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@, , Gi), , Thus, the electronic configuration of chromium (Z = 24), and copper (Z =29)are 1s*2s* 2p* 3s?3p* 3d 4s' and 1s*2s?, 2p* 3s‘ 3p* 3d" 4s! respectively., , ies, , , , 1.3 Prope, , Atomic Raddii, The atomic radii of the transition metals lie in-between those, , of s- and p-block elements, , Generally the atomic radii of d-block elements in a series, decreas with increase in atomic number but the decrease in, atomic size is small after midway., , Explanation, , (iii), , The atomic radius decreases with the increase in atomic, number as the nuclear charge increases whereas the, shielding effect of d-electron is small. After midway, as the, electrons enter the last but one (penultimate) shell, the added, d-electron shields (screens) the outermost electron Hence,, with the increase in the d-electrons, screening effect, increases. This counterbalances the increased nuclear, charge due to inerease in atomic number. As a result, the, atomic radii remain practically same after chromium. For, example in Fe, the two opposing tendencies almost, counterbalance and there is no change in the size ffom Mn, to Fe., , At the end of the period, there is a slight increase in the, atomic radii., , Explanation, , (iv), , (iui), , Near the end of series, the increased electron-electron, repulsions between aided electrons in the same orbitals, are greater than the attractive forces due to the increased, nuclear charge. This results in the expansion of the electron, cloud and thus the atomic radius increases., , The atomic radii increase down the group. This means that, the atomic radii of second series are larger than those of, first transition series. But the atomic radii of the second, and third transition series are almost the same, , The atomic radii of the elements of the second and third, transition metals are nearly same due to lanthanide, contraction (or also called lanthanaid contraction), discussed later., , The trend followed by the ionic radii is the same as that, followed by atomic radii., , Tonic radii of transition metals are different in different, oxidation states., , The ionic radii of the transition metals are smaller than those, of the representative elements belonging ta the same period., , , , , , 1.5 Metallic character, , Except for mercury which is a liquid, all the transition, elements have typical metallic structure. They exhibit all, the characteristics of metals. ductile, have high melting and, boiling points, high thermal and electrical conductivity and, high tensile strength., , Transition elements have relatively low ionization energies, and have one or two electrons in their outermost energy level, (ns! orns’), Asaresult, metallic bonds are formed. Hence, they, behave as metals. Greater the number of unpaired d electrons,, stronger is the bonding due to the overlapping of unpaired, electrons between different metal atoms., , 1.6 Melting Point, , Transition metals have high melting paints which is due to, their strong metallic bond. The metallic bond. The metalic, bonding depends upon the number of unpaired ¢~. The, melting point first increases (Sc-Cr), reaches a maximum value, (Cr) and then decreases (Fe-Zn), , * Tungsten (W) has the highest melting peint., , * Mercury (Hg) has the lowest melting point., , * Mn has the Jowesi melling point in 3d series typicl, transition elements., , , , Joi, , , , ation energies or lonisation enthalpies, , The first ionisation enthalpies of d-block elements lie between, s-block and p-block elements. They are higher than those of'sblock elements and are lesser than thase of p-bleck elements, The ionisation enthalpy gradually increases with increase in, atomic number along a given transition series though some, irregularities are observed, , Explanation, , 0, , The increasing ionization enthalpies are due to increased, nuclear charge with increase in atomic number which reduces, the size of the atom making the removal of outer electron, difficult., , Ina given series, the difference in the ionisation enthalpies, between any two successive d-block elements is very much, Jess than the difference in case of successive s-block or pblock elements., , Explanation, , (a), , (wv), , The addition of d electrons in last but one [(n — 1) or, penultimate] shell with increase in atomic number provides, a screening effect and thus shields the outer s electrons, from inward nuclear pull. Thus, the effects of increased, nuclear charge and addition of d electrons tend to oppose, each other., , The first ionization enthalpy of Zn, Cd and Hg are, however,, very high because of the fully filled (n-1) d!® ns?, configuration., , Although second and third ionization enthalpies also, in, general, increase along a period, but the magnitude of, increase for the successive elements is much higher., , , , Scanned with Cases

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(v) The high values of 3rd ionization enthalpies for Cu, Ni and In‘3d’ series all element contain 2 electrons in‘4s’ and hence, Znexplain why they show a maximum oxidation state of +2. they all give a common minimum oxidation state of +2, (vi) The first ionisation enthalpies of Sd elements are higher as (Except’Cr’ and 'Cu‘ where minimum oxidation state ist 1], compared ta those of 3d and 4d elements. This is because The maximum oxidation state is given by Mn i.e. Mn‘? in, the weak shielding of cucleusby dfelectron¢in Sd elements which two electrons are removed from 4s and five unpaired, results in greater effective nuclear charge acting on the electrons are removed from 3d orbitals, outer valence electrons, (4) The highest oxidation state is shown by Ruthenium (Ru), — and Osmium (Qs) i.e. +8., 1.8. Oxidation state, (5) _ Across the period oxidation state increases and it is maximum, “The net numerical charge assigned to an atom of an element at the centre and than decreases even if atomic number, in its combined state is known as its Oxidation state or increases. The element which shows highest oxidation state, Oxidation number”, occur in the middle or near the middle of the series and than, (1) With the exception of few elements, most of the d-block decreases., elements show more than one oxidation state i.e. they show (6) Transition metals also show zero oxidation states in metal, variable oxidation states. The elements show variable carbonyl complex. (Nickel tetracarbony]),, oxidation state because of following reasons Example: Ni in Ni (CO), has zer0 oxidation state, () “(n-1)d’ and ‘ns’ orbitals in the atoms of d-block elements | (7) The bonding in the compounds af lower oxidation state, have almost same energies and therefore electron can be (+2, +3) is mostly ionic and the bonding in the compounds, removed from ‘(n-1)d° orbitals as easily as ‘s’ orbitals of higher oxidation state is mostly covalent., electrons. Ls y rage, (8) The relative stabilities af some oxidation states can be, (ii) After removing ‘s’ electrons, the remainder is called Kernel explained on the basis of rule extra stability, according to, of the metal cations. In d-block elements, the kernel is which , d° and d'” are stable configurations,, unstable and therefore it lases one or more electrons ftom s , lity order of ‘ons isas oll, (n= 1)d electrons. This results in formation of cations with or exaimple, the stability order of some ions is as follows;, different oxidation states Ti (30°, 4s°)> Ti (3d, 4s"), (2) All transition elements show variable oxidation state except Mn™ (3d°, 48°) > Mn** (3d*, 4s"), last element in each series Fe®, (3d°, 4s") > Fe" (3d, 45°), (3) Minimum oxidation state= ‘Total number ofelectrons in4s | (9) Cu’? is more stable than Cu! even when Cu"! is 34" while, lost, Maximum oxidation state = (Total number of electrons Cu’ is 3d due to high heat of hydration, in 4s+ ber of ired electrons in 3d Jost). - ‘ 7, in de Sugb a oR unpagcapettroms in 3d.tost), Variable oxidation states shown by 3d-series of d-block elements., Oxidation States, ELECTRONIC Se Ti v cr Mn Fe Co Ni Cu Zn, CONFI rion | d's? ¢?s? 3s? dis? ds? ds? ds? ds? ds? "52, ds! 93!, +1 +1, Wn, E 42 +2 2 42 42 42 42 +2 aod ad, a 43 43 43 +3 43 43 43 43 43, Zz, E 4 4 44 +4 +4 +4, 5 +5 +5 +5 +5 45, 3 +6 +6 +6, +7, , , , , , , , , , , , Scanned with Cases

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@, , (i), , (iii), , , , EBLOCK ELEMENTS, , , , , , , , 1.9 Standard electrode potentials (E*) and chemical, reactivity, , , , , , Thermodynamic stability of the compounds of transition, elements can be evaluated in terms of the magnitude of, ionisation enthalpies of the metals — smaller the ionisation, enthalpy of the metal, stabler is its compound., , In solution, the stability of the compounds depends upon, electrode potentials rather than ionization enthalpies, Electrode potential values depend upon factors such as, enthalpy of sublimation (or atomisation) of the metal, the, ionisation enthalpy and the hydration enthalpy, ic.,, , M(s) "5 M(g), (A,,H is enthalpy of sublimation/, , atomisation), , M(s)—“5M*(g) +e", (4, H is ionisation enthalpy), , M*(g)+aq—*"_5 mM’ (aq), (Ag H is enthalpy of, hydration), , The total energy, A, H, for the process involving, sublimation, ionisation and hydration simultaneously, i.e.,, , for the process, M(s)—» M*(aq) +e", will be the sum, of the three types of enthalpies, ie.,, , ApH=A,4H +A,H+0j4H., , Thus, A,H, is the total enthalpy change when solid meal,, M is brought in the aqueous medium in the form of, monovalent ion, M* (aq), , Me) A> (a), , ‘AH, , n| Ana H, M(e) —an> ~-M* (e), , Trends in the M*/M Standard Electrode Potentials, , There is no regular trend in the E* (M**/M) values. This is, because their ionization enthalpies (I, + IE,)and sublimation, enthalpies do not show any regular trend., , The general trend towards less negative E* values along, the series is due to the general increase in the sum of first, and second ionization enthalpies., , Copper shows a unique behaviour in the series as it is the, only metal having positive value for E*, This explains why, is does not liberate H, gas from acids. [t reacts only with, the oxidizing acids (HNO, and H,SO,) which are reduced, The reason for positive E° value for copper is that the sum, of enthalpies of sublimation and ionization is not balanced, by hydration enthalpy., , , , (iv), , The values of E° for Mn, Ni and Zn are more negative than, expected from the generall trend. This is due to greater, stability of half-filled d-subshell (d°) in Mn**, and completely, filled d-subshell (d'°) inZn**. The exceptional behaviour of, Ni towards E* value from the regular trend is due to its high, negative enthalpy of hydration., , Trends in the M>*/M* Standard Electrode Potentials, , oO, , ai), , (ul), , (iv), , ), , O, , (ii), , (iv), , A very low value for E* (Sc**/Sc™) reflects the stability of, Sc** ion which has a noble gas configuration., , The highest value for Zn is on account of very high stability, of Zn** ion with d' configuration. It is difficult to remove, an electron from it to change it into +3 state., , The comparatively high value of E* (Mn**/Mn™) shows, that Mn™ is very stable which is on account of stable d°, configuration of Mn**., , ‘The comparatively low value of E* (Fe¥*/Fe™) ison account, of extra stability of Fe (¢?), ie,, low third ionizationenthalpy, ofFe., , The comparatively low value for V is on account of the, , stability of V?* ion due to its half-filled Us configuration, (discussed in unit 9),, , Cheinical Reactivity and E* Values, The transition metals, vary very widely in their chemical reactivity. Some of them, are highly electropositive and dissolve in mineral acids, whereas a fewofthem are ‘noble’, ie., they do notreact with, simple acids, Some results of chemical reactivity of transition, metals as related to their E” values are given below:, , The metals of the first transition series (except copper) are, relatively more reactive than the othr series. Thus, they are, oxidized by H* ions though the actual rate is slow, ¢.g., Ti, and V are passive to dilute non-oxidizing acids at room, temperature., , As already explained, less negative E* values for M2*/M, along the series indicate a decreasing tendency to form, divalent cations,, , More negative E* values than expected for Mn, Niand Zn, show greater stability for Mn**, Ni** and Zn**., , E° values for the redox couple M**/M”* indicate that Mn*, and Co™ ions ar the strongest oxidizing agents in aqueous, solution whereas Ti*, V"* and Cr* are strongest reducing, agents and can liberate hydrogen from a dilute acid, e.g., 2, Cr* (aq) + 2 H* (aq) —> 2 Cr (aq) +H, (2), , 10 Catalytic Proper, , Most transition elements and their compounds have good, catalytic properties because, , (a) They possess variable oxidation state., , , , Scanned with Cases

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(b) They provide a large surface area for the reactant to be, , , , , , , , absorbed., Catalysts Uses, , TIC, +AL(C,H,) Ziegler-Natta catalyst, used in, polymerisation of ethylene, , V0; Contact process SO, > SO, , Fe Haber Bosch process, , PdCl, Wacker’s process for CH CHO, manufacturing, , Pd Hydrogenation of alkene, alkyne, ete, , PUPtO Adam’s catalyst for selective, reduction, , Pt Catalytic convertor, for, cleaning, car exhaust fumes, , PUR Ostwald’s process :, NH +NO, , Cu Oxidation ofalcohols, , , , , , , , 1.11 Magnetic Behaviour, , When a substance is placed in a’ magnetic field of strength, H, the intensity of the magnetic field in the substance may, be greater than or less than fH. If the field in the substance, is greater than H, the substance is a paramagnetic material, and attracts line of force. If the field in the substance is less, than H, the substance is diamagnetic. Diamagnetic materials, tend to repel lines of force. The magnetic moment of a, substance depends on the strength of magnetic field, generated due to electronic spin, there is a change in electric, flux which leads ta induction of magnetic field. The electron, is also under orbital angular momentum or orbital spin. It, leads to generation of magnetic moment., , In firsttransition elements series the orbital angular magnetic, moment is insignificant the orbital contribution is quenched, by the electric fields of the surrounding atoms so magnetic, moment is equal to the spin magnetic moment only., , Heer = n(n +2)BM, , n— no. of unpaired electron., , Maximum transition elements and compounds are, paramagnetic due to presence of unpaired electrons., , , , , , 2. COMPLEX FORMATION, , Transition metal ions farm a large number of complex, compounds, Complex compounds are those which have, a metal ion linked to anumber of negative ions (anions) or, neutral molecules having lone pairs of electrons. These, ions or molecules are called ligands. They donate lone, pairs of electrons to the central transition metal ion forming, coordinate bonds, , A few examplesare given below:, , [Fe(CN),}*, [Fe(CN),]*, [Cu(NH,),}*, [Zn(NH,),]*,, [Ni(CN), "and [Ptel, =, , Such complex compounds are not formed by s - and p block elements., , Explanation, The transition elements form complexes, because of the following reasons :, , (0 Comparatively smaller size of their metal ions, (i) Their high ionic charges,, (Because of (i) and (ii), they have large charge/size ratio), , (iii) Availability of vacant d-orbitals so that these orbitals can, accept lone pairs of electrons donated by the ligands., , 2.1 lnterstitital Compounds, , The transition metals form a large number of interstitial, compounds in which small atoms such as hydrogen,, carbon, boron and nitrogen occupy the empty spaces, (interstitial sites) in their lattices (Fig.), , They are represented by formulae like TiC, TiH,, Mn,N,, Fe,H, Fe,C ete. However, actually they are nonstoichiometric materials, e.g., TiH, ,, VH,;, ete. and the, bonds present in them are neither typically ionic nor, covalent. Some of their important characteristics are as, follows:, , () ‘They are very hard and rigid, e.g, ste! which is an, interstitial compound of Fe and Cis quite hard. Similarly,, some borides are as hard as diamond., , (i) They have high melting points which are higher than those, of the pure metals., , (ii) They show conductivity like that of the pure metal., , (iv) They acquire chemical inertness., , 2.2 Alloy formation, , Alloys are homogencous solid solutions of two or more, metals obtained by melting the components and then, cooling the melt. These are formed by metals whose atomic, radii differ by not more than 15% so that the atoms of one, metal can easily take up the positions in the crystal lattice, of the other (Fig,), , Now, as transition metals have similar atomic radii and, , , , Scanned with Cases