d & f Block Elements Part - 2 | Class 12th

·   Melting and Boiling Points: The transition metals have very high m.p. and b.p. The high melting points of these metals are attributed to the involvement of greater number of electron from (n-1)d in addition to the ns electrons. This can also be explained in terms of metallic bond strength which depends on the number of unpaired electrons. As the number of unpaired electron increases, the metallic bond strength increases. Hence the melting point also increases.

In a given transition series, the number of unpaired electrons increases up to the middle and then decreases.

Another factor which affects the m.p is heat of atomization. Mn and Tc have low melting point even though they have d⁵ configuration. This is because of their low heat of atomization.

·   Ionisation Enthalpy:  There is slight and irregular variation in ionization energies of transition metals due to irregular variation of atomic size. The I.E. of 5d transition series is higher than 3d and 4d transition series because of Lanthanoid Contraction.

Lanthanoid Contraction:  The steady decrease in the atomic and ionic radii of the transition metals as the atomic number increases. This is because of filling of 4f orbitals before the 5d orbitals. This contraction is size is quite regular. This is called lanthanoid contraction. It is because of lanthanoid contraction that the atomic radii of the second row of transition elements are almost similar to those of the third row of transition elements.

·   Metallic Character: All transition elements are metallic in nature, i.e. they have strong metallic bonds. This is because of presence of unpaired electrons. This gives rise to properties like high density, high enthalpies of atomization, and high melting and boiling points.

·   Oxidation State: Transition metals show variable oxidation states. This is because in these elements d and s electrons have comparable energies. So, in chemical reaction along with s-electrons, d-electrons also participate. In a given transition series, the maximum oxidation state increases up to the middle and then decreases.

F Most common oxidation states are in circles.

·       Electrode Potential: The electrode potential values of first row transition series generally increases from left to right with some exceptions. The E⁰(Cu²⁺/Cu) is positive (+0.34V), while the E⁰ values of all the other first row transition elements are –ve. This is because the high energy to transform Cu(s) to Cu²⁺(aq) is not balanced by its hydration enthalpy. So, Cu does not easily react with acid and liberate H₂. Only oxidizing acids [HNO₃ and hot Conc. H₂SO₄] react with Cu and the acid get reduced.
  ·    Magnetic properties Paramagnetic: Due to presence of the unpaired electrons in  (n-1)d orbitals, most of the transition metal ions and their compounds are paramagetic. They are attracted by the magnetic field. Diamagnetic - They have paired electrons and are repelled by magnetic field.
      Magnetic moment µ_s = √n(n+2),  n =  number of unpaired electrons Magnetic moment or paramagnetic property increases with increase in the number of unpaired electrons. Ferromagnetic -  Substance which are attracted very strongly are said to be ferromagnetic. Ferromagnetism is an extreme form of paramagnetism.