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  • Last Modified 25-08-2022

NCERT Solutions Class 9 Science Chapter 3 Atoms and Molecules


NCERT Solutions for Class 9 Science Chapter 3: The chapter covers all the basics of Atoms and Molecules. The chapter is of significant importance since the primary understanding of atoms and molecules is necessary to understand its dependent and complex topics in Class X, XI, and XII. The topic is also important regarding career prospects in Chemistry and Biotechnology. Students are advised to thoroughly study this chapter and get all their basic doubts cleared. 

Atoms and molecules chapter mainly talks about Law of Conservation of Mass, Law of Definite Proportions, Postulates of Dalton’s Atomic Theory, Atoms, Molecules and Ions and many more. Embibe delivers over 470 practice questions for all the sub-topics of chapter 3 Class 9 Science. Students must study all the in-text questions curated by Embibe to achieve good marks. Keep scrolling to access the NCERT solutions for Class 9 Science chapter 3.

NCERT Solutions for Class 9 Science Chapter 3: Important Topics

Atoms and Molecules is one of the easiest concepts to understand. Students are advised to go through the concepts first so that they can solve the questions easily. Solving as many questions as possible is advisable to develop speed and accuracy in problem-solving. The more the questions are practised, the easier it becomes to answer the questions in the final exam.

Embibe offers solutions to all the questions in the Class 9 Science chapter 1 textbook, considering the latest syllabus. Students can solve the questions at Embibe for free. Along with the free NCERT solutions, students can also access NCERT 3D Videos, NCERT Exemplars, Embibe Explainers, etc., for free at Embibe.

Students can go through the list of topics of Class 9 Science chapter 1 as mentioned below:

SectionTopic Name
1Laws of Chemical Combination
5Writing Chemical Formulae
6Molecular Mass and Mole Concept

NCERT Solutions for Class 9 Science Chapter 3: Points to Remember

We have provided some of the important points to remember for NCERT Class 9 Science chapter 3 as mentioned below:

  • Laws of chemical combination:
    (i)The combination between the elements are governed by laws of chemical combination.
    (ii) The law of conservation of mass was given by Lavoisier.
    (iii) The law of constant proportions was given by Louis Proust.
    (iv) The law of constant proportions is applicable only to pure chemical compounds.
  • Atom:
    (i) John Dalton was the first to state that atom is the smallest indivisible part of matter.
    (ii) Atom is the smallest tiny particle of matter which can take part in chemical combination. It may or may not exist independently.
    (iii) Dalton’s Atomic theory has certain limitations or defects.
    (iv) Scanning tunneling microscope (STM) can be used to have a pictorial view of the atoms.
    (v) Symbols of elements give the shorthand representation of the name of elements.
    (vi) Chemical symbols for the elements were given by Berzelius.
    (vii) No two elements can have the same symbol. Isotopes of an element have same symbol.
    (viii) Atomic masses of the elements are relative masses and not their actual masses.
  • Molecule:
    (i) In a molecule, the atoms of same or different elements are chemically bonded to each other.
    (ii)The number of atoms present in one molecule of element represent its atomicity.
    (iii) Noble gases are monoatomic.
    (iv)An ion is an atom or group of atoms having positive or negative charge.
    (v)A cation has positive charge whereas an anion has negative charge
    (vi)Number of electrons and protons in a neutral atom is equal. Cations as well as anions of an atom do not have equal number of electrons and protons.
  • Writing chemical formula:
    (i) Chemical formula is the representation of a molecule in terms of the symbols of the constituting atoms.
    ii) Valency of an element is the combining capacity of its atoms

NCERT Solutions for Class 9 Science: All Chapters

FAQs on NCERT Solutions for Class 9 Science Chapter 3

Q1: Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3g of hydrogen gas? 

Ans: We know hydrogen and water mix ratio 1: 8.
For every 1g of hydrogen, it is 8g of oxygen. Therefore, for 3g of hydrogen, the quantity of oxygen = 3 x 8 = 24g. Hence, 24g of oxygen would be required for the complete reaction with 3g of hydrogen gas. 

Q2: Which postulate of Dalton’s atomic theory results from the law of conservation of mass? 

Ans: The postulate of Dalton’s Atomic theory, which is a result of the law of conservation of mass, is “Atoms can neither be created nor destroyed”.

Q3Which postulate of Dalton’s atomic theory can explain the law of definite proportions? 

Ans: The postulate of Dalton’s atomic theory that can explain the law of definite proportions is – the relative number and kinds of atoms are equal in given compounds.

Q4: Write down the formulae of (i) sodium oxide, (ii) aluminium chloride, (iii) sodium sulphide and (iv) magnesium hydroxide?

Ans: The following are the formulae: (i) sodium oxide – Na2O, (ii) aluminium chloride – AlCl3, (iii) sodium sulphide – Na2S and (iv) magnesium hydroxide – Mg (OH)2.

Q5: How many atoms are present in a (i) H2S molecule and (ii) PO43- ion? 

Ans: The number of atoms present is as follows: (i) H2S molecule has 2 atoms of hydrogen and 1 atom of sulphur, hence 3 in totality and (ii) PO43- ion has 1 atom of phosphorus and 4 atoms of oxygen hence 5 atoms in totality. 

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