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| Transition Elements, (3d Series), , , , 11. INTRODUCTION, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Transition Elements of d-Block Elements, ges } IVE | VB | VIB | vo8 |<——vins——| 38] Old numbering, 3 a1) s 6/7 8| 9] 10] 11 | 12 |Newnumbering, 21 22] 33 | a os 26 RI” [zea ao o|) SO" 1e sd daries, se | Ti VfCe | Mn | Fe} .co | Nil cu}-zn, S| 40|) 41 |04a) frag ae | nas a7 ‘, x Zr Nb Mo} Te Ru | Rh ne Ag “a Sister, i By iy UN alle «Sel ae a I 3 16.) 77 7% | 79 | 80 ., ita | He} Te} Ww] Re | os} tr | m| Au| ny | 9°, 89 | 104] 105, ni a Ha © 6d series, , , , , , , , , , , , , , , , Transition elements or d-block elements may be defined as those elements which have, partly filled d-orbitals in their elementary form or in their commonly occurring oxidation, States,, , The d-block of the periodic table (shown above) consists of four series, called as first, second,, third and the fourth (3d, 4d, Sd, and 6d) transition series. The first three series are complete, containing 10 elements each and the fourth one is an incomplete series. All these elements are, placed in eight sub-groups, viz., II] B, 1V B, V B, VIB, VII B, VIII B, | B and II B as shown in, the periodic table along with the atomic numbers. Now, these groups are called 3rd, 4th, Sth.......12th, groups. Elements of sub-group II B, viz. Zn, Cd and Hg, are not included in transition elements, as they do not have a partly filled d- sub-shell in their clementary state. They do not show the, Properties of transition elements to any appreciable extent except for their ability to form complexes, with ligands such as ammonia, amines, and halide ions, ete. However, they are generally studied, with the d-block elements in order to maintain a simple classification of elements,
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ae r, 4 CHEMISTRY FOR DEGREE STUDENTS SEMESTER —IV (INORGANIC CHEMISTRY), , 12) THE ELECTRONIC CONFIGURATION OF 4-BLOCK, , , , , , , , , , , , , , , , ELEMENTS Z, Electronic configurations of the first transition series of d-block elements are given jn Th, Table 1.1. Progressive filling of electrons in 3d orbitals takes place 3d, Table 1.1 ", Element Symbol At. No. Electronic Configurations, Scandium Se 21 Is", 257 2p*, 37 3p! 3a! 4g?, Or [Ar] 3d", 457, Titanium Ti 22 1s*, 28? 2p®, 3573p 3a? 4g?, Vanadium v 23 Is’, 28? 2p°,3s? 3p! 3a? 49?, Chromium Cr 24 Is?, 25? 2p, 357 3p° 3a*, 4!, Manganese Mn 25 Is’, 2s? 2p®, 3973p" 3a, 452, Iron Fe 26 1s? 257 2p*, 39°3p* 3%, 45?, Cobalt Co 27 1s, 29° 2p*, 3573p" 37, 42 |, Nickel Ni 28 Is?, 2s? 2p 3s73p*3a", 42, Copper Cu 29 1s?, 2572p*, 39 3p* 3a", 45!, Zinc Zn 30 Is, 25? 2p, 35°3p3d" 452 |, 1.2.1 Electronic Configuration of Cr and Cu, The unexpected electronic configuration of Cr (Ar) 3d° 4.5! (in place of (Ar) 3d‘ 48°) and, Cu (Ar) 3d" 4s! (in place of (Ar) 3d? 4s) can be explained on the basis of half-filled and completely, filled sub-shells which are found to be more stable arrangements. The extra stability of half-filled, sub-shells results from the occupation of each orbital by one electron resulting in s ing and, , equal distribution of charge around an atom. In case of copper, it appears that Cu (Ar) 3d" 4s" with, filled 3d sub-shell and half-filled 4s sub-shell is more stable than (Ar) 3d? 4s?, Another important 1, factor which explains the stability of these configurations is their large exchange energy., , The electrons in various orbitals of the same sub-shell or in different sub-shell (having similar, energy) can exchange their respective positions. During this exchange process a small amount of, energy is released. The released energy is called exchange energy. It has been found that exchange, energy is maximum when all the orbitals of the same sub-shell are either completely filled or, , exactly half-filled. This is because maximum exchanges take Place under these condtions. So the, configurations of Cr (Ar) 3d“4s! and Cu (Ar) 3d"° 4s! have extra stability,, , 1.2.2 Electronic Configurations of Ions of Transition Metals, , Ions of the transition elements are formed by the loss of electrons from the atoms. The, configuration obtained after the loss of electrons is the electronic configuration of the ion, Whether, the electrons will be lost from the (n ~ 1) d-orbital or ns -orbital depends upon the stability of the, electronic configuration of the ion. For example, consider the first transition series. As 49 orbitals, are occupied first and 3d-orbitals later, we may expect that 4s orbitals are more stable than 4, and hence the elements of first transition series would be expected to form ions by the oss of, 3d electrons. In fact ions are formed by the loss of 4y electrons first, This is because prior, , as, , a? 2m, , ’, , tae
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TRANSITION ELEMEN TS (3d SERIES) 5, , occupation of electrons, the 49 orbital is energetically more stable I, , the 3¢@ orbitals are occupied by electrons, they repel the 4s I . ahi, Thus, when transition metal atoms form ions, they lose ele f So, 3d orbitals (Table 1.2). Pah eel, , d orbitals. But. once, (0 the higher energy level, from the 4s orbitals before the, , Table 1.2 Electronic configuration of the ions of the elements of first tra iti, : nsition series, , , , , , , , , , , , , , , , , , , , Atom Electronic I, Configuration He Electronic Configuration, Sc (Z = 21) [Ar] 3d '4s? Sc?* ae: =, o'56 : Ar] 3d, Ti (Z = 22) Ar] 3d 74s? i2*, [Ar] 3d“4s Ti [Ar] 3d?, 3+, Ti Ar] 3d!, Tiny 0, Ar] 3d, V (Z=23) 3452 +, [Ar] 3d°4s Vv? Ar] 3d°, ve [Ar] 3d?, Cr(Z=24) s - ae, = 1 +, [Ar] 3d> 4s Gr Ar] 3d4, Cet [Ar] 34°, Mn (Z = 25) [Ar] 3d°4s? Mn?” [Ar] 3d°, Fe (Z = 26) [Ar] 34%4s? Fe** [Ar] 3d®, Fe** [Ar] 3d°, fnt2 27) [Ar] 3d’4s” Co** At] 3d”, Ni (Z = 28) [Ar] 354s? Ni** . [Ar] 3a, Cu (Z = 29) [Ar] 3d!°4s! Cu** Ar] 3d°, Zn (Z = 30) [Ar]3 d!°43? Zn** [Ar] 3a'°, , , , 13> GENERAL CHARACTERISTICS OF d-BLOCK ELEMENTS, , Some important properties of d-block elements are given below:, (i) They are practically all hard, strong, high melting, high boiling metals and conduct heat, and electricity well., (ii) They form alloys with one another and with other metallic elements., , (iii) They are all ductile metals., , (iv) They form interstitial compounds with non-metals like C, B, N, H, ete., (v) Because of unpaired electrons they form paramagnetic compounds,, (vi) Most of the elements have great tendency to form stable complex ions,, , (vii) Many of them are electropositive enough to dissolve in mineral acids., (viii) They exhibit catalytic properties and are wonderful catalysts for hydrogenation, oxidation,, , dehydration etc., ts form compounds with Lewis acid ligands like CO, NO ete. in, , (ix) The transition elemen like, which the elements exist in low, zero or even formally negative oxidation states., , (x) They exhibit variable oxidation state and their ions and compounds are coloured,
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, —_—_, 6 DEGREE STUDENTS SEMESTER ~IV (INORGANIC CHEMISTRY), , 14 VARIATION IN ATOMIC RADILIN A PERIOD AND DOWN THE, GROUP IN CASE OF TRANSITION ELEMENTS, Along a period, The atomic radius of an element decreases while moving from left to righ, in a period. This can be understood in terms of increases in nuclear charge with consequen, contraction in size, However, the decrease is very small within some range in the period, For example, in the firs:, , transition series, the decrease in atomic radius is very small from chromium to nickel. The sam:, thing happens in other transition series, Another remarkable observation is that the atomic radius, starts increasing after that, For example, the valuc increases from nickel through copper to zinc, (See Table 1.3)., , __As a matter of fact, two opposing forces are simultancously taking place. In moving from lef:, to right, nuclear charge increases leading to decrease in atomic radius. At the same time, repulsion, takes place between the electron population resulting in increase in the atomic radius. Net value, of atomic radius would be determined by the combined effect of these opposing phenomena. It is, found that the radius decreases but very slightly from Cr to Ni. After that repulsive forces outweigh, and an increase in radius is observed., , Along a group. While there is some increase in atomic radius in moving from first to second, member, the value remains unchanged while moving from second to third member. This can be, explained in terms of lanthanide contraction which is described in detail in the chapter on Fbolock, , elements., , 1.4.1 Variation in Atomic Volume and Density of Transition Elements, , in a Period and a Group, , Atomic volume shows the same trend in the period and the group as the atomic radius. Thus, atomic radius volume decreases in the period up to a certain point and then increases thereafter up, to the end of the period (See Table 1.3). Reasons are the same as explained in previous section., , In a group the volume increases first and then becomes constant,, , Density is the ratio of mass and volume. We observe that the density increases (Density =, Mass/volume) in the period as the atomic mass increases and volume shrinks. Towards the end of, the period, the trend is reversed. Because of interelectronic repulsion, the atomic volume increases,, , thereby reducing density (Sce Table 1.3)., Down the group, the density increases because there is not much variation in atomic volume, , whereas atomic mass increases., , 1.4.2 Variation in Melting and Boiling Points of Transition Elements, , Transition metals have high melting and boiling points. This is due to strong overlapping, between (n — I)d orbitals and formation of covalent bonds by the d-electrons. Greater the numbet, of unpaired d-electrons, greater the possibility of covalent bond formation, and hence higher the, melting and boiling point (See Table 1.3)., , Zn, Cd and Hg (melting points 420, 320, and —39° C respectively) are notable exceptions, because in these elements the (m —1)d orbitals are complete and covalent bond formation i”, d-orbitals is not expected, — ,, , The melting and boiling points of these clements increase down the group due to increasing, atomic mass, And in a period melting and boiling points reach a maximum near group VB, 8%, VIB. In moving across a transition series from left to right, the number of unpaired d-clectrons 15°, to maximum of five in group VI beyond which electron pairing begins. The number of unpaite’
