trends in ionisation enthalpy of d-block elements part- 2

 

Ionisation enthalpy- M(g) -----> M⁺(g) + Δ_iH (ionisation enthalpy) 

·      Ionisation potential values increase in a period from left to right with increase in  nuclear charge. The increase, however, is not regular.

·      In vertical columns, i.e., groups, the ionisation potential decreases from first member to second member in most of the cases as expected,

·       however, the third member has higher value than second member. This is due to lanthanide contraction, i.e., the atomic radii of the elements of the same group of the second and third transition series are nearly the same but atomic numbers differ by 32. Thus, the outer electrons are held very firmly and the ionisation potential values are very high.   

·      On account of this, the members of 5d are inert under ordinary conditions, i.e., Pt, Au, Hg, etc., are noble metals.

·      Ionisation enthalpy depends upon

      a) Nuclear charge α I.E   α 1/size of atom     

      b) electronic configuration

  (order of ionisation enthalpy  à partially filled < half-filled < completely filled orbitals)

                                                       Fe                           Mn                     Zn

  Electronic configurationà3d⁶4s²                 3d⁵4s²           3d¹⁰4s²      

  Stability order                      less                 stable         most stable

·      I^st ionisation enthalpy of 5d series element are higher than those of 3d or 4d elements

 Ans it is due to lanthanoid contraction or the ineffective shielding of the nucleus by 4f electrons over effective nuclear charge.

·      Ist ionisation enthalpy of Mn is greater than Fe

Ans-due to presence of half-filled d-orbital in Mn(3d⁵4s²⁾ that is most stable than partially filled d-orbital present in Fe(3d⁶4s²)

·      Third ionisation energy of manganese is very high.

 Ans This is because the third electron has to be removed from a stable configuration, (Mn^{2 +} 3d⁵), i.e;, half-filled 3d-subshell.

·      The second ionisation potential values of Cu and Cr are sufficiently higher than those of neighbouring elements.

Ans This is because of the electronic configuration of Cu⁺ which is 3d¹⁰ (completely filled) and of Cr⁺ which is 3d⁵ (half-filled), i.e., for the second ionisation potential, the electron is to be removed from very stable configurations.

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