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

List any four characteristics of a good fuel.

What are non-renewable resources of energy? Give two examples of such resources.

(i) How do you calculate the possible valency of an element from the electronic configuration of its atoms?

(ii) Calculate the valency of element X whose atomic number is 9.

State the two laws of reflection of light.

The stars appear higher from the horizon than they actually are. Explain why it is so.

Explain why the planets do not twinkle but the stars twinkle.

Write any two differences between binary fission and multiple fission in a tabular form as observed in cells of organisms.

Why is DNA copying an essential part of the process of reproduction?

Explain the terms:

(i) Speciation

(ii) Natural selection