Alkynes usually show which type of reaction ?

Alkynes and Alkadienes express which type of isomerism

Alkene and alkyne gives following types of polymerisation-

The element which usually show variable oxidation states is :

Which type of compounds show iodoform reaction ?

What is meant by "brittleness"? Which type of elements usually show brittleness: metals or non-metals?

Why do alkenes and alkynes undergo addition reactions ? Describe some important addition reactions of alkenes and alkynes.