Chemistry Introduction and Position of d-block Elements in Periodic Table

### Topics Covered :

● Introduction
● Position of d-block Elements in Periodic Table
● Electronic Configurations of the d-Block Elements

### Introduction :

=> The color{red}(d)-block of the periodic table contains the elements of the groups 3-12 in which the color{red}(d)-orbitals are progressively filled in each of the four long periods.

=> The elements constituting the color{red}(f)-block are those in which the color{red}(4 f) and color{red}(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 color{red}(f)-block of the periodic table.

=> The names transition metals and inner transition metals are often used to refer to the elements of color{red}(d)-and color{red}(f)-blocks respectively.

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

=> The two series of the inner transition metals, (color{red}(4f) and color{red}(5f)) are known as lanthanoids and actinoids respectively.

=> A transition element is defined as the one which has incompletely filled color{red}(d)-orbitals in its ground state or in any one of its oxidation states.

● Zinc, cadmium and mercury of group 12 have full color{red}(d^(10)) 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 color{red}(d) or color{red}(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.

### Position in the Periodic Table :

=> The color{red}(d)–block occupies the large middle section flanked by color{red}(s)- and color{red}(p)- blocks in the periodic table.

=> The very name ‘transition’ given to the elements of color{red}(d)-block is only because of their position between color{red}(s-) and color{red}(p-) block elements.

=> The color{red}(d-)orbitals of the penultimate energy level in their atoms receive electrons giving rise to the three rows of the transition metals, i.e., color{red}(3d), color{red}(4d) and color{red}(5d). The fourth row of color{red}(6d) is still incomplete. These series of the transition elements are shown in Table 8.1.

### Electronic Configurations of the d-Block Elements :

=> In general the electronic configuration of these elements is color{red}((n-1)d^(1–10) ns^(1–2)). The (color{red}(n–1)) stands for the inner color{red}(d)-orbitals which may have one to ten electrons and the outermost color{red}(ns)-orbital may have one or two electrons.

=> However, this generalisation has several exceptions because of very little energy difference between color{red}((n-1)d) and color{red}(ns) orbitals.

● Furthermore, half and completely filled sets of orbitals are relatively more stable.

● A result of this factor is reflected in the electronic configurations of color{red}(Cr) and color{red}(Cu) in the color{red}(3d) series.

● Consider the case of color{red}(Cr), for example, which has color{red}(3d^5 4s^1) instead of color{red}(3d^4 4s^2); the energy gap between the two sets (color{red}(3d) and color{red}(4s)) of orbitals is small enough to prevent electron entering the color{red}(3d) orbitals.

● Similarly in case of color{red}(Cu), the configuration is color{red}(3d^(10) 4s^1) and not color{red}(3d^9 4s^2). The outer electronic configurations of the transition elements are given in Table 8.1.

=> The electronic configurations of color{red}(Zn), color{red}(Cd) and color{red}(Hg) are represented by the general formula color{red}((n-1)d^(10) ns^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 color{red}(d)-orbitals of the transition elements project to the periphery of an atom more than the other orbitals (i.e., color{red}(s) and color{red}(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 color{red}(d^n) configuration (color{red}(n = 1 - 9)) have similar magnetic and electronic properties.

● With partly filled color{red}(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.

=> 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.
Q 3070191916

On what ground can you say that scandium (Z = 21) is a transition element but zinc (Z = 30) is not?

Solution:

On the basis of incompletely filled 3d orbitals in case of scandium atom in its ground state (3d^1), it is regarded as a transition element. On the other hand, zinc atom has completely filled d orbitals (3d^(10)) in its ground state as well as in its oxidised state, hence it is not regarded as a transition element.