Unit 8: The d- and f-Block Elements PDF

Unit Objectives The d- d - and f- f- 8 After studying this Unit, you...

Unit Objectives The d- d - and f- f- 8 After studying this Unit, you will be able to Block Element Elementss learn the positions of the d– and f- block elements in the periodic table; Iron, copper, silver and gold are among the transition elements that know the electronic configurations have played important roles in the development of human civilisation. of the transition (d-block) and the The inner transition elements such as Th, Pa and U are proving inner transition (f-block) elements; excellent sources of nuclear energy in modern times. appreciate the relative stability of various oxidation states in terms of electrode potential values; The d-block of the periodic table contains the elements describe the preparation, of the groups 3-12 in which the d orbitals are properties, structures and uses progressively filled in each of the four long periods. of some important compounds The elements constituting the f -block are those in such as K2Cr 2O 7 and KMnO4 ; which the 4 f and 5 f orbitals are progressively filled understand the general in the latter two long periods; these elements are formal characteristics of the d– and members of group 3 from which they have been taken f–block elements and the general out to form a separate f-block of the periodic table. horizontal and group trends in The names transition metals and inner transition them; metals are often used to refer to the elements of d-and describe the properties of the f-blocks respectively. f-block elements and give a There are mainly three series of the transition comparative account of the lanthanoids and actinoids with metals, 3d series (Sc to Zn), 4d series (Y to Cd) and 5d respect to their electronic series (La to Hg, omitting Ce to Lu). The fourth 6d configurations, oxidation states series which begins with Ac is still incomplete. The two and chemical behaviour. series of the inner transition metals, (4f and 5f) are known as lanthanoids and actinoids respectively. Strictly speaking, a transition element is defined as the one which has incompletely filled d orbitals in its ground state or in any one of its oxidation states. Zinc, cadmium and mercury of group 12 have full d10 configuration in their ground state as well as in their common oxidation states and hence, are not regarded as transition metals. However, being the end members of the three transition series, their chemistry is studied along with the chemistry of the transition metals. The presence of partly filled d or f orbitals in their atoms sets the study of the transition elements and 2015-16(20/01/2015)  their compounds apart from that of the main group elements. However, the usual theory of valence as applicable to the main group elements can also be applied successfully to the transition elements. Various precious metals such as silver, gold and platinum and industrially important metals like iron, copper and titanium form part of the transition metals. In this Unit, besides introduction, we shall first deal with the electronic configuration, occurrence and general characteristics of the transition elements with special emphasis on the trends in the properties of the first row (3d) transition metals and the preparation and properties of some important compounds. This will be followed by consideration of certain general aspects such as electronic configurations, oxidation states and chemical reactivity of the inner transition metals. THE TRANSITION ELEMENTS (d-BLOCK) 8. 1 Position in the The d–block occupies the large middle section flanked by s– and Periodic Table p– blocks in the periodic table. The very name ‘transition’ given to the elements of d-block is only because of their position between s– and p– block elements. The d–orbitals of the penultimate energy level in their atoms receive electrons giving rise to the three rows of the transition metals, i.e., 3d, 4d and 5d. The fourth row of 6d is still incomplete. These series of the transition elements are shown in Table 8.1. 8. 2 Electronic In general the electronic configuration of these elements is Configurations (n-1)d1–10 ns 1–2. The (n–1) stands for the inner d orbitals which may have one to ten electrons and the outermost ns orbital may have one or two of the d-Block electrons. However, this generalisation has several exceptions because Elements of very little energy difference between (n-1)d and ns orbitals. Furthermore, half and completely filled sets of orbitals are relatively more stable. A consequence of this factor is reflected in the electronic configurations of Cr and Cu in the 3d series. Consider the case of Cr, for example, which has 3d5 4s 1 instead of 3d44s2; the energy gap between the two sets (3d and 4s) of orbitals is small enough to prevent electron entering the 3d orbitals. Similarly in case of Cu, the configuration is 3d104s1 and not 3d94s 2. The outer electronic configurations of the transition elements are given in Table 8.1. Table 8.1: 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 Chemistry 210 2015-16(20/01/2015)  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 the general formula (n-1)d10ns 2. The orbitals in these elements are completely filled in the ground state as well as in their common oxidation states. Therefore, they are not regarded as transition elements. The d orbitals of the transition elements project to the periphery of an atom more than the other orbitals (i.e., s and p), hence, they are more influenced by the surroundings as well as affecting the atoms or molecules n surrounding them. In some respects, ions of a given d configuration (n = 1 – 9) have similar magnetic and electronic properties. With partly filled d orbitals these elements exhibit certain characteristic properties such as display of a variety of oxidation states, formation of coloured ions and entering into complex formation with a variety of ligands. The transition metals and their compounds also exhibit catalytic property and paramagnetic behaviour. All these characteristics have been discussed in detail later in this Unit. There are greater horizontal similarities in the properties of the transition elements in contrast to the main group elements. However, some group similarities also exist. We shall first study the general characteristics and their trends in the horizontal rows (particularly 3d row) and then consider some group similarities. On what ground can you say that scandium (Z = 21) is a transition Example 8.1 element but zinc (Z = 30) is not? On the basis of incompletely filled 3d orbitals in case of scandium atom Solution in its ground state (3d1), it is regarded as a transition element. On the other hand, zinc atom has completely filled d orbitals (3d10) in its ground state as well as in its oxidised state, hence it is not regarded as a transition element. 211 The d- and f- Block Elements 2015-16(20/01/2015)