question_answer

  Explain the following observations. (i) Many of the transition elements are known to form interstitial compounds. (i) There is general increase in density from titanium (Z =22) to copper (Z =29). (iii) The members of the actinide series exhibit a larger number of oxidation states than the corresponding members of the lanthanide series.                                                                                                                          [3 x 1]

Answer:

  (i) Interstitial compounds Transition metals can trap some atoms like hydrogen, boron, carbon, nitrogen etc., in vacant spaces in their crystal lattice forming interstitial compounds. Carbon and nitrogen always occupy octahedral holes; hydrogen is smaller and always occupies tetrahedral holes. As only transition metals form such compounds, the d electrons are, therefore, presumably involved in the bonding. The structure of the metal often changes during the formation of such compounds. The composition of these compounds is generally non-stoichiometric, e.g.,but may approach regular stoichiometry and a regular structure, e.g., TiC and VN. The later transition elements of the first series form non-stoichiometric carbides with irregular structures, such as which are more reactive than the interstitial carbides of the early transition elements. These interstitial compounds are of much importance, i.e., carbon steels are interstitial iron-carbon compounds in which the interstitial carbon prevents the iron atoms from sliding over one another, making iron harder, stronger but more brittle.                 [1] (ii) When we move along a period left to right, force of attraction between valence shell electron and nucleus increases. Thus, atomic size decreasesIn a period, n remains constant Thus,                     Atomic volume   is Thus directly related to radius. Since, density                              [1] Volume also decreases along a period, Thus density increases. (iii) The sum of the first three ionisation energies of the lanthanoids is comparatively low, so the elements are highly electropositive. They readily form  ions by losing both the 6s and one 5d electrons. In the absence of 5d electron, one of the electrons present in the (n -2)f orbitals is lost. Besides + 3 oxidation state, some of the lanthanides show other oxidation state                                                 Actinides generally show + 3 oxidation state. But due to comparable energies of 5f, 6d and 7s level, variety of oxidation states is shown by actinoids. The maximum oxidations state increase from + 4 (in Th) to + 5 in Pa, + 6 in U and + 7 in Np and Pu.                                      [1]