Catalysts play a crucial role to allow chemical processes to change reaction steps and proceed at an economical rate. 7.Melting and Boiling points: – All transition elements have high melting and boiling point because of strong metallic bond between atoms of metals. (iii) Enthalpy of atomization is the amount of heat required to break the metal lattice to get free atoms. Although electrons do not fill up ‘d’ orbital in the group 12 metals, their chemistry is similar in many ways to that of the preceding groups, and so considered as d block elements. Catalytic property of 3d series elements also explained as follow. Elements having electrons (1 to 10) present in the d-orbital of the penultimate energy level and in the outer most ‘s’ orbital (1-2) are d block elements. The partially filled subshells of d-block elements incorporate (n-1) d subshell.All the d-block elements carry a similar number of electrons in their furthest shell. Transition elements are those elements that have partially or incompletely filled d orbital in their ground state or the most stable oxidation state. Hence, they possess similar chemical properties. In d-block, oxidation states differ by one, whereas in p-block… d- Block Elements Short Answer Questions: **1. Osmium has highest density because atomic volume of d-block elements are low and electron enter into (n-1)d sub-shell. Transition metals and their compounds also possess catalytic properties. Explain Werner’s theory of coordination compounds with suitable examples A: The important postulates of Werner’s theory are: i) In coordination compounds the metal atom exhibit two types of valency a) Primary valency b) secondary valency Properties and Trends in Transition Metals. The elements with incompletely filled d-subshell in their ground state or most stable oxidation state are named as D-block elements.They are additionally named as transition elements.The partially filled subshells incorporate the (n-1) d subshell.All the d-block elements have a similar number of electrons in the furthest shell. So effective nuclear charge is more, volume is less and density is higher. This indicates that the transition elements are less electropositive than s-block elements. Transition Elements. It is due to similar energy of (n – 1 )d and ns orbitals, electrons from both can be lost. In p-block, lower oxidation state is more stable due to inert pair effect, whereas in ehblock elements higher oxidation states are more stable. The question is the catalytic properties of d block elements and the uses of d block element catalyst For example, they exhibit variable oxidation states, form coloured complexes with different anions and neutral molecules and show paramagnetic behaviour. The elements of the second and third rows of the Periodic Table show gradual changes in properties across the table from left to right as expected. Electrons in the outer shells of the atoms of these elements have little shielding effects resulting in an increase in effective nuclear charge due to the addition of protons in the nucleus. (i) The ionisation energies of these elements are high, and in the most cases lie between those of s- and d-block elements. Formation of coloured ions 4 Catalytic Properties 5 Variable oxidation states 6 Magnetic Properties 7 Complex formation 8 Formation of alloys 1 Atomic and Ionic Radii The atomic and ionic radii of transition elements are smaller than those of p- block elements and larger than those of s-block elements. * in kJ mol-1. i) Due to variable oxidation states of transition metals they can absorb and re-emit a wide range of energies. The following generalizations can be obtained from the ionisation energy values given above. Transition elements having partly filled d-orbitals exhibit several interesting properties. (ii) d-block elements exhibit more oxidation states due to small energy gap between ns and (n – 1)d subshell while f – block elements show less oxidation state due to large energy gap between ns and (n -2)f subshell. Thus, they can provide necessary activation energy. 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