Comment upon the statement that:
(i) The electronic configurations of lanthanoid metals are not known with certainty.
(ii) Strictly speaking, the lanthanoids should include only thirteen elements with atomic number ranging from 58 to 70.

The electron configuration of lanthanoid elements is not known with certainty. Explain.

(a). Why second asnd third transition series elements show similar size? (b). Why electronic configuration of lanthanoids not known with certainty?

The electronic configuration of an element 'X' is 2,8,8,2. Write its (i) group number, (ii) period number and (iii) the number of valence electrons in its atom. Justify your answer in each case.

The periodicity is related to the electronic configuration. That is, all chemical and physical properties are a manifestation of the electronic configuration of the elements. The atomic and ionic radii generally decrease in a period from left to right. As a consequence, the ionization enthalpies generally increase and electron gain enthalpies become more negative across a period. In other words, the ionization enthalpy of the exterme left element in a period is the least and teh electron gain enthalpy of the element on the exterme right is the highest negative. This results into high chemical reactivity at the two extermes and the lowest in he centre. Similarly down the group, the increase in atomic and ionic radii result in gradual decrease in ionization enthalpies and a regular decrease (with exception in some third period elements) in electron gain enthalpies in the case of main group elements. The loss and gain of electrons can be co-related with the reducting and oxidising behaviour, and also with metallic and non-metallic character respectively, of the elements. Which of the following statements is correct?

The transition element ( with few exceptions ) show a large number of oxidation states . The various oxidation states are related to the electronic configuration of their atoms. The variable oxidation states of a transition metal is due to the involvement of (n-1)d and outer ns electrons . For the first five elements of 3d transition series , the minimum oxidation state is equal to the number of electrons in 4s shell and the maximum oxidation state is equal to the sum of 4s and 3d electrons. The relative stability of various oxidation states of a given element can be explained on the basis of stability of d^(0),d^(5) and d^(10) configuration . Identify the correct statement

Consider the following statements (I) Zr^(4+) and Hf^(4+) have almost same ionic radii (II) Lanthanoids liberate H_2 when treated with dilute acids. (III) The ionic radii of trivalent lanthanoids steadily increase with increase in atomic number. The correct statement(s) is/ are

The outer electronic configuration of two members of the lanthanoid series are as follows: 4f^(1)5d^(1)6s^(2) and 4f^(7) 5d^(0) 6s^(2) What are their atomic number ? Predict the oxidation states exhibited by elements in their compounds.

(a) Complete the following chemical equations for reactions : (i) MnO_(4(aq))^(-) + S_(2)O_(3(aq))^(2-) + H_(2)O_((l)) to (ii) Cr_(2)O_(7 (aq))^(2-) + H_(2)S_((g)) + H_((aq))^(+) to (b) Give an explanation for each of the following observations : (i) The gradual decrease in size (actinoid contraction) from element to element is greater among the actinoids than that among the lanthanoids (lanthanoid contraction.) (ii) The greatest number of oxidation states are exhibited by the members in the middle of a transition series. (iii) With the same d-orbitals configuration (d^(4)) Cr^(2+) ion is a reducing agent but Mn^(3+) ion is an oxidising agent .

Give an explanation for each of the following observations: (i) The gradual decrease in size (actinoid con-traction) from element to element is greater among the actinoids than that among the lanthanoids (lanthanoid contraction). (ii) The greater number of oxidation states are exhibited by the members in the middle of a transition series. (iii) With the same d-orbital (d^4) , Cr^(2+) ion is a reducing agent but Mn^(3+) ion is an oxidising agent.

The periodicity is related to the electronic configuration. That is, all chemical and phyical properties are a manifestation of the electronic configurfation of the elements. The atomic and ionic radii generally decrease in a preiod from left or right. As a consequence , the ionisation enthalpies generally increase and electron gain enthalpies become more negative across a period. In other words, the ionisation enthalpy of the extreme left element in a period is the least and the electron left element on the extreme right is the highest negative. This results into high chemical reactivity at two extgremes and the lowest in the centre. Similarly down the group, the increase in atomic and ionic radii result in gradual decrease (with excetption in some third period elements) in electron gain enthalpies in the case of main group elements . These properties can be related with the (i) reducing and oxidising behacviour of the elements (ii) metallic and no-metallic charcter of element (iii) acidic, basic , amphoteric and neutral character of the oxides of the elements. Among Al_(2)O_(3), SiO_(2),P_(2)O_(3) and SO_(2) the correct order of acid strenght is :