(i) Except for mercury, which is a liquid at room temperature all other elements are solid metals exhibiting all the characteristics of a metal.
(ii) The show variable oxidation states unlike s and p block elements.
(iii) They, and some of their compounds, show catalytic properties.
(iv) Their compounds are coloured.
(v) They have great tendency to form complex compounds.
(vi) They form alloys and interstitial compounds.
All the transition metals are good conductors of heat and electricity. Silver is the best conductor of electricity.
Because of small size of their atoms and strong metallic bonding the density and hardness of transition elements are high.
The ionization energy (IE) of transition elements are higher than those of s-block elements but lower than p-block elements. In a particular transition series, ionization energy although increases gradually as we move from left to right but this increase is not appreciable. The increase in ionization energy is due to increase in nuclear charge; the effect of increase in nuclear charge is partly balanced by the increase in screening effect. Consequently, the increase in ionization energy along the period of d-block elements is very small.
Illustration 1. K2PtCl6 is a well known compound whereas corresponding Ni compound is not known. State a reason for it.
Solution: This is because Pt4+ is more stable than Ni4+ has the sum of four ionization energies of Pt is less than that of Ni.
VARIABLE OXIDATION STATES
Transition elements usually exist in several different oxidation states and the oxidation states changes in units of one, e.g. Fe2+ and Fe+3, Cu+1 and Cu+2.
Scandium can have an oxidation number of (+II) if both s electrons are used for bonding and (+III) when two s and one d electrons are involved. Similarly all the elements show variable oxidation states depending upon the number of electrons available for bonding in their s and d sub-shells.
Illustration 2. Why do transition elements show variable oxidation states?
Solution: In the transition elements, the energies of (n-1)d orbitals and ns orbitals are very close. Hence electrons from both can participate in bonding.
Illustration 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.
Solution: The sum IE1 + IE2 increases. As a result the standard reduction potentials (E0) becomes less and less negative. Hence the tendency to form M2+ ion decreases. The greater stability of +2 state for Mn is due to half-filled d-subshell (d5), that for zinc is due to completely filled d-subshell (d10) and half that for nickel is due to highest negative enthalpy of hydration.
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