1. d-Block Elements: The d-block elements are those elements in which the last electron enters the d–subshell of penultimate shell. The general electronic configuration of these elements is (n – 1) d 1–10 ns1–2, where n is outermost shell. The d-block consisting of groups 3–12 occupies the large middle section of the periodic table.
2. Transition Elements: The elements of d-block are known as transition elements as they possess properties that are transitional between the s and p block elements. A transition element is defined as an element which has incompletely filled d-orbitals in its ground state or any one of its oxidation states. There are four series of transition elements spread between group 3 and 12.
First transition series or 3d-series: Scandium (21Sc) to Zinc (30Zn)
Second transition series or 4d-series: Yttrium (39Y) to Cadmium (48Cd)
Third transition series or 5d-series: Lanthanum (57La) and Hafnium (72Hf) to Mercury (80Hg), (Omitting 58Ce to 71Lu)
Fourth transition series or 6d-series: Begins with Actinium (89Ac) is still incomplete.
Zinc, cadmium and mercury of group 12 have full 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 elements of the three transition series, their chemistry is studied along with the chemistry of the transition elements.
3. General Characteristics of Transition Elements
(a) Atomic radii: The atomic radii of transition elements are smaller than those of s-block elements and larger than those of p-block elements in a period. In a transition series, as the atomic number increases, the atomic radii first decreases till the middle, becomes almost constant and then increases towards the end of the period. The decrease in atomic radii in the beginning is due to the increase in the effective nuclear charge with the increase in atomic number. However, with the increase in the number of electrons in (n – 1) d–subshell, the screening effect of these d-electrons on the outermost ns-electrons also increases. This increased screening effect counterbalances the effect of increased nuclear charge, therefore, the atomic radii remain almost constant in the middle of the series. Increase in atomic radii towards the end may be attributed to the electron–electron repulsion. The pairing of electrons in the d-orbitals of the penultimate shell occurs only after the d–subshell is half filled. The repulsive interactions between the paired electrons in d-orbitals (of the penultimate shell) become very dominant towards the end of the series and causes the expansion of the electron cloud and thus, resulting in increased atomic size.
The atomic radii usually increase down the group. But the atomic radii of the elements of second and third transition series belonging to a particular group are almost equal. This is due to lanthanoid contraction.
(b) Ionic radii: The ionic radii of the transitional elements follow the same order as their atomic radii. In general, the ionic radii decrease with increase in oxidation state.
(c) Ionisation enthalpies: The first ionisation enthalpies of transition elements are higher than those of s-block elements but lower than p-block elements. In a particular transition series, ionisation enthalpy increases gradually with increase in atomic number, though some irregularities are observed.
Reason: The increase in ionisation enthalpy is due to increase in nuclear charge with increase in atomic number which tends to attract the electron cloud with greater force.
The addition of d-electron in penultimate shell with increase in atomic number provides a screening effect and shield the outer s-electrons from inward nuclear pull. Thus, the effect of increased nuclear charge and increased magnitude of screening effect tend to oppose each other. Consequently, the increase in ionisation enthalpy along the series of transition element is very small. The irregular variations of first and second ionisation enthalpies in the first transition series is mainly due to varying degree of stability of different 3d-configuration. For example, Cr has low first ionisation enthalpy because loss of one electron gives stable 3d5 configuration and Zn has very high first ionisation enthalpy because the electron has to be removed from 4s-orbital of the stable 3d104s2 configuration.
The first ionisation enthalpies of 5d-transition elements are higher than those of 3d and 4d elements. This is due to greater effective nuclear charge acting on the outer valence electrons in these elements because of the ineffective shielding of the nucleus by 4f-electrons.
(d) Metallic character: All the transition elements are metallic in nature. They show gradual decrease in electropositive character in moving from left to right in a series. The metallic bond in transition metals are very strong. This is due to greater effective nuclear charge, low ionisation energies and large number of vacant orbitals in their outermost shell. Nearly, all the transition metals are hard, possess high density and high enthalpy of atomisation. This is due to presence of strong metallic bonds.
(e) Melting and boiling points: Except zinc, cadmium and mercury all the other transition elements generally have high melting and boiling points. This is due to strong metallic bonds and presence of partially filled d-orbitals in them. Because of these half-filled orbitals some covalent bonds also exist between atoms of transition elements. As zinc, mercury and cadmium have fully filled d-orbitals, therefore, there is no covalent bonding amongst the atoms of these elements. This accounts for their low melting and boiling points.
In moving along series from left to right, the melting and boiling points of transition elements first increase to a maximum and then decrease towards the end of the period. As the number of unpaired electrons increases, the tendency to form metallic and covalent bonds also increases. In first transition series after chromium, the number of unpaired electrons decreases, hence the melting point also decreases. Manganese possesses anomalous melting and boiling points because it has stable 3d 54s2 configuration, i.e., electrons are held tightly by nucleus so that the delocalisation is less and the metallic bond is much weaker than that of preceding element.
(f) Oxidation states: All transition elements except first and last member of the series exhibit variable oxidation states as (n – 1)d and ns orbitals have comparable energies so that both can enter into chemical bond formation. The maximum oxidation state shown by first series increases from Sc to Mn and then decreases. The common oxidation state of first series is +3 (except Sc). The highest oxidation state of transition elements is 8 (Os and Ru).
The compounds of transition elements in lower states +2 and +3 are mostly ionic and of higher oxidation states are covalent, e.g., ZnCl2 and CdCl2 are ionic whereas Cr2O72– and MnO4– are covalent in nature, higher oxidation state of transition elements are shown in oxides and oxoacids (e.g., MnO4–). Transition metals with fluorine and oxygen exhibit higher oxidation state due to higher electronegative nature of fluorine and oxygen. Transition metals also exhibit +1 and 0 oxidation states. For example:
Cu2Cl2, AgCl, Hg2Cl2 (OS of metal is +1)
Ni(CO)4 , Fe(CO)5 (OS of metal is 0)
When the metal exhibit more than one OS, their relative stabilities can be known from their standard electrode potential, e.g.,
Cu2+(aq) + 2e– → Cu(s) Eored = 0.34 volt
Cu+(aq) + e– → Cu(s) Eored = 0.52 volt
Lower standard reduction potential indicates Cu2+ is more stable than Cu+ in aqueous medium.
(g) Standard electrode potential: Electrode potential is the electric potential developed on a metal electrode when it is in equilibrium with a solution of its own ions, taking electrons from the electrode. There is irregular variation in electrode potential due to irregular variation in ionisation enthalpy, sublimation energy and energy of hydration. The E° value decreases from left to right across the series; Mn, Ni and Zn have higher values than expected because of their half-filled or completely filled 3d-orbitals in case of Mn2+ and Zn2+ respectively and the highest negative enthalpy of hydration, Ni2+.
(h) Magnetic properties: Substances containing unpaired electrons are said to be paramagnetic. A diamagnetic substance is one in which all the electrons are paired. Except the ions of d 0 (Sc3+, Ti4+) and d10 (Cu+, Zn2+) configurations, all other simple ions of transition elements contain unpaired electrons in their (n – 1) d subshell and are, therefore, paramagnetic. The magnetic moments (µ) of the elements of first transition series can be calculated with the unpaired electrons (n) by the spin only formula.
(i) Complex formation: The tendency to form complex ions is due to
(i) the high charge on the transition metal ions,
(ii) the availability of d-orbitals for accommodating electrons donated by the ligand atoms.
(j) Catalytic property: Most of the transition metals and their compounds possess catalytic properties. The catalytic activity of transition metal ions is attributed to the following two reasons:
(i) Variable oxidation states due to which they can form a variety of unstable intermediate products.
(ii) Large surface area so that the reactants are adsorbed on the surface and come close to each other facilitating the reaction process.
(k) Colour: Most of the transition metal ions in solution as well as in solid states are coloured. This is due to the partial absorption of visible light. The absorbed light promotes the electron from one orbital to another orbital of the same d-subshell. Since the electronic transition occurs within the d-orbitals of the transition metal ions, they are called d–d transitions. It is because of these d–d transitions occurring in a transition metal ion by absorption of visible light that they appear coloured.
(l) Alloy formation: The transition metals have similar radii and other characteristics. Therefore, these metals can mutually substitute their position in their crystal lattices and form alloys. The alloys so formed are hard and often have high melting point. Various types of steel, brass, bronze are examples of this type of alloy.
(m) Interstitial compounds: Interstitial compounds are those in which small atoms occupy the interstitial sites in the crystal lattice. Interstitial compounds are well known for transition metals because small-sized atoms of H, B, C, N, etc., can easily occupy positions in the voids present in the crystal lattices of transition metals.
4. Some Important Compounds of Transition Elements: Though the transition elements are sufficiently electropositive, yet they are not very reactive because of
(i) their high enthalpies of sublimation, and
(ii) their high ionisation enthalpies.
Oxides: Transition metals form oxides of the general composition MO, M2O3, MO2, M2O5 and MO6. Oxides in the lower oxidation states are generally basic in nature and those in the higher oxidation states are amphoteric or acidic in nature. For example,
+2 MnO Basic | +3 Mn2O3 Amphoteric | +8/3 Mn3O4 Amphoteric | +4 MnO2 Amphoteric | +7 Mn2O7 Acidic |
(a) Potassium Dichromate, K2Cr2O7: It is prepared from the chromite ore. Different reactions involved in the preparation of potassium dichromate from chromite ore are:
K2Cr2O7 is separated by fractional crystallisation.
Properties: When potassium dichromate is heated with any ionic chloride (e.g., NaCl, BaCl2, etc.) and concentrated H2SO4, red vapours of chromyl chloride are obtained.
Potassium dichromate is a powerful oxidising agent. In acidic medium, its oxidation action can be represented as follows:
K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O+ 3[O]
Cr2O72– + 14H+ + 6e– → 2Cr3+ + 7H2O (Eo = +1.31 V)
(i) It oxidises ferrous to ferric.
(ii) It oxidises stannous to stannic.
(iii) It oxidises sulphur dioxide to sulphuric acid.
(iv) It oxidises hydrogen sulphide to sulphur.
(v) It oxidises iodides to iodine.
Structures of chromate and dichromate ions:
The chromate and dichromate ions are interconvertible in aqueous solution depending upon the pH of the solution.
Potassium dichromate is used as primary standard in volumetric analysis.
(b) Potassium permanganate, KMnO4: It is prepared by fusion of pyrolusite, MnO2, with KOH in the presence of an oxidising agent like KNO3. This produces the dark green potassium manganate, K2MnO4 which disproportionates in a neutral or acidic solution to give purple permanganate.
3MnO32– + 4H+ → 2Mn
Commercially, it is prepared by alkaline oxidative fusion of MnO2 followed by the electrolytic oxidation of manganate (VI).
In the laboratory, KMnO4 is prepared by oxidation of manganese (II) ion salt by peroxodisulphate.
Properties:
Potassium permanganate is a dark purple crystalline solid.
On heating, it decomposes at 513 K and O2 is evolved.
Potassium permanganate acts as a powerful oxidising agent in acidic, alkaline and neutral media. Few important oxidation reactions of KMnO4 are given below:
1. In acidic medium potassium permanganate oxidises:
(i) Iodide to iodine
(ii) Ferrous to ferric
(iii) Oxalate to carbon dioxide
(iv) Hydrogen sulphide to sulphur
(v) Sulphite to sulphate
(vi) Nitrite to nitrate
2. In neutral or faintly alkaline solutions potassium permanganate oxidises:
(i) Iodide to iodate
2MnO4– + I– + H2O → IO3– + 2MnO2 + 2OH–
(ii) Thiosulphate to sulphate
8MnO4– +
(iii) Manganous salt to MnO2 in presence of zinc sulphate or zinc oxide
2MnO4– + 3Mn2+ + 2H2O → 5MnO2 + 4H+
The MnO42– and MnO4– are tetrahedral; the green MnO42– is paramagnetic with one unpaired electron but the purple MnO4– is diamagnetic.
Inner Transition Elements (f-Block Elements)
The inner transition elements consist of lanthanoids and actinoids. They are characterised by filling of the ‘f ’ orbitals.
5. Lanthanoids
The series involving the filling of 4f-orbitals following lanthanum La (Z = 57) is called the lanthanoid series. There are 14 elements in this series, starting with cerium Ce (Z =58) to lutetium Lu (Z = 71). The lanthanoids
are highly dense metals.
have high melting points.
form alloys easily with other metals.
are soft, malleable and ductile with low tensile strength.
(i) Oxidation state: The most characteristic oxidation state of lanthanoid elements is +3. Some of the elements also exhibit +2 and +4 oxidation states.
(ii) Colour: Some of the trivalent ions of lanthanoids are coloured. This is due to the absorption in visible region of the spectrum, resulting in f-f transitions because they have partly filled orbitals.
(iii) Magnetic properties: Among lanthanoids, La3+ and Lu3+, which have 4f0 or 4f14 electronic configurations are diamagnetic and all the other trivalent lanthanoid ions are paramagnetic because of the presence of unpaired electrons.
(iv) Reactivity: All lanthanoids are highly electropositive metals and have an almost similar chemical reactivity.
(v) Lanthanoid contraction: In lanthanoids, with increasing atomic number, the atomic and ionic radii decreases from one element to the other, but the decrease is very small. It is because, for every additional proton in the nucleus, the corresponding electron goes into a 4f-subshell, which is too diffused to screen the nucleus as effectively as the more localised inner shell. Hence, the attraction of the nucleus for the outermost electrons increases steadily with the atomic number.
(vi) Uses of lanthanoids: The pure metals have no specific use. So they are used as alloys or compounds.
As alloys lanthanoids are used in making a misch metal which consists of lanthanoid metals (~95%) and iron (~5%) and traces of sulphur, carbon, calcium and aluminium. Magnesium mixed with 3% misch metal is used for making jet engine parts.
Steel mixed with La, Ce, Pr and Nd is used in the manufacture of flame throwing tanks.
Lanthanoid oxides are used for polishing glass. Neodymium and praseodymium oxides are used for making coloured glasses for goggles.
Cerium salts are used in dyeing cotton and also as catalysts.
Lanthanoid compounds are used as a catalyst for hydrogenation, dehydrogenation and petroleum cracking.
Pyrophoric alloys are used for making tracer bullets and shells.
6. Actinoids: The elements following actinium, Ac (Z = 89), up to lawrencium (Z = 103), are called actinoids. The actinoids
are highly dense metals with a high melting point and form alloys with other metals, specially iron.
are silvery white metals, which are highly reactive.
get tarnished when exposed to alkalis and are less reactive towards acids.
(i) Actinoid contraction: The atomic and ionic size decreases with an increase in atomic number. Electrons are added to the 5f-subshell, as a result the nuclear charge increases causing the shells to shrink inwards.
(ii) Electronic configuration: The actinoids involve the filling of 5f-subshells. Actinium has the electronic configuration 6d1 7s2. From thorium (Z = 90) onwards, 5f-orbitals get progressively filled up. Because of equal energy of 5f and 6d subshells, there are some uncertainities regarding the filling of 5f and 6d subshells. Most of their properties are comparable to that of lanthanoids.
(iii) Oxidation state: Generally +3 oxidation state is preferred in actinoids. The elements in the first of actinoid series frequently exhibit higher states. For example, the maximum oxidation increases from +4 in Th to +5, +6 and +7 in Pa, U and Np, respectively, but decreases in succeeding elements.
(iv) Colour: The actinoid ions are coloured.
(v) Magnetic properties: Many of the actinoid ions are paramagnetic.
(vi) Reactivity: They are also highly electropositive and form salts as well as complexes. Many of these elements are radioactive.
(vii) Uses of actinoids:
l Thorium is used in the treatment of cancer and in incandescent gas mantles.
l Uranium is used in the glass industry, textile industry, in medicines and as nuclear fuel.
l Plutonium is used in atomic reactors and in atomic bombs.
7. Differences between Lanthanoids and Actinoids
S.No. | Lanthanoids | Actinoids |
(i) | 4f-orbital is progressively filled. | 5f-orbital is progressively filled. |
(ii) | +3 oxidation state is most common along with +2 and +4. | They show +3, +4, +5, +6, +7 oxidation states. |
(iii) | Only promethium (Pm) is radioactive. | All are radioactive. |
(iv) | They are less reactive than actinoids. | They are more reactive. |
(v) | Magnetic properties are less complex. | Magnetic properties are more complex. |
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