NCERT Class 12 Chemistry The d and f Block Elements

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The d-block of the periodic table contains the elements of the groups 3-12 in which the d orbitals are progressively filled in each of the four long periods. The elements constituting the f -block are those in which the 4 f and 5 f orbitals are progressively filled in the latter two long periods; these elements are formal members of group 3 from which they have been taken out to form a separate f-block of the periodic table. The names transition metals and inner transition metals are often used to refer to the elements of d-and f-blocks respectively.

There are mainly three series of the transition metals, 3d series (Sc to Zn), 4d series (Y to Cd) and 5d series (La to Hg, omitting Ce to Lu). The fourth 6d series which begins with Ac is still incomplete. The two 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 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  onfiguration, 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.

Position in the Periodic Table

The d–block occupies the large middle section flanked by s– and 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.

Electronic Configurations of the d-Block Elements

In general the electronic configuration of these elements is (n-1)d1–10ns1–2. 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 electrons. However, this generalisation has several exceptions because 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 4s1 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 3d94s2.

The electronic configurations of Zn, Cd and Hg are represented by the general formula (n-1)d10ns2. 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 surrounding them. In some respects, ions of a given dn 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.

8.1 Write down the electronic configuration of:

(i) Cr3+ (iii) Cu+ (v) Co2+ (vii) Mn2+

(ii) Pm3+ (iv) Ce4+ (vi) Lu2+ (viii) Th4+

8.2 Why are Mn2+ compounds more stable than Fe2+ towards oxidation to their +3 state?

8.3 Explain briefly how +2 state becomes more and more stable in the first half of the first row transition elements with increasing atomic number?

8.4 To what extent do the electronic configurations decide the stability of oxidation states in the first series of the transition elements? Illustrate your answer with examples.

8.5 What may be the stable oxidation state of the transition element with the following d electron configurations in the ground state of their atoms : 3d3,3d5, 3d8 and 3d4?

8.6 Name the oxometal anions of the first series of the transition metals in which the metal exhibits the oxidation state equal to its group number.

8.7 What is lanthanoid contraction? What are the consequences of lanthanoid contraction?

8.8 What are the characteristics of the transition elements and why are theycalled transition elements? Which of the d-block elements may not be regarded as the transition elements?

8.9 In what way is the electronic configuration of the transition elements different from that of the non transition elements?

8.10 What are the different oxidation states exhibited by the lanthanoids?

 

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