Important Questions for Class 9 Chapter 3 - Atoms and Molecules

Important questions based on NCERT syllabus for Chapter 3 - Atoms and Molecules:

Question-1: Why do elements combine? What is meant by the term chemical formula? Give one example to explain it.

Solution: 1. Elements combine to attain stability.
2. The chemical formula of a compound means the symbolic representation of the composition of a compound. From the chemical formula of a compound, we can know the number and kinds of atoms of different elements that constitute the compound.
3. For example, from the chemical formula
CO2 of carbon dioxide, we come to know that one carbon atom and two oxygen atoms are chemically bonded together to form one molecule of the compound, carbon dioxide.

Question-2: What is an atom, atomic mass and atomic number?

Solution: 1. An atom is the smallest fundamental particle of a chemical element.
2. Atomic mass(A) is equal to number of protons plus number of neutrons.
3. Atomic number(Z) is the number of protons in an atom.

Question-3: Find the chemical formula of carbon tetrachloride.

Solution: Carbon and chlorine will combine in such a manner that their valencies should get satisfied.

Valency of Carbon = 4
Valency of Chlorine = 1

To satisfy the valency of Carbon, 4 chlorine atoms each having a valency of 1 would be required. Hence, the formula is CCl4.