a) It is because nuclear charge increases due to increase in atomic number, therefore, force of attraction between nucleus and valence electrons increases, i.e. effective nuclear charge increases, hence atomic radii decrease from Li to F. (b) (i) Most metallic element is ‘Li’ as it can lose electrons easily due to larger atomic size. (ii) Most non-metallic element is ‘F’ because it can gain electrons easily due to smallest atomic size.