The lighter elements of Group I and II have distinctly different chemistry from their heavier congeners. For example, Be and Li both have a strong tendency to form polymer-like structures with bridging atoms whereas the heavier elements do not. (a) What makes Be and Li different from the heavier group members? (b) What is the physical origin of the tendency towards polymerization for Be and Li complexes, and how can it be overcome?

Explanation:

(A) Heavier members of group 1A are usually more reactive than the lighter members e.g. lithium react very slowly with liquid bromine whereas CS  react violently.

Similarly Be of group 2A has smaller size, heavier member of this group show different colour in flame test but Be and Mg emit light when heated. Reactivity of group 2A metals decreases down the group

(B) In ethylene polymerization, the stronger catalytic action is shown by the alkyl of metal having less diameter e.g Be, Li etc.

A lithium alkyl in a hydrocarbon solvent is stereospecific catalyst for polymerisation of isoprene