(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.  

(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.