Short Notes: The d and f Block

The transition metals (d-block elements)

There are three complete series (from period 4-6 in the group 3-12) and one incomplete series (period 7th) of the transition elements.

3d series (4th period) : Scandium (21SC) to Zinc (30Zn)

4d series (5th period) : Yttrium (39Y) to Cadmium (48Cd)

5d series (6th period) : Lanthanum (57La) to Mercury

(80Hg) except 58Ce to 71Lu

6d series (7th period) : Begins with Actinium (89AC) and is still complete. It does not include actinoids i.e., the elements from 90Th to 103Lr.

The general outer most electronic configuration of d-block elements is (n − 1)d1 − 10 ns1−2, where n = outermost shell. Following chart describes the outer shell electronic configurations of transition elements in ground state:

Table: Outer Electronic Configurations of the Transition Elements (ground state)

1st Series

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Z

21

22

23

24

25

26

27

28

29

30

4s

2

2

2

1

2

2

2

2

1

2

3d

1

2

3

5

5

6

7

8

10

10

 

2nd Series

Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Ag

Cd

Z

39

40

41

42

43

44

45

46

47

48

5s

2

2

1

1

1

1

1

0

1

2

4d

1

2

4

5

6

7

8

10

10

10

 

3rd Series

La

Hf

Ta

W

Re

Os

Ir

Pt

Au

Hg

Z

57

72

73

74

75

76

77

78

79

80

6s

2

2

2

2

2

2

2

1

1

2

5d

1

2

3

4

5

6

7

9

10

10

 

4th Series

Ac

Rf

Db

Sg

Bh

Hs

Mt

Ds

Rg

Uub

Z

89

104

105

106

107

108

109

110

111

112

7s

2

2

2

2

2

2

2

2

1

2

6d

1

2

3

4

5

6

7

8

10

10

 

The electronic configurations of Zn, Cd and Hg are represented by general formula (n − 1)d10 ns2 and these elements have completely filled d-orbitals in ground state as well as in their common oxidation states. Hence, they are not considered as transition elements. On the other hand, silver (47Ag) has also completely filled d-orbitals in ground state, but in +2 oxidation state it has 4d9 configuration which is incomplete, thus silver is considered to be transition element.

General characteristics of transition elements

The transition metals show almost similar physical and chemical properties due to similar outer electronic configuration however they differ in some properties due to difference in number of electrons in their d-orbitals.

 

(i) Metallic character: Transition elements are malleable and ductile metals having metallic lustre. These are good conductors of heat and electricity. Except mercury (a liquid) all are solid.

(ii) Atomic volume and density: They have low atomic volume and high density.

(iii) Melting and boiling points: Melting points and boiling points are directly related to the strength of metallic bonds which in turn depends on the number of unpaired electrons. It rises to the maximum and then decreases in a period. It increases in a group. Mn and Tc have abnormally low melting points. Hg melts at ?38?C. Tungston (W) has the maximum MP and Re has second higher melting point. Partially filled especially half filled d-orbitals form additional covalent bonding due to overlapping of d-subshells of adjacent atoms resulting high MP and BP and also hard and brittle nature.

(iv) Atomic radii: Atomic radii of transition elements in a period are smaller than s-block elements and larger than p-block elements. In a series atomic radii decreases initially and varies irregularly showing near constancy till 11th group and then increases in the 12th group with increasing atomic number. Nuclear charge and also the shielding effect of electrons increase and counter balance the effects resulting small decrease in atomic radii. But in Zn, Cd and Hg there is increase in atomic radii due to electron repulsion which exceeds the attraction between nucleus and the electrons of outer most orbit.

The atomic radii of 5th period elements are greater than that of the 4th period elements due to increase in the number of outer most orbit. But 5th and 6th period elements have nearly the same radii due to the lanthanoid contraction.

Ionic Radii: Ionic radii decrease from left to right with increase in oxidation state. For the same oxidation state the ionic radii generally decrease with increase in nuclear charge.

(v) Ionization energy: Ionization energies of transition metals are generally very high. Because of the irregular variation of atomic size, transition metals show very little and irregular variation in their ionization energies.

Ionisation enthalpy and electrode potential: The first ionization enthalpy of d-block elements are higher than those of s-block elements and lesser than those of p-block elements. In the series IE increases with increase in atomic number (i.e., decrease in size). Zn has the highest IE due to fully filled orbitals though the size is larger.

IE3 > IE2 > IE1 due to positive charge on them.

IE2 of V < Cr > Mn and Ni < Cu > Zn

IE3 of Mn is much higher than that of Fe due to its electronic configuration.  IE1 of 5d series is much higher than those of 4d and 3d due to weak shielding of 4f electrons.

(vi) Oxidation states: All transition elements except first and last number of the series exhibit variable oxidation states as (n ? 1)and ns 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 oxidation states +2 and +3 are mostly ionic and of higher states are covalent e.g., ZnCl2 and CdCl2 are ionic  are covalent in nature, higher oxidation state of transition elemnets are shown in oxides and oxoacids (e.g., ). 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.

Cu2Cl2, AgCl, Hg2Cl2(O.S. of metal is +1)

Ni(CO)4, Fe(CO)5(O.S. of metal is 0)

When the metals exhibit more than one oxidation states their relative stabilities can be known from their standard electrode potential e.g.,

Lower standard reduction potential Cu2+ indicates that Cu2+ is more stable than Cu+ in aqueous medium.

(vii) Standard electrode potential: Electrode potential is the electric potential developed on a metal electrode when it is in equilibrium with a solution of its ions, taking electrons from the electrode. There is irregular variation in electrode potential due to irregular variation in ionization 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+and the highest negative enthalpy of hydration Ni2+.

(viii) Reducing properties: The electrode (oxidation) potentials of the first transition series metals (except Cu) are quite high. Hence, they are expected to be oxidised easily to their ions.

Hence, they should be good reducing agents but they are not so because of their high heat of vaporisation, high ionization potentials and low heat of hydration. Since the oxidation potential of Cu (?0.34 volt) is negative, it has a low tendency to change to Cu+2 ions and does not displace H+ ions from acid solutions.

(ix) Magnetic properties: A diamagnetic substance is one in which all the electrons are paired while the substances containing unpaired electrons are said to be paramagnetic. Except the ions of d0(Sc+3, Ti+4) or d10(Cu+, Zn+2) 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.

μ = √n(n + 2) BM(Bohr Magneton)

(x) Complex formation: The tendency to form complex ions is due to:

(a) the high charge on the transition metal ions and

(b) the availability of d-orbitals for accommodating electrons donated by the ligand atoms.

(xi) 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:

(a) Variable oxidation states due to which they can form a variety of unstable intermediate products.

(b) Large surface area so that the reactants are adsorbed on the surface and come closer to each other facilitating the reaction process.

(xii) 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 dd transitions.

(xiii) Non-stoichiometric compounds: They form non-stoichiometric compounds having indefinite compositions and unique structures e.g., FeO0.84 to FEO0.98etc.

(xiv) Alloy formation: Most of the transition metals form alloys because of their similar radii and other characteristics, transition metals can mutually substitute their positions in their crystal lattices.

(xv) 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.

(xvi) Reactivity: Though the transition elements are sufficiently electropositive, yet they are not very reactive because of:

(a) their high heats of sublimation and

(b) their high ionization energies

(xvii) 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,

 

Some industrially important compounds

(i) K2Cr2O7 (Potassium dichromate)

 

A Preparation: Dichromates are generally prepared from chromate which is the fusion product of chromite ore (FeCr2O4) and sodium or potassium carbonate.

When sodium chromate is acidified with H2SO4, it gives orange sodium dichromate.

Orange crystals of potassium dichromate is obtained by treating sodium dichromate with potassium chloride.

B Properties:  K2Cr2O7 is an orange coloured crystal with melting point 669 K. It is moderately soluble in cold water and freely soluble in hot water. It is less soluble than  Na2Cr2O7

The structure of dichromate ion is the combination of two tetrahedral structures of chromate ion sharing one oxygen atom at one corner with Cr?O?Cr bond angle of 126° (see following figure).

(a) Action of heat:  K2Cr2O7 decomposes on heating with evolution of O2.

(b) Reaction with alkalies: Potassium dichromate reacts with alkali to give a yellow solution of chromate which on acidifying again changes to orange coloured dichromate.

The chromate and dichromate ions are interconvertible in aqueous solution because they are in equilibrium at pH = 4.

Here, we should note that the oxidation state of chromium in chromate as well as dichromate is same i.e., +6.

(c) Reaction with conc. H2SO4In cold, red crystals of chromic anhydride (chromium trioxide) is formed.

On heating the mixture,  gas is evolved.

(d) Oxidising behaviour:  is a powerful oxidizing agent because  takes up electrons. One molecule of potassium dichromate furnishes
3 atoms of available oxygen in the presence of dilute H2SO4.

It is clear from the above equation that Cr(+6) is reduced to Cr(+3).

Some of the oxidising properties of K2Cr2O7 in presence of dil H2SO4 are given by the following examples:

? It liberates I2 from KI,

? It converts sulphides into sulphur,

? It oxidises tin (II) to tin (IV) and iron (II) salts to iron (III),

? It oxidises nitrites to nitrates,

? It oxidises halogen acids to halogen,

C Uses:

(a) In industry, K2Cr2O7  is used in the preparation of chrome alum K2SO4.Cr2(SO4)324H2O and other industrially important compounds such as Cr2O3, CrO3, CrO2Cl2, K2CrO4, CrCl3etc.

(b) In volumetric analysis, it is used as primary standard for the estimation of Fe2+ and I in redox titration. Na2Cr2O7 being deliquescent is not used for this purpose.

(c) Both sodium and potassium dichromate are used as oxidising agents in organic chemistry but sodium salt being more soluble in water, is extensively used.

(ii) KMnO4 (Potassium permanganate)

A. Preparation: KMnO4 is prepared by the fusion reaction of MnO2 (Pyrolusite) with an oxidising agent like KNO3. This produces potassium manganate (K2MnO4) which in neutral or acidic solution oxidises to form potassium permanganate (KMnO4).

Commercial preparation of KMnO4 is as under:

B. Properties: Potassium permanganate crystallizes to form dark crystals which are isostructural with KClOi.e., tetrahedral.

KMnO4 has two typical physical properties, first is its intense colour and secondly it shows weak temperature dependent paramagnetism.

The structure of manganate as well as permanganate ions is tetrahedral as shown below:

(a) Action of heat: On heating at 746 K, KMnO4 decomposes to give O2.

(b) Reaction with conc. H2SO4:

? With cold H2SO4 it gives Mn2O7  which on warming decomposes to MnOand O2.

? With warm conc. H2SO4, O2 gas is liberated.

(c) Oxidising behaviour: Potassium permanganate is a powerful oxidising agent. A few important oxidising reaction of KMnO4  are as follows:

In acidic solutions:

In faintly alkaline or neutral solution:

It converts iodide to iodate,

It oxidises thiosulphate to sulphate,

It oxidises manganous salt to ,

It is important to note that permanganate titrations do not give the satisfactory result because HCl is oxidised to Cl2.

C. Uses:

(a) Being a strong oxidising agent, it is used for organic synthesis in industry.

(b) It is used for bleaching of wool, cotton, silk and other fibres and for decolourisation of oils.

(c) It is often used in volumetric analysis for the estimation of famous salts, oxalates, iodides and hydrogen peroxide.

(d) Alkaline KMnO4  as Baeyer?s reagent is used in organic chemistry for testing unsaturation

The f-block elements (Lanthanoids and Actinoids)

The f-block elements are those elements in which the last electron enters the (n? 2) f-orbitals or antepenultimate energy level. These elements are also called inner transition elements because they form a transition series within the transition elements or d-block elements. Their general electronic configuration is:

(n − 2)f1 − 14(n − 1)d0 − 1nS2

This means, they have three incomplete shells, (n ? 2), (n ? 1) and nth shell. The elements in which the last electron enters 4f-orbital are called Lanthanoids (from At. no. 58 to 71) because they come immediately after Lanthanum (57) in the periodic table. Similarly the 5f-block elements are called Actinoids (from At. no. 90 to 103) because they come after Actinium (89). 4f-block elements are also called rare earth elements.

(i) Lanthanoids (first inner transition series): 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 (Z=58) to Lutetium (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.

(a) Oxidation state: The most characteristic oxidation state of lanthanoid elements is +3. Some of the elements also exhibit +2 and +4 oxidation states.

(b) Colour: Some of the trivalent ions of lanthanoids are coloured. This is due to partial absorption of visible region of the spectrum, resulting in ff transitions because they have incompletely filled orbitals.

(c) 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.

(d) Reactivity: All lanthanoids are highly electropositive metals and have an almost similar chemical reactivity.

(e) Lanthanoid Contraction: In lanthanoids, with increasing atomic number, the atomic as well as ionic radius 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 elecetrons increases steadily with the atomic number.

(f) Uses of Lanthanoids: The pure metals have no specific use. So they are used as alloys or compounds. The alloys are called ?misch metals?.

? 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.

(ii) Actinoids: The elements following Actinium (Z=89), upto 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 alkali and are less reactive towards acids.

(a) Actinoid contraction: The atomic and ionic size decreases with an increase in atomic number. Electrons are added to the 5fsubshell, as a result the nuclear charge increases causing the shells to shrink inwards.

(b) Electronic configuration: The actinoids involve the filling of 5f subshells. Actinium has the electronic configuration 6d17s2. From thorium (Z=90) onwards, 5f-orbitals get progressively filled up. Because of equal energy of 5f and 6d subshells, there are some uncertainties regarding the filling of 5f and 6subshells. Most of their properties are comparable to that of lanthanoids.

(c) Oxidation state: Generally a +4 oxidation state is preferred in actinoids. A few of the actinoid elements exist in a +6 oxidation state, e.g., uranium, neptunium and plutonium.

(d) Colour: The actinoid ions are coloured.

(e) Magnetic properties: Many of the actinoid ions are paramagnetic.

(f) Reactivity: They are also highly electropositive and form salts as well as complexes. Many of these elements are radioactive.

(g) Uses of actinoids

? Thorium is used in the treatment of cancer and in incandescent gas mantles.

? Uranium is used in the glass industry, textile industry, in medicines and as nuclear fuel.

? Plutonium is used in atomic reactors and in atomic bombs.

(iii) Differences between Lanthanoids and Actinoids

Lanthanoids Actinoids
(i) 4f-orbital is progressively filled. (i) 5f-orbital is progressively filled.
(ii) +3 oxidation state is most common along with +2 and +4. (ii) They show +2, +3, +4, +5, +6 and +7 oxidation states.
(iii) Only promethium (Pm) is radioactive. (iii) All are radioactive.
(iv) They are less reactive than actinoids. (iv) They are more reactive.
(v) Magnetic properties are less complex. (v) Magnetic properties are more complex.

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