## Disimilarities between Lanthanides and Actinides preparation and properties, Group 18 d – and f – Block Elements

Disimilarities between Lanthanides and Actinides :

Despite good many points of similarities between two series there are certainly some points of difference as regards the nature of 4f and 5f orbitals in relation to other neighbourly orbitals. The basic points of difference between actinides and lanthanides have brought out in table 8.17.

Table 8.17: Differences between Lanthanides and Actinides

 Lanthanoids Actinoids Except promethirem, they are non-radioactive All actinoides are radio active. It show very less number of oxidation states. They show only +2 and +4 O.S. in few cases besides +3. They show higher number of oxidation state. They show +4, +5, +6, +7 besides +3. Lanthanoides show higher shielding effect of 4f electron. Hence it show lesser lanthanoid contraction and show less decrease in ionic radii Actinides show lesser shielding effect of 5f electrons. Hence there is larger decrease in ionic radius. Bonding energy of 4f electrons are higher. Bonding energy of 5f electrons are comparatively lower. They do not form oxo ions. They form oxo ions. The compounds of lanthanoids are less basic. The compounds of actinoids are more basic. They have less tendency of complex formation. They have greater tendency of complex formation.

## Similarities between Lanthanides and Actinides preparation and properties, Group 18 d – and f – Block Elements

Similarities between Lanthanides and Actinides :

• Oxidation state of +3is not common in both the series.
• In both the series, f-orbitals are being successively filled.
• As lanthanide contraction, we find actinide contraction because of electrons still passes very ineffective power as compared to 4f-electrons.
• Because off-transitions, sharp line like bonds are observed in the absorption spectra of the elements of both the series.
• Beside possessing low electronegativities they are very highly reactive.
• Nitrates, perchlorates and sulphates of trivalent actinides and also lanthanides are soluble whereas the hydroxide, fluorides and carbonates are not soluble in water.
• Just like lanthanides actinides show ion exchange behaviour.
• Due to presence of unpaired electron, the ions of both are paramagnetic.
• The elements of both the series are highly electro positive hence these elements are chemically highly reactive and behave as strong reducing agent.

## Actinoides or 5f-series preparation and properties, Group 18 d – and f – Block Elements

Actinoides or 5f-series :

Introduction :

→ This series contain 14 elements from Thorium (atomic number 90) to Laurencium (atomic number -103). These elements are radioactive in nature the earlier members have relatively long half-lives, but the latter ones have half-life values ranging from a day to 3 minutes. The half life of Lawrencium is 3 minutes. The latter members could be prepared only in nanogram quantities. Due to these reasons their study is more difficults.

→ Electronic configuration Their general electronic configuration is (n – 2)f1-14 (n – 1)d0-1ns2 or 5f1-14 6d0-17s2. All the actinoides have 7s2 only the configuration of 5f and 6d is variable. Actually the differentiating electron enters in 5f-subenergy level. The fourteen electrons are formally added to 5f, though not in thorium (z = 90) but form Pa onwards the 5f orbitals are completed at element 103.

→ The electronic configuration of actinides show irregularities because the energies of 5f and 6d subshell are almost equal. Due to extra stability of f7 configuration, the electronic configuration of Am and Cm are [Rn] 5f77s2 and [Rn] 5f76d18s2.

→ Some important points related to electronic configuration are given below :

• All the actinides have 7s2 configuration while 5f and 6d subshells are filled variably.
• The filling of 5f-subshell starts from protoactinium not from thorium.
• Irregularities in configuration is due to extra stability of f0, f7 and f14 configuration. For example :

Am : [Rn] 5f77s2
Cm : [Rn] 5f76d1782
No: [Rn] 5f146d17s2

## Oxidation state

→ They show variable oxidation states this is because in these elements the energies of 5f and 78 orbitals are almost similar and electrons from all the energy levels may be used for bond formation. There is a greater range of oxidation states.

→ But the actinides show in general +3 oxidation state. The elements in the first half of the series frequently show higher oxidation states because the energy required for the conversion of 5f → 6d is lesser than that required for the conversion of 4f → 5d. Hence they show higher oxidation states for example +4, +5 +6 and +7 as compared to lanthanides.

→ In last half of the actinide series, the energy required for the conversion 5f → 6d is more than that required for the conversion 4f → 5d, these actinides, therefore show more lower oxidation states. The most stable oxidation state of these elements is +3 and their stability increases with increase in atomic number. In addition they also show +2, +4, +5, +6 and +7 oxidation states.

→ The actinides resemble the lanthanides in having more compounds in +3 state than in the +4 state. However +3 and +4 ions tend to hydrolyse. Because the distribution of oxidation states among the actinides is so uneven and so different for the earlier and latter elements. It is unsatisfactory to review their chemistry in terms of oxidation states.

## General Characteristics of Actinoides or 5f-series

The general characteristics of actinides are as follow:

→ Physical properties: All the actinides are silvery white metals. They are highly electropositive in nature. Their melting points are lower than the melting points of transition metals. Except thorium and americium, they have high densities.

→ Colour of lons : The ions of actinides are generally coloured. The colour of ions depends on the number of 5f-electrons. When their salts in the solid solution state are exposed to light, the f-electrons of these elements absorb light from visible region and are excited from f-orbitals of lower energy state to the f-orbitals of hight from visible region and are excited from f-orbitals of lower energy state to the f-orbitals of higher energy state. It is known as f-f transition. All those which have empty, half completed or completed f-subshells are colourless.

→ The colours of some of the actinide ions in aqueous solutions are shown in the table 8.16.

Table 8.16 : Colour of some actinide ions

 Ion Colour Ac3+ (5f°) Colourless U3+ (5f3) Red Np3+ (5f4) Blue Th4+ (5f°) Colourless U4+ (5f2) Green Np4+ (5f3) Yellowish green – Pu3+ (5f5) Violet Pu 4+ (5f4) Orange Am3+ (5f6) Pink Am4+ (5f5) Pink Cm3+ (5f7) Colourless Cm4+ (5f6) Pale yellow

→ Magnetic properties : Most of the ions of actinide series are paramagnetic ie, they are attracted into the magnetic field. It is due to the presence of unpaired f-electrons in these ions. Ac3+(5f°), Th4+(5f°) and Lw+ (5f14) are dimagnetic and are repelled by the magnetic field.

→ In these elements of 5f orbitals are deep inside the metal ion and are shielded from the surroundings by 6s and 6p subshells. Therefore, it is not possible to explain their magnetic moments in terms of number of unpaired electrons alone.

→ The magnetic moment of these ions can be calculated by using equation

→ where u is the magnetic moment in Bohr Magnetons (B.M.) calculated by using both the spins and orbital momentum contribution. S is the resultant spin quantum number and L is the resultant orbital momentum quantum number, The magnetic moments of actinides are less than expected values.

→ This is due to the fact that 5f-electrons have lesser shielding effect which results in the quenching of orbital contribution.

This magnetic properties can also be calculated by spin only formulas

→ Complex formation: They form complex compounds and their tendency to form complex compounds is greater than lanthanides. This is because of higher nuclear charge and small size of their ions. Most of the actinide halides form complexes with alkali metal halides. For example ThCl4 reacts with KCl to form complexes KThCl5, K2ThCl6.

→ Thorium tetra chloride also form complexes with pyridine, E.D.T.A. and oxime. The degree of complex formation decreases in the order :

(where M = actinides)

→ Chemical properties : Due to less ionisation potential and high electropositive character the chemical reactivity of actinides are generally high. Actinides are highly reactive metals when they are finely divided. They react with boiling water and form oxides and hydroxides. They react with various non-metals at normal temperature.

→ They rapidly tarnished in air due to the formation of oxide layer on their surface. They are less reactive with acids and resist the action of alkalies. They forin compounds which give simple trivalent ions in aqueous solutions.

## Uses of Actinoides or 5f-series

The main uses of actinides are as follow:

• Pu-239 is used as a nuclear fuel.
• U-235 is used as nuclear fuel in atomic reactors and atom bombs.
• Thorium salts are used in medicines for the treatment of cancer.
• Thorium is used for the production of fissionable material need for atomic reactors.
• Salts of uranium are used in glass industry, textile industry, ceramic industry and in medicines.

## Lanthanides or 4f – series preparation and properties, Group 18 d – and f – Block Elements

Lanthanides or 4f-series :

Introduction :

It includes 14-elementa i.e. from atomic number 58 to atomic number 71. In this series electrons enter in 4/-orbitals (perpenultimate orbital). These element are also known as rare earth elements because in past these elements were very rare but now a days this name is not appropriate as various elements of this series are not rare. Promethium (Pm) is the only element which is artificial and radioactive.

## Electronic configuration of Lanthanides or 4f – series

→ Its general electronic configuration is (n – 2)f1-14 (n – 1)d0-1ns2 or 4f1-14ed0-16s2. As energies of 5d and 4/-orbitals are nearly similar hence their is various irregularities in their filling.

→ In the electronic configuration of all the elements of this occupancy. The electronic configuration of these elements having +3 oxidation state is similar i.e.4fn.

→ Where the value of n increases from 1 to 14 on increasing atomic number. The electronic configuration of ground state of Lanthanides is given in table 8.14

Table 8.14: Electronic configuration of Lanthanides with their stable oxidation state.

Some important striking features of electronic configuration of Lanthanides are as follow :

• 6s-subshell of these elements have two electrons but the number of electrons in 48-subshell is variable.
• The electronic configuration of Lanthanum (z = 57) is [Xe]5d16s2 i.e. Its f-subshell have no electrons. But it is considered that in the other elements off-block have electrons in their f-subshell.

Besides 5d1 in Gd and Lu, 4f-subshell is either half filled or completely filled hence the stability of these elements are higher

• Gd (z = 64) [Xe] 4f7 5d16s2
• Lu (Z = 74) : [Xe] 4f14 5d1 6s2

The stability of Eu (z = 63) and Yb (z = 70) is also higher due to half filled and full filled configuration.

• Eu (z = 63) : [Xe] 4f7 6s2
• Yb (z = 70) : [Xe] 4f14 6s2

## Atomic and lonic radii (Lanthanide contraction)

In lanthanide series, there is a regular decrease in the atomic as well as ionic radii of trivalent ions (M ) as the atomic number increases from Cerium to Lutetium leaving some exceptions. This regular atomic and ionic radii on increasing atomic number is called lanthanide contraction. This decrease in size is very small. Atomic and ionic radil of Lanthanide series is given in table 8.15.

Table 8.15: Atomic and ionic radii of Lanthanides

 Element Atomic raddi (pm) Ionic radii (pm) La 187 106 Ce 183 103 Pr 182 101 Nd 181 99 Pm 181 98 Sm 180 96 Eu 199 95 Gd 180 94 Tb 178 92 Dy 177 91 Ho 176 90 Er 175 89 Tm 174 88 Yb 173 87 Lu 172 86

## Cause of Lanthanide contraction

→ In Lanthanide series, as atomic number increases, nuclear charge also increases by one unit and one electron also increases in 4/-orbitals of prepenultimate shell. But outer electronic configuration remain 5s2 5p6 6s2 ie. there is no change in outer electronic configuration.

→ On account of the very diffused shape of f-orbital, the 4f-electrons shield or screen each other quite poorly from the nuclear charge. Thus the effect of increased nuclear charge is somewhat larger than charged shielding effect.

→ Due to heigher nuclear charge than shielding effect, valane shell come close to nucleus, Hence the size of atom or ion goes on decreasing on increasing atomic number, However, this decraease in atomic radii is very small. This contraction is only about 10 pm from Ce to Lu.

## Effects of Lanthanoid Contraction

→ Similarity in Atomic radius of one group in second (4d) and third (5d) transition series : The size of one group of 4d and 5d transition series is almost similar. In group 3 elements, there is regular increase in size from Sc to Y and from Y to La.

→ But after this, the size of one group in 4d to 5d series is almost same because Lanthanoids come in between them and size decreases due to Lanthanoid contraction. With increase in number of shells, atomic size stiould be increased but due to Lanthanoid contraction it is not so.

→ Separation of Lanthanoids: There is similarity in chemical properties of Lanthanoids due to Lanthanoid contraction, as a result, their separation is very difficult. There is difference in solubility, complex formation capacity of Lanthanoids due to Lanthanoid contraction.

→ Basicity of Lanthanoid hydroxides : With an increase in the atomic number, the basic strength of the oxides and hydroxides decreases. This contraction causes a decrease in the size of Lanthanoide cations and, therefore, the polrising power of the cations increases. This further decreases the ionic character of the oxides and hydroxides. Thus Ce(OH)4 is maximum and Lu(OH)4 is least basic.

→ Ionisation potential : The values of ionisation potential should be lower and should decrease regularly down the group. But due to Lanthanoide contraction there is a trend seen in the values of ionization potential, this regularity occurs after the element tungsten.

→ Oxidation States General oxidation state of all the Lanthanides is +3. However, occasionaly they show +2 and +4 oxidation state in solution or in solid compounds in the form of ions. This irregularities on oxidation states are due to extra stability of empty. half filled or fullfilled of f-subshel.

→ For example : Ce4+(4f0), Tb4+ (4f7), Eu2+ (4f7) Yb2+ (4f14) are very stable ions +2 or +4 oxidation states tend to revert to the more stable oxidation state of +3 Stable +3 oxidation state can be achieved by lossing or gaining an electron.

→ That is why Eu2+ and Yb2+ ions are very good reducing agents in solutions. On the other hand Ce4+ and Tb4+ ions are good oxidising agents in solutions. The E° value of Ce4+/Ce3+ and Tb4+ is +1.7 V which suggests that it can oxidise water. However, the reaction rate is very slow and hence Ce (iv) is a good analytical reagent.

→ The compounds of lanthanides are generally ionic in nature. There are some lanthanides which show +2 or +4 oxidation states yet they do not show f0, f7 and f14electronic configuration.

→ Examples : Pr4++ (4f1), Nd2+ (4f4), Nd4+ (4f2), Sm2+ (4f6), Dy4+ (4f8) etc. Pr, Nd, Tb and Dy also show +4 oxidation state but only in oxides, MO2. Europian forms Eu2+ ions by losing the two s-electrons and its f7 configuration accounts for the formation of this Eu2+ ion.

→ But it is a strong reducing agent and converts into Eu3+ form. Similarly Yb2+ (f14) is a reductant and Tb4+(f7) is an oxidant. Similarly like Europium exhibit both +2 and +3 oxidation state.

Genearl Characteristics :

The general characteristics of lanthanides are as follow:

Physical properties :

• All the lanthanides are silvery white metal but get tarnished rapidly in presence of air.
• They are good conductor of heat and electicity.
• Their density is generally high. the range of density is from 6.77 g cm to 9.74 g cm3. As atomic number increases density also increases.
• Their melting points are generally high. They melts at about 100 K to 1200 K temperature. The melting point of Smarium (Sm) is 1623 K.

→ Ionisation Enthalpy: Lanthanides have generally low ionisation enthalpies. The first ionisation enthalpies of the lanthanides are around 600 kJ mol-1. The second ioniation enthalpy is about 1200 kJ mol-1.

→ These values are quite comparable with those of calcium. Due to very low value of ioniation enthalpy, these elements are highly electropositive so these elements react cold and hot water to liberate hydrogen. However the reaction is slow with cold water but fast with hot water.

→ The standard reduction potential of these elements lie in between -22 to -24. Only Eu has the value -20 Due to favourable values of reduction potential these elements behave as strong reducing agent. Their reducing power decreases from La to Lu.

→ Colour: Many of the lanthanoid ions are coloured in solid as well as in solutions. The colour of Lanthanoid ions is due to f-f Transition as they have partially filled f-orbitals. The absorption bands of lanthanoid ions are narrow. It is because of the excitation within f-level. The lanthanoid ions, which do not have unpaired electron, are colourless. For example La3+ (4f0) and Lu3+ (4f14).

The colour fo some lanthanoid ions are given in table :

 Lathanoid ions Colour Pr3+ (4/2) Green Tm3+(4/12) Green Nd3+ (4/3) Pink Er3+(4/11) Pink Sm3+ (4/5) Yellow Dy3+ (4/9) Yellow Eu3+(4/6) Light pink Tb3+ (4/8) Light pink

→ Magnetic behaviour: The lanthanoid ions which have unpaired electrons are paramagnetic while the ions like La3+(f0), Ce4+(f0), Yb2+ (f14) and Lu3+ (f14) are dimagnatic due to the absence of unpaired electron.

→ Chemical reactivity : Lanthanoids are highly electropositive and these elements are quite reactive similar to calcium. But as atomic number increases they start to behave like aluminium. These metals get tarnished on exposure to air. On heating with air they form oxides of the type M2P3– Only cerium show exception as it from CeO2.

The chemical reactivity of lanthanoides can be represented as:

→ The values of first three ionisation energies of transition elements are very less. Hence, these elements are ionic and +3 is the most stable oxidation state in these compounds. Their chemistry is also depend on Ln3+ ion.

→ Reducing Property : Lanthanoides easily oxidise losing its three electrons and act as strong reducing agents.

Ln → Ln3+ + 3e

→ Electropositive character : Their electron donating capacity is the indication of their strong electropositive character or metallic nature.

→ Reaction with water : These elements react with water and release hydrogen gas. With cold water, reaction is slow whereas it becomes fast with hot water.

2Ln + H2O → 2Ln(OH)2 + 3H2

The baisc nature of hydroxides decreases from Ce to Lu.

Reaction with Oxygen: These elements react with atmospheric oxygen and forms oxides.
2Ln + 3O2 → 2Ln2O3

Reaction with Hydrogen : These elements react with hydrogen at 300-400°C and forms non-stoichiometric hydrides like LnH2 and LnH3.

Reaction with Halogens:anthanides forms trihalides ion reacting with halogens.

2Ln + 3X2 → 2LnX3

With Non-metals : These elements react with non-metals like carbon, nitrogen and sulphur to form compounds.

## Uses of Lanthanoids

Lanthanoides are used in various industrial processes. Some important uses are as follow :

Alloys:

• The alloys formed from Lanthanoides are known as Misch Metal.
• It contains 94-95% Lanthanides metals, 5% iorn and traces of sulphur, carbon, silicon, calcium and aluminium main lanthanide metal in misch metal is cerium which is about 40%. Lanthanum and neodymium also present in it.
• About the precentage of both is about 44%. These alloys are used for making tracer bullets, shells and flints for lighters.
• Cerium : Magnessium alloys are used in flash lights powers.
• An alloys of magnesium and 3% misch metal is used in making jet engine parts.
• 3% misch metal forms a strong alloy with magnesium and 1% zirconium, which has high strength and strong resistance at more than 3000°C. It is used in preparing jet engines.

In nuclear science : Because of high thermal properties, certain lanthanide elements are very useful in some nuclear applications. These are as follow:

• Gadolinium-titanium alloy is used in reactor shielding.
• European-samarium oxides have been dispersed in stainless steel control rods for use in nuclear reactor
• Thulium and samarium are used as portable X-ray sources.

In glass and ceramics:

• Cerium-neodymium oxides are used in goggles to filter out bright yellow sodium light in glass blowing processes.
• Lanthanum oxide is used in optical glass of high refractive index.
• Praseodymium-zirconium oxides are used in staining cermic tiles.
• Some oxides, carbides, sulphides of lanthanides are good high temperature resistant refractories.

As a catalyst :

• Cerium is used as catalyst in hydrocarbon oxygenation reactions.
• Lanthanium oxide is used for hydrogenation, dehydrogenation and oxidation of various organic compounds.
• Ceric sulphate and chloride are used as catalyst in petroleum cracking,

In magnetic and electronic instruments:

• Lanthanoids are useful in microwave devices.
• Lanthanoide selenide and tellurides are used in semi-conductors.
• Godolinium is used to produce low temperature by magnetic cooling during certain reactions.
• Cerium, praseudymium and samarium are used for preparing permanent magnets.

Miscellaneous uses :

• Lanthanoide fluoride is used in arc lamps as an alloy of carbon electrode that gives intense white light for searchlights and are lights.
• Neodymium-praseodymium oxides is used to produce atrificial gems.
• Europium oxide is used in picture tubes of colour televisions.
• Fluorescent lamps coated with phosphorus containing traces of europium and terbium oxides have higher light output and better colour balance.

## General characteristics of transition elements preparation and properties, Group 18 d – and f – Block Elements

General Characteristics of Transition elements :

General characteristics of transition elements are as follow :

## Electronic configuration

→ In the transition elements, the last differentiating or valence electron enters in penultimate d-orbitals i.e.. (n-1) d-orbitals are successively filled. The general electronic configuration of transition elements is

(n – 1)d1-10 ns0, 1 or 2

→ Due to very small energy gap between (n-1) d and ns orbitala, transition elements show various exceptions. the most common elements are Cr and Cu. They show exceptions in these electronic configurations due to extra stability of half filled and completely filled orbitals.

→ Table 8.2(a) : Electronic configurations of 3d-series

→ Table 8.2(b) : Electronic configurations of 4d-series

The exception shown above are attributed to factors like nuclear electron and electron-electron forces.

→ Table 8.2(c) Electronic configurations of 5d-series

→ 8.2 (d): Electronic configurations of 6d-series

→ The elements from atomic number 106 to 112 have recently been reported but these heavy elements are very unstable. They are radoiactive in nature and these elements are known as trans actinoid series.

## Characteristics of Transition elecments

→ The characteristics of transition elements are as follow:

• These all elements are metallic in nature. They are good conductors of heat and electricity.
• They do not show vertical similarity as other elements of periodic table show similarities in properties.
• They show property of catalysis.
• They form complex compounds.
• They form coloured complexes.
• They show paramagnetism.
• They form interstitial compounds
• They form many types of alloys.
• Generally, all the elements are strong, hard, high melting and boiling solids.

## General characteristics of elements of first transition series

The study of general characteristics of elements of first transition series can be done as follow :

→ Atomic Radii : The atomic radii of transition metal are lesser than s-block elements but greater than p block elements. On moving left to right in any series of transition metals the atomic radii decrease up to middle ie, from group 3 to group-7 atomic radii decrease regularly, then from group-7 to group-10. The atomic radii are approximately same then after it, upto group-12 atiomic radii increase slightly.

→ The main factor for this change in atomic radii is shielding effect and nuclear force of attraction. The decrease in atomic radii in each series from group-3 to group-7 is due to an increase in nuclear charge from element to element which tends to pull the ns-electrons towards the nucleus i.e., the size decreases. At the same time, due to addition of extra electrons to (n-1)d-orditals screening effect developes. As the number of d-electrons increases, the screening or shielding effect also increases.

→ This screening effect opposes the nuclear force of attraction. In the beginning nuclear force of attraction is powerful than screening effect hence size decreases. In the mid way or middle of the screening effect and nuclear force of attraction is couter balanced by each other, hence in the middle of the series there is no change in atomic radii inspite of the fact that atomic number increaes gradually.

→ At the end of the series, there is slight increase in the atomic radii due to powerful screening effect which creates electron-electron repulsion, among (n-1)d-electrons as after d5 – configuration pairing of electrons starts The group, on moving upto down the atomic size increases i.e., on moving from 3d-series to 4d-series the atomic radii increase due to increase in number of shells. But the atomic size of the elements of 4d-and 5d-transition series is approximate equal due to lanthanoid contraction.

→ Due to inclusion of fourteen elements of lanthanoid series between lanthanum and hafinium, there is regular decrease in atomic size from cerium to Lutetium due to poor shielding of 4f-electrons. Hence, the size of Haffnium becomes nearly equal to the size of Zirconium. For example: The atomic radii of Ti of 3d, Zr of 4d and Hf of 5d is 132, 160 and 159 pm repectively. The atomic radii of the elements of 3d-series are given in table 8.3.

Table 8.3 : Atomie radii of the atom of transition metals in picometer (pm)

 Series 3d Element Atomic radii 2iSc 144 22Ti 132 23V 122 24Cr 117 25Mn 117 26 Fe 117 27Co 116 28Ni 115 19 Cu 117 30Zn 125

→ Ionic Radii : On moving left to right i.e., on increasing atomic number, the ionic radii of the ion (MP) with oxidation number +2 follow the same trend as by atomic radii for first transition series or 3d-series. The values of ionic radii are given in table 8.4.

Table 8.4 : Ionic radii of transition metals of 3d-series

 Element Ionic radii (pm) — m2+ m3+ Sc — 81 Ti 90 76 V 88 74 Cr 84 69 Mn 80 66 Fe 76 64 Co 74 63 Ni 72 — Cu 72 — Zn 74 —

→ Generally the ionic radii of transition elements are different for different oxidation state. As oxidation number increases ionic radius decreases because effective nuclear charge increases.

→ Ionisation enthalpy : Transition elements are present in between the elements of s-block and pblock hence the value of the ionisation enthalpies are also lie in between the elements of s-block and p block i.e., the value of first ionisation enthalpy of the elements of d-block is higher than the s-block elements but lower than the p block elements because d-block elements are less electropositive than pblock elements.

→ On moving left to right in each series the ionisation enthalpies increase regularly. However this increase is not regular. It is observed that for a perticular tarnsition series, the difference in the ionisation enthalpies between any two successive d-block elements is much less than the difference in case of any two successive s-block or p block elements.

→ It can be explained on the basis of screening effect created by (n-1)d electrons on ns electrons and increased nuclear charge. As the number of electrons in (n-1)d subshell increases, the shielding of ns electrons also increases. Thus the effect of increased nuclear charge is somewhat neutralised by the increased screening effect. As a result the value of ionisation enthalpy increases but quite slowly among a period of d-block elements.

→ The value of first ionisation tnthalpy of Zinc (Zn). Cadmium (Cd) and Mercury (Hg) of group-12 is appreciably higher due of extra stability associated with completely filled d-orbitals. Their general electronic configuration is (n-1)d10ns2. Slight variations in the value of ionisation enthalpy are mainly due to the slight change in atomic radii. The first ionisation enthalpy values are given in table 8.5

Table 8.5: Ionisation enthalpies (in kJ/mol) of the elements of 3d-series

→ Metallic Nature : All the transition elements are metals because their atoms have one or two electrons in their outermost s-sub shell while their outer p-subshell is empty. They all exhibit most of the properties of metals. They possess malleability, ductility, metallic lusture, high tensile strength, high density, brittleness, hardness etc. Except mercury (Hg) other elements have high melting and boiling points.

→ They are good conductors of heat the electricity. With the exceptions of Zn, Cd, Hg and Mn, they have one or more typical metallic structures at normal temperature. They exhibit all the three types of structure : face centred cubic (fcc), hexagonal closed packed (hcp) and body centred cubic (bcc). These properties confirm that both metallic and covalent bonding are present in the atoms of d-block.

The metallic or lattice structure of these elements are given in table 8.5

Table 8.5: Lattice structures of transition Metals

• (bcc – boby centred cubic)
• (hep – hexagonal close packing)
• (ocp – cubic close packing)
• (X- a typical metal structure)

→ Oxidation states : All transition metals with some exception show a large number of oxidation states. The oxidation states of transition elements are given in table 8.6.

Table 8.6 : Various oxidation states of transition elements

Note: Very rare oxidation states are given in parentheses and most common ones are enclosed in boxes for first transition series (3d series)]

→ The various oxidation states of these elements are related to their electronic congifurations. The less common and unstable oxidation states are given in parentheses. The stability of the oxidation state depends upon the nature of the elements with which the transition elements forms the compounds. It is observed that the highest oxidation states are found in fluorides and oxides because fluorine and oxygen are the most electronegative in nature and have small size.

→ The variable oxidation states of a transition metal are due to the participation of (n-1)d-and ns-electrons in bonding because the energy difference of ns and (n-1)d-subshells are very less. The higher oxidation states are generally shown when only ns-electrons participate in bonding.

→ It should be noted that the oxidation states of transition element differ from each other by unity whereas oxidation states of non-transition elements differ from each other by two because of inert pair effect. Some important points related to oxidation states of transition elements are as follow:

→ The highest oxidation states are shown by the elements lying in middle of the series i.e. the highest oxidation state increases with increase in atomic number, reaches a maximum value in the middle and then starts decreasing. For example, In 3d series maximum oxidation state is shown by manganese (Mn). Manganese shows oxidation state from +2 to +7.

→ The elements lying in the beginning of the series show fewer oxidation states because they have small number of electrons which they can lose or share. The elements lying at the last of the series also show fewer oxidation states because they have fewer vacant d-orbitals which can be participated in bonding.

→ The elements in lower oxidation state (+2 and +3) generally from ionic bonds while in higher oxidation state form ionic bond for example In CrO42- ion there is covalent bond between Cr and oxygen.

→ Some transition metals can also show zero oxidation state in their compounds. For example in (Ni(CO)4) and Fe(CO)5) the oxidation state of Ni and Fe is zero. Different oxidation states of transition metals are given in table 8.6

→ The oxides formed by the transition elements with lower oxidation state have basic nature while the oxides formed by the transition elements with higher oxidation state have acidic nature. For example:

etc in which the oxidation number and group number of the elements are same.

→ In a group of d-block elements, the higher oxidation states are more stable for heavier element. For example in group 6, Mo (VI) and W (VI) are more stable than Cr(VI). For example dichromate having Cr(VI) is a strong oxidation agent in acidic medium, while MoO3 and WO3 with VI oxidation state are stable oxides.

→ Melting and boiling points : Transition metals generally have very high melting and boiling points. It is due to strong metallic bonds and the presence of half-filed d-orbitals in them. Due to these half filled orbitals, some covalent bonds also exist between atoms of transition metals.

→ On moving left to right in a period, the melting points of transition metals first increase to a maximum value and then decrease with increase in atomic number regularly towards the end of the period.

→ However, manganese and technetium have abnormally low melting points. It is observed that except the elements of group-12 i.e. Zn, Cd and Hg the melting points of most of the transition metals are above 1173 K.

[Note : Tungsten (W) has the highest melting point (3683 K) amongest transition metals]

→ The high melting and boiling points of transition metals are due to the strong interatomic forces which are roughly related to the number of unpaired electron i.e. half filled orbitals in the begining. On moving left to right the number of half-filled d-orbitals increases up to the middle of the period which causes the increase in strength of interatomic forces that bring their atoms together.

→ But therefore, the pairing of electrons in d-orbitals occurs and the number of half filled orbitals decreases which also decrease the melting points. When no unpaired electrons are present, the melting points also become very low as in the case of Zn, Cd and Hg.

→ However this concept does not explains why manganese (Mn) having five unpaired d-electrons possesses lower melting point than that of vanadium (V) or cobalt (Co) which have only three unpaired electrons. Actually its complex structure is responsible for its low melting point.

The value of enthalpy of atomisation in kJ mol-1 and melting point (K) of 3d-series are given in table 8.7

Table 8.7 : Heat of atomisation (kJ mol-1) and melting points (K) of 3d-series

→ From above given table 8.7 it is clear that manganese have abnormally low value of melting point. It is probably due to its stable electronic configuration (3d-half filled, 4s-fully filled). In Mn, 3d electrons are held very tightly by the nucleus. Due to this strong nuclear force of attraction electrons are not avialable for metallic bonding which result very weak metallic force of attraction and hence very low melting point.

→ Catalytic Property : Catalytic property is an important property of transition elements. The catalytic activity of transition metals and their compounds is associated with their variable oxidation states. Generally, the metals used as catalyst, alloys and many other compounds are transition metals only. In transition elements, partially filled (n – 1)d orbitals are responsible for catalytic activity of transition elements.

→ Transition elements form unstable compounds with active substances by using their empty orbitals. These unstable compounds decompose to give products and catalyst is obtained as such In the presence of catalyst, the reaction proceeds via the way in which value of activation energy is minimum. Thus, reaction takes place smoothly and readily.

→ Some examples are there in which catalyst provide suitable surface for the reaction. The reactants are absorbed on the surface of catalyst. Some of the examples in which transition elements and compounds acts as catalyst are :

→ In short we can say that they act as catalyst because of the following reasons:

• They have variable oxidation state.
• They easily absorb and re-emitt the wide range of energies to provide the necessary activation energy.
• Because of variable oxidation states, they easily combine with one of the reactant to form an intermediate compound which reacts with the second reactant to form final product.

→ There are some examples in which transition elements act as catalyst.

They are given below:

→ In conversion of SO2 into SO3, V2O5 act as catalyst.

→ In conversion of unsturated compounds into sturated compounds. Ni catalyst is used.

→ During the formation of ammonia, finely divided iron is used.

→ In some cases, transition metals provide a suitable surface where the reactants are adsorbed and thus come closer to one another so the reaction can proceed with fast rate.

Colour :

→ Elements of d-block form coloured salts in solid state and in solution. The colour of state is due to the presence of unpaired electron. The compound containing unpaired electron absorbs some radiation from visible region which promotes the electron from one of the d-orbitals to another. It can be explained as follow: The colour of compounds is due to the presence of incomplete (n-1) d-subshell.

→ When any anion approaches the metal ion then degeneracy of d-orbitals destroy and they split into two sets i.e., t2g set and eg set The set which contain two d-orbitals i.e. $$d_{x^{2}-y^{2}}$$ and $$d_{z^{2}}$$, is knwon as eg set, while the set which contain three d-orbitals i.e. dxy, dyz and dxz, is knwon as t2g-set.

→ The energy of eg set is higher than the energy of ta-set. When light falls on the compounds then the electron present in d-orbitals having lower energy absorbs radiation and becomes excited.

→ This excited electron jumps from d-orbitals having lower energy to d-orbitals of higher energy. This is called d-d transition.

→ When white light falls on any compound of transition metal having unpaired electron, then some of energy corresponding to a particular colour is absorbed by this electron and it becomes excited. This excited electron raised to higher energy set of d-orbitals and rest of colours of white light are transmitted. This transmitted light makes the compound coloured. The observed colour of the compounds is always the complementary colour of observed colour.

→ Thus we can say that the transition metal ions and the compounds, which have unpaired electrons, are coloured, while those which do not have unpaired electrons are colourless. The colour and outer electronic configuration of some important transition metal ions are given in table 8.8

Table 8.8 : Colours of ions of the first transition series

 Configuration Example Colour 3d0 Sc3+ Colourless 3d0 Ti4+ Colourless 3d1 Ti3+ Purple 3d1 V4+ Blue 3d2 V3+ Green 3d3 V2+ Violet 3d3 Cr3+ Violet 3d4 Mn3+ Violet 3d4 Cr2+ Blue 3d5 Mn2+ Pink 3d5 Fe3+ Yellow 3d6 Fe2+ Green 3d6 3d7 Co3+, Co2+ Bluepink 3d8 Ni2+ Green 3d9 Cu2+ Blue 3d10 Zn2+ Colourless

Magnetic Properties :

→ The magnetic properties of compounds are due to the presence of unpaired electrons. Magnetic properties are very helpful in determination of structure of transition element compounds. The main properties of magnetism are given here: The reason for magnetic property is two type motion of electrons.

• Orbital motion and
• Spin motion

→ Magnetic moment is sum of two motions :

μ = μL + μS B.M.

→ Hence, μL is orbital magnetic moment and is spin magnetic moment. Thus, electron itself works as a small magnet. Magnetic moment is expressed in Bohr Magneton (B.M.).

1 B. M = $$\frac{\text { eh }}{4 \pi m c}$$

→ Armc In transition elements.(n-1)d-electrons are held on the surface only, hence are affected more by external force. As a result, their orbital motion is restricted and becomes negligible. In this conditions,

µ ≅ µS

→ Magnetic moment of magnetic material can be calculated in B.M., Bohr Magneton) by using the following expression,

µ = √n(n + 2) B.M.

→ Where n is the number of unpaired electrons. This formula is known as ‘spin only formula. B.M. -Bohr magneton (a unit of magnetic moment)

1 B.M. = 9.27 × 10-24 A m2 or JT-1

→ The magnetic moment increases with increase in number of unpaired electron. Large the number of unpaired electrons in a substance, the greater is its magnetic moment in Bohr magneton, thus larger will be paramagnetism. The magnetic moment of some important ions are listed in table 8.9

Table 8.9: Observed and calculated magnetic moment (BM) of some ionic species

where n is the number of umpaired electrons.

→ There are many types of magnetic properties are found in substance. Main three types are given as:

• Diamagnetism
• Paramagnetism
• Ferromagnetism

→ Diamagnetism : The substance in which paired electrons are present have zero magnetic moment because both the electrons neutralise effects of each other. Such substances are known as diamagnetic substances.

→ They do not show any type of magnetic behaviour in absence of magnetic field. When these substances are placed in magnetic filed, their resultant magnetic moment becomes zero but orbital magnetic moment is induced in the opposite direction of the field.

→ Hence these substance are repelled in magnetic field. Diamagnetism does not depend on temperature and almost all the substance show this property. It is weaker than para magnetism. Due to this, it is difficult to observe this property in substances having unpaired electrons.

→ Paramagnetism : It is found in the substances in which unpaired electrons are present. The electrons do not cancel effect of each other. Such substances are show paramagnetic properties in substances. An atom, ion or molecule of substance can be considered as a magnet. Their spin and orbital motion generates small electric and magnetic field.

→ When electron are paired, their electronic field is equal and opposite. Hence, they neutralise each other and resultant magnetic filed is zero. The magnetic field produced by unpaired electron is small but effective. Therefore, paramagnetism depends on number of unpaired electrons present in that substance. When paramagnetic substance are placed in magnetic field, than they are strongly attracted towards magnetic field.

→ If the field is heterogeneous, the substance will move in that directions in which the speed of field is maximum so that maimum lines can be passed through the substance. Paramagnetism is inversaly proportional to the temperature.

→ The paramagnetic and dimagnetic behaviour of a substance can be calculated by weighing the substance first in air and then suspending it between the poles of magnetic field. If the weight of the substance is more in magnetic field then it is paramagnetic because this substance is attractcted by the magnetic field. If the weight of substance is less in the magnetic field then it is dimagnetic because it is repelled by the magnetic field.

→ Ferromagnetic substances: The substances which are very strongly attracted by the magnetic field, then these are known as ferromagnetic substances. Ferromagnetism is the extreme condition of paramagnetism. These substances show permanent magnetism even in the absence of magnetic field. Hence ferromagnetic substances are used to form magnet. Example- iron, cobalt, nickel, godolinium, CrO2 Fe3O4, Mn ete.

→ On increasing or decreasing the temperature or beating etc, the arrangement of ionic magnets is disturbed and As a result, their magnetic property vanishes.

→ Formation of Interstitial Compounds The size of transition element are quite large. When they are arranged in lattice then larger interstitial voids are formed in these voids smali non metallic atoms like H, B, C, N, He etc are trapped. The compounds formed by the trapping of smaller atoms in the interstitial voids of these transition elements are known as interstitial elements are known as interstitial compounds.

→ When H, He, B, C, N etc are trapped then interstitial compounds are known as Hydrides, Helidea, Borides, Carbides, Nitrides etc. respectively. These interstitial compounds are generally non-stoichiometric compounds having formula like Tic, Mn4N, Fe3H, VH0.50, VH0.56 TiH1.7 etc. The bonds formed between these element are neither ionic nor covalent in naute. These formulas are also not correspond to any normal oxidation state to the metal.

Some of the important chemical and physical properties of these interstitial compounds are as follow :

• The melting points of these compounds are generally high.
• They are very hard and rigid. For example-steel is very hard in nature.
• It is an interstitial compounds of Fe and C.
• Simililarly some of the borides are as hard as diamond.
• They retain metallic conductivity.
• They are chemically inert.
• In interstitial compound, the malleability and ductility of the metals decreases but tensile strength increases, due to the presence of these atoms.

Formation of Alloys :

→ A large number of alloys are formed by transtition metals. The size of transition metals are quite similar hence the atom of one metal can substitute the atoms of other metal in its crystal lattice. In this way when more than one metal are melted mixed well and then on cooling this solution of mixture of more than one transition metals, a smooth solid alloy is obtained.

→ Alloys are generally hard, strong and have high melting point. they are resistant to corrosion Stainless steel, normal steel, vanadium steel, c-steel etc. are alloys of iron with chromium, vanadium, molybdenum, tungsten, nickel, manganese etc with some amount of carbon. Some other examples of alloys are brass, (copper-Zine), Bronze (Copper-tin) etc.