Study Materials

# NCERT Solutions for Class 9th Science

Page 3 of 5

## Chapter 3. Atoms and Molecules

Chapter– 3 ATOMS AND MOLECULES

Additional And Important Questions with Solution:

Ques.1 – Who had found two important laws of chemical combination.

Ans. - Antoine L. Lavoisier .

Ques.2 – Write the name of two important laws of chemical combination which is found by Antoine L. Lavoisier.

Ans. – (i) Law of conservation of mass.    (ii)    Law of constant proportions.

Ques.3 – Write the law of conservation of mass.

Ans. - Mass can neither be created nor destroyed in a chemical reaction.

Ques.4 – Write the law of constant proportions.

Ans. - In a chemical substance the elements are always present in definite proportions by mass.

Ques.5 – Define atoms according to Dalton atomic theory.

Ans. – All matter, whether an element, a compound or a mixture is composed of small particles called atoms.

Ques.6 – Write the Dalton’s atomic theory.

Ans. – Dalton’s atomic theory as follow.

(i)  All matter is made of very tiny particles called atoms.

(ii)  Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

(iii) Atoms of a given element are identical in mass and chemical properties.

(iv) Atoms of different elements have different masses and chemical properties.

(v) Atoms combine in the ratio of small whole numbers to form compounds.

(vi) The relative number and kinds of atoms are constant in a given compound.

Ques.7 - Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Ans. – The second postulate of Dalton’s atomic theory is the result of the law of conservation of mass.

Ques.8 - Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Ans. – The forth postulate of Dalton’s atomic theory can explain the law of definite proportions.

Ques.9 – Define atomic mass.

Ans. – The sum of numbers of protons and neutrons presence in a nucleus of an atom is known as atomic mass.

Ques.10 – Define atomic number.

Ans. – The number of protons presence in a nucleus of an atom is known as atomic number.

Ques.11 – What is ion?

Ans. – The charged atom is known as ion.

Example – Na +, Mg+, Cl-.

Ques.12 – Write the differences between atom and ion.

Ans –

 Atom Ions It is neutralized. All the atoms are active insteed of  noble gas. It has uncompleted electronic structure in spite of noble gas. It is charged. All ions are reactive. It has completed electronic structure.

Page 3 of 5

Chapter Contents: