What is the main characteristic of the last elements in the periods of the periodic table ? What is the general name of such elements? (b) What is the number of elements in: (a) 1st period, and (b) 3rd period, of the modern periodic table?

(a) The main characteristic of last elements in a period is that they all have 8 valence electrons in their atoms except helium. Such elements are generally called noble elements which have stable electronic configuration.

(b) (i)On first period = 2 (ii) On 3^rd period= 8.