(a) On the basis of electronic configuration, how will you identify the first and the last element of a period?

(b) The stars appear higher from horizon then they actually are. Explain why it is so ?

(a) The element will be present in which period will depend on the number of valence electrons of that element. Since number of valence electrons varies from 1 to 8, therefore, the first element present in period 1 will have 1 valence electron and element present in last period will have 8 valence electrons (except in the first period where last element helium has only 2 valence electrons).

(b) The light rays coming from the star travels through different layers of atmosphere and due to varying optical density, there is bending of light continuously. Due to phenomenon of atmospheric refraction, this bending of light results in curved path of light rays. It seems to the observer that the light is coming straight from source and that’s why the stars appear higher than the actual position of them.