Introduction occurance and abundance of block elements.

Introduction

Three series of the element are formed by filling 3d, 4d and 5d shells of electrons . Together these comprise the d-block elements. They are often called “transition element” because their position in the periodic table is between the s-block and p-block elements. Their properties are transitional between the highly reactive metallic element of the s-block , which typically form ionic compounds , and the elements of the p-block, which are largely covalent. In the s-block and p-block electrons are added to the outer shell of the atom. In the d-block , electrons are added to the penultimate shell, expanding it from 8 to 18 electrons. Typically the transition elements have an incompletely filled d level. Group 12 (the zinc group) has a d10 configuration and since the d shell is complete, compounds of these elements are not typical and show some differences from the others. The elements make up three complete rows of ten elements and an incomplete fourth row. The position of the incomplete fourth series is discussed with the f-block elements.

Metallic character

In the d-block elements, the penultimate shells of electrons are expanding . Thus they have many physical and chemical properties in common. Thus all the transition elements are metals. They are therefore good conductors of electricity and heat, have a metallic lustre and are hard , strong and ductile . They also form alloys with other metals.

Density

The atomic volumes of the transition elements are low compared with elements in neighbouring Groups 1 and 2. This because the increased nuclear charge is poorly screened and so attracts all the electrons more strongly. In addition , the extra electrons added occupy inner orbitals. Consequently, the densities of the transition metals are high. Practically all have a density greater than 5gcm-3.(The only exceptions are Sc 3.0 gcm-3 and Y Ti 4.5 gcm-3 and iridium 22.61gcm-3. To get some feel for how high this figure really is , a football made of osmium or iridium measuring 30cm in diameter would weigh 320 kg or almost one-third of a tonne.

Melting and boiling point

The melting and boiling point of transition elements are generally very high (see appendencies B and C). Transition elements typically melt above 1000c. Ten elements melt above 2000c are three melt above 3000c (Ta 3000c ,W 3410c and Re 3180c). There are a few exceptions . The melting point of LA and Ag are just under 1000c (920c and 961c respectively). Other notable exceptions are Zn(420c), Cd(321c) and Hg which is liquid at room temperature and melts at -38c . The last three behave typically because the d shell is complete, and d electrons don’t participate in the melting bonding. The high melting point is in marked contrast to the low melting points for the s-block metal Li(181c) and Cs(29c).

Reactivity of metals

Many of the metals are sufficiently electropositive to react with mineral acids , liberating H2. A few have low standard electrode potentials and remain unreactive or novel . Noble character is favoured by high enthalpies of sublimation , high ionisation energies and low enthalpies of solvation. The high melting point indicates high heats of sublimation . The smaller atoms have higher ionisation energies , but this is offset by small ions having high solvation energies . This tendency to the noble character is most pronounced for the platinum metals (Ru, Rh,Pd ,Os Ir, Pt) and gold.

Abundance.

Three of the transition metals are very abduent in the earth’s crust.Fe is the fourth most abundant element by weight,Ti the ninth and Mn the twelfth. The first row of transition elements largely follows Harkin’s rule that elements with an even atomic number are in general more abduent than their neighbours with the odd atomic number.Manganese is an exception. The second and third-row elements are much less abundant than the first row. Tc does not occur in nature.Of the last six elements in the second and third rows (Tc, Ru, Pd, Ag, Cd, Re, Os, Ir, Pt, Au, Hg, none occur to an extent of more than 0.16 parts per million (ppm) in the earth’s crust.

The size of Atoms and ions.

The covalent radii of the element decreased from left to right across a row in the transition series, until near the end when the size increased slightly. On passing from left to right, extra protons are placed in the nucleus and extra-orbital electrons are added. The orbital electrons shield the nuclear charge incompletely (d electrons shield less efficiently) than p electrons, which in turn shield less effectively than s electrons.Because of the poor screening by d electrons, the nuclear charge attracts all of the electrons more strongly;hence a concentration in size occurs.

Atoms of the transition elements are smaller than those of the group 1 or 2 elements in the same horizontal period. This is partly because of the usual character in size across a horizontal period discussed above and partly because the orbital electrons are added to the penultimate d shell rather than to the outer shell of the atom.

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.

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