. When they are added to boiling water, they give a mixture of oxide and hydride. Actinoids combine with most non-metals at moderate temperatures. Alkalies have no action on these actinoids. In the case of acids, they are slightly affected by nitric acid (because of the formation of a protective oxide layer).