A)

(a) Mention any three demerits of Mendeleev's periodic law.  

(b) Explain how and why the metallic character of elements varies on moving from  

1. Left to right in a period  
2.  From top to bottom in a period of the modern periodic table.  

 

                                                                                                 OR

B)

Explain how the trends in the following properties of elements vary on moving from left to right in a period, in the modern periodic table.

 (a)Variation of valency. 

 (b) change of atomic radius. 

(c)  Metallic to the non-metallic character.

(d) Electronegative character. 

(e) Nature of oxides.  

A)

(a) The three observations that challenged Mendeleev's Periodic Law were:  

Arranging elements according to the increasing order of atomic mass could not be maintained. The chemical properties of an element do not depend on atomic mass.  

Isotopes were not given any place in the table as they have different atomic masses but the same chemical properties.  

A correct position could not be assigned to hydrogen in the periodic table. 

(b) (i) On moving from left to right in a period, the effective nuclear charge increases and hence the attraction between the nucleus and the valence electron increases. And it becomes difficult for an atom to donate electrons. Therefore, the metallic character of elements decreases as we move from left to right as the electropositive character decreases. 

(ii) Ongoing down in a group of the periodic table, atomic size increases as the number of shells increases. So, the force with which the nucleus is holding electrons also decreases, and it becomes easier for an atom to donate electrons. Therefore, the metallic character of elements increases because the electropositive character of elements increases. 

                                                                                                 OR

B)

The trend in the properties of elements when we move from left to right in the period.  

1. Valency: Along the period from left to right, the valency of elements increases first and then decreases. It first increases from 1 to 4 and then decreases to zero. This is because the first element of every period has 1 valence electron and the last element in every period has 8 electrons except for the first period.  
2. Atomic radii: Along the period from left to right, atomic size decreases. This is because on moving from left to right, effective nuclear charge increases as the number of electrons and protons increases. As a result, the attraction between the nucleus and electron increases and the size of the atom decreases.  
3. Metallic to non-metallic character: On moving from left to right in a period, the effective nuclear charge increases and hence the attraction between the nucleus and the valence electron increases. And it becomes difficult for an atom to donate electrons. Therefore, the metallic character of elements decreases as we move from left to right as the electropositive character decreases. But the non-metallic character increases because the size of the atom decreases and the removal of the electron becomes difficult, but the addition of the electron becomes easier.  
4. Electronegativity character: Electronegativity is the tendency of an atom to accept the electron. When we move from left to right, the effective nuclear charge increase, and the tendency of the nucleus to accept the electron increases hence the electronegativity of the atom increases.  
5. Nature of oxides: On moving from left to right in a period, the effective nuclear charge increases and hence the attraction between the nucleus and the valence electron increases. And it becomes difficult for an atom to donate electrons. Therefore, the metallic to non-metallic character increases in the period. So, the nature of oxide formation across the period changes from basic to acidic.