Important Questions for CBSE Class 9 Science Atoms and Molecules

CBSE Class 9 Science Chapter 3 Question Answer – Atoms and Molecules

Science is an integral part of our life. It can be an easy or difficult subject to understand if students have no idea how to approach it. Students face problems in understanding the concepts and theories if they lack an understanding of fundamental concepts and theories studied in previous classes. Therefore, students will have to try to understand the underlying concepts of Biology, Chemistry, and Physics taught earlier.  To make these subjects easier for students, Extramarks subject matter experts have compiled a complete list of important questions for Class 9 Science Chapter 3. There are more than 30 questions, including multiple-choice, long, and short answer types. It will also help students learn various complicated topics, including the molar mass and valency of the atoms. The subject matter experts at Extramarks understand the importance of solving questions and practice exercises to step up students’ preparation and help them excel in their academics. These sets of important questions definitely help in clarifying their doubts early on and prepare a strong foundation.

At Extramarks, we highlight important concepts and questions from each chapter which help students with their studies right before their examinations. They need not look elsewhere for any reference material to supplement their studies. Extramarks provides a one-stop solution to all your problems.The answer to Class 9 Science Chapter 3 important questions is written by experienced subject experts while meticulously following the latest CBSE guidelines.    Students can solve important questions in Class 9 Science Chapter 3 by learning and practising step-by-step solutions to numerical problems. Therefore, they don’t make those minor slips or skip any step while answering those tricky questions. . Remember the more you practice, the easier it will get.

CBSE Class 9 Science Chapter 3 Question Answer   each question is taken from f the NCERT textbook, NCERT exemplar, and other reference books. It has questions from past years’ exam papers. Further, students can refer to important questions in Class 9 Science Chapter 3.  Moreover, students can register on Extramarks website and access our complete set of Chapter 3 Class 9 Science important questions to maximise their output  without having to look elsewhere.

 It goes without saying that students need to prepare from multiple sources to excel in academics. Hence, they can also refer to other course material on our website, including NCERT solutions, NCERT books, and past year question papers and stay ahead of the pack. 

Get Access to CBSE Class 9 Science Important Questions 2022-23 with Solutions

Also, get access to CBSE Class 9 Science Important Questions for other chapters too:

CBSE Class 9 Science Important Questions

Sr No Chapters Chapter Name
1 Chapter 1 Matter in Our Surroundings
2 Chapter 2 Is Matter Around Us Pure
3 Chapter 3 Atoms and Molecules
4 Chapter 4 Structure of Atom
5 Chapter 5 The Fundamental Unit of Life
6 Chapter 6 Tissues
7 Chapter 7 Diversity in Living Organisms
8 Chapter 8 Motion
9 Chapter 9 Force and Laws of Motion
10 Chapter 10 Gravitation
11 Chapter 11 Work and Energy
12 Chapter 12 Sound
13 Chapter 13 Why Do We Fall ill
14 Chapter 14 Natural Resources
15 Chapter 15 Improvement in Food Resources

Atoms and Molecules Class 9 Extra Questions  for Science Chapter 3 with Solutions 

Long Answer Questions 

Q1. What is radioactivity? What are radioisotopes used for?


Radioactivity is the process in which spontaneous emission of radiation in the form of particles or high-energy photons results from a nucleus decay. Radioactivity causes the release of energy from the decay of the nucleus atoms or isotopes. Radioisotopes are used in the following applications:

  • Co 60 is an isotope that emits radiation that can be used to treat cancer. 
  • I 131 – used for the diagnosis and treatment of thyroid gland diseases.
  • P 32 – used for the diagnosis and treatment of leukaemia.
  • C 14 – This is used for studying biochemical processes

Q2. Create an experiment to prove that cathode rays travel in a straight line.


A fluorescent-coated discharge tube, a source of cathode radiation, an opaque object and a high voltage source can all be used to demonstrate that cathode beams travel in a straight line. The experiment’s setup:

  • A fluorescent substance-coated discharge tube can initiate the production of cathode radiations by using a high voltage source.
  • Place an opaque object in the cathode radiation path to observe fluorescence phenomena.
  • Fluorescence is produced when cathode radiation strikes the screen. Because of the position of the opaque object on the screen, we will see a sharp shadow that is shaped by the object.
  • The shadow of an object can be created if the cathode radiations travel in a straight line and not around its edges.
  • This experiment shows that cathode radiation travels in a straight line.

Q3. What were the limitations of Dalton’s Atomic Theory?

Answer: (i) It doesn’t account for subatomic particles. It said that atoms were a unit of matter. This postulate was disproved by discovering subatomic particles, namely protons, electrons and neutrons.

(ii) It doesn’t account for isotopes. For example, hydrogen 1H1, deuterium 2H1 and tritium h3H1 all have the same atomic numbers but different mass numbers.

(iii). It doesn’t account for isobars. Example: (40 Ar18) vs (40 Ca 20) – they have different atomic numbers but the same mass.

(iv) Elements do not have to combine in simple whole-number combinations to make compounds. Complex organic compounds are composed of many constituent atoms and cannot be combined in simple proportions. Example: sugar/sucrose ( C H O 11 22 11 ).

(v) It doesn’t account for allotropes. Dalton’s atomic theory can’t explain the differences in properties of graphite and diamond, even though they contain carbon.

Q4. What do you mean by atomicity? What is the atomicity of ozone?

Answer: The atomicity of a molecule is defined as the number of atoms constituting that molecule. 

Q5. What does the term chemical formula mean?


The chemical formula is the symbol for a compound’s composition. It can also be a notation that indicates the type and number of atoms within a compound’s molecule using atomic symbols or numbers. These numbers provide information about the elements that make up the molecules of a compound, as well as the ratio of the elements to create the molecules.

Example: A molecule containing water, a chemical compound, has two molecules of hydrogen and one molecule of oxygen. Its chemical formula is H O2.

Q6. Write the chemical formulae of the following. 

(a) Magnesium chloride 

Ans: Magnesium chloride – MgCl2

(b) Calcium oxide 

Ans: Calcium oxide – CaO 

(c) Copper Nitrate 

Ans: Copper Nitrate – CuNO3

(d) Aluminium chloride 

Ans: Aluminium Chloride – AlCl3 

(e) Calcium carbonate 

Ans: Calcium carbonate – CaCO3

Q7. What is Avogadro Constant? Write its value. 

Answer: Avogadro’s numbers tell us the number of particles that make up one mole (or mole) of an element. They could be molecules, electrons, or atoms. The Avogadro number is around 6.022140857×1023 mol−1. With the help of this formula, we measure the total volume of a substance, the mass or temperature of a substance and general things along those lines. The substances are measured at the atomic level per atomic mass unit. 

Besides, the mass unit is defined as the 1/12th weight of the mass of one carbon atom. For example, the atomic mass unit of hydrogen is 1.00794 amu. Now to calculate the ability of a single particle to say, carry out a reaction isn’t possible. Therefore, experts came up with the atomic mass unit and the gram.

1 amu = 1.66 x 10 grams 

Q8. Write a short note on the following:

(a) Law of conservation of mass

Answer: A law on conservation of mass says that the mass in closed systems remains constant throughout time.

Mass cannot be created, nor can it get destroyed; however, it can be transformed from one form to another. Based on the conservation laws of mass, reactants must be the same as their mass in the product to have a low-energy thermal process.

It is believed that few theories of classical mechanics determine mass conservation. Later, the conservation of mass law was modified with quantum mechanics and special relativity, which states that mass and energy are a single conserved quantity. Wood burning is a way to conserve mass since burning wood requires oxygen, carbon dioxide, water vapour and ashes.

(b) Law of constant proportion:


A constant proportion law says that in all chemical compounds, the elements will always be present in a constant and specific proportion in mass. This means that the same chemical composition will contain the same elements in the exact proportion, regardless of the source, the compound, the method of making the compound, or the quantity of the substance.

A law called constant composition says that chemical compounds comprise elements found in a particular proportion to their mass. This means that a pure substance will always have the same elements and proportion in the mass regardless of origin.

Q9. State the postulates of Dalton’s Theory.


Dalton’s atomic theory states every particle, be it an element, compound, or a mix, is made up of atoms, which are tiny particles. The tenets of the theory include:

  • Everything is composed of tiny particles known as atoms. They are involved in chemical reactions.
  • Atoms are unbreakable particles that cannot be made or destroyed by the course of a chemical reaction.
  • The elements that make up an element have the same mass and chemical characteristics.
  • Atoms from different elements are different in mass and physical properties.
  • Atoms mix in the form of small whole numbers to make compounds.
  • The proportions and types of atoms remain constant within the same compound.

Short Answer Questions 

Q10. What exactly is the chemical formula?


The chemical formulas used in the process explain the various types of atoms and their number within a compound or an element. The atoms of an element are represented as a combination of two letters or sometimes as just one letter. A set of chemical symbols that represent the elements that comprise compounds and their proportions. For instance, the chemical formula of the acid hydrochloric is HCl.

Q11. What are polyatomic ions? Give examples.


A polyatomic ion develops when a neutral molecule acquires or loses electrons in the same manner as ions form when neutral atoms lose or gain electrons. In the end, a polyatomic ion is a collection of covalently bonded atoms with net charges because the number of electrons within the molecule doesn’t equal the number of protons. 

The total of all the charges of every atom in the Lewis dot structure of a polyatomic ion should be equal to the net charge of the Ion. Polyatomic ions have more than one atom but function in a singular unit. Example CO32- H2PO4

Q12. How to calculate the molecular masses of:

H2, O2, Cl2, CO2, CH4, C2H6, NH3, CH3OH

Answer: Molecular mass of H2: 2 x Atomic mass of H = 2 x 1u = 2u 

Molecular mass of O2: 2 x Atomic mass of O = 2 x 16u = 32u

Molecular mass of Cl2: 2 x Atomic mass of CL = 2 x 35.5 u = 71u

Molecular mass of CO2: Atomic mass of C + 2 x Atomic mass of O = 12 + 2 x 16 = 44 u 

Molecular mass of CH4: Atomic mass of C + 4 x Atomic mass of H = 12 + 4 x 1 = 16 u

Molecular mass of C2H6: 2 x Atomic mass of C + 6 x Atomic mass of H = 2 x 12 + 6 x 1 = 30 u

Molecular mass of NH3: Atomic mass of N + 3 x Atomic mass of H = 14 + 3 x 1 u = 17 u 

Molecular mass of CH3OH = Atomic mass of C + 4 x Atomic mass of H + Atomic mass of O = 12 + 4 x 1 + 16 = 32 u 

Q13. The oxygen and hydrogen mix in a ratio of 1:18 in mass to create water. What is the volume of oxygen gas needed to react with 3 grams of hydrogen gas completely?


We know that water and hydrogen are mixed in the proportion of 1:8. For each 1 gram of hydrogen, it’s 8 grams of oxygen. 

Thus, for 3 grams of hydrogen, the amount of oxygen is 3 x 8 = 24 grams. 

Hence, 24 grams of oxygen would be needed for the complete reaction with 3 grams of hydrogen gas.

Q14. In a reaction, 5.3 grams of sodium carbonate was reacted with 6.0 grams of ethanoic acid. The products included 2.2 grams of carbon dioxide, 0.9 grams of water, and 8.2 grams of sodium ethanoate. The results show that these measurements are in line with the conservation law.


The chemical reaction that produces results is sodium carbonate + the acid ethanoic. The reaction is sodium ethanoate plus carbon dioxide plus water.

The mass of the reactants is (5.3 + 6.0) = 1.3 grams

Mass of the product is (8.2 + 2.2 + 0.9) = 11.3 g.

The products and reactants are of identical mass. This implies that it was impossible to lose mass in the reaction. Thus, the results are in line with the conservation law.

Q15. Is it impossible to see an atom with your naked eyes?

Answer: It is impossible to observe an atom through the naked eye due to its size. For instance, it is estimated that the radius composed of hydrogen is on the order of 10-10 millimetres. In reality, an atom can be described as a microscopic part. They are microscopic and cannot be seen by the naked eye.

Q16. If 3.0 grams of carbon are burned in 8.0 grams of oxygen, 11.0 grams of carbon dioxide are produced. What mass of carbon dioxide will be created when 3.0 grams of carbon are burned in 50.0 grams of oxygen? What law of chemical mixture will be the determining factor?


Oxygen and carbon react to create carbon dioxide in accordance with this equation: CO2 = Carbon (C) + oxygen (O 2) > Carbon dioxide (CO 2)

In the first case, the 3.0 gram of carbon is burnt in 8.0 grams of oxygen to form 11.0 grams of CO2.

In the second case: 3.0 grams of carbon must also combine such that 8.0 grams of oxygen blend with each other. This means that (50-8) = 42 grams of oxygen will remain unreacted. Here, the mass of CO2, in this case, will become 11 grams. This answer is based on the law of constant proportions. In another case, only 8.0 grams of oxygen react, although 50.0 grams are available. It shows that the mass of carbon dioxide formed depends upon the mass of carbon. 

The substance present in smaller amounts in a reaction limits the participation of the other reactants. It is known as the limiting reactant. Carbon is the limiting reactant in this case. 

Q17. Give the names of the respective elements present in the following compounds: 

(a) Quick lime

(b) Hydrogen bromide

(c) Baking powder 

(d) Potassium sulphate 


The names of the elements present are only available when the compound’s chemical formula is already known. For example, 

(a) Quick lime: It is the trade name for the chemical. The chemical name of the compound is calcium oxide.

Chemical formula: CaO.

Elements present: calcium (Ca) : oxygen (O).

(b) Hydrogen bromide, the chemical formula for the compound, is HBr.

There are elements in the form of hydrogen (H) and bromine (Br).

(c) Baking powder: It is used to commercialise the chemical. The chemical name of the compound is sodium carbonate, and its chemical formula is NaHCO3.

Elements present sodium (Na), hydrogen (H), carbon (C), and oxygen (O).

(d) Potassium sulphate: The chemical formula of this substance is 2SO4

Elements present: potassium (K), sulphur (S), and oxygen (O).

Q18. Calculate the molar mass of the respective substances: 

  1. a) Ethyne
  2. b) Sulphur molecular 
  3. c) Phosphorus molecule
  4. d) Hydrochloric acid 
  5. e) Nitric acid 


  1. a) Molar mass of ethyne = 2 x 12 + 2 x 1 = 28 gram
  2. b) Molar mass of sulphur molecules = 8 x 32 = 256 gram
  3. c) Molar mass of phosphorus molecules = 4 x 31 = 124 gram
  4. d) Molar mass of hydrochloric acid = 1 + 35.5 = 36.5 gram
  5. e) Molar mass of nitric acid = 1 + 14 + 3 x 16 = 63 gram 

Q19. Calculate the formula unit masses of ZnO, Na2O, and K2CO3, given the atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C= 12 u, and O = 16 u.


Given that the atomic mass of Zn = 65 u,

The atomic mass of Na = 23 u

The atomic mass of K = 39 u 

The atomic mass of C = 12 u

The atomic mass of O = 16 u 

Now, formula unit mass of ZnO = Atomic mass of Zinc + Atomic mass of O = 65 u + 16 u = 81 u 

Formulas unit mass of Na20 = 2 x Atomic mass of Na + Atomic mass of O = 2 x 23 u + 16 u = 62 u 

Formula unit mass of K2CO3 = 2 x Atomic mass of K + Atomic mass of C + 3 x atomic mass of O = 2 x 39 u + 12 u + 3 x 16 u = 138 u.

Q20. Which is the mass of: 

(a) 0.2 mole oxygen molecules? 

(b) 0.5 mole of water molecules? 


The mass is as follows: 

(a) Mass of 1 mole of oxygen atoms = 16u so it weighs 16g. From 0.2 moles oxygen molecules = 0.2  x 16 = 3.2g 

(b) A mole of water molecules mass is 18u, weighing 18g. 0.5 moles of water molecules equals 0.5 + 18 = 9g

Multiple Choice Questions 

Q21. Do the following observations apply to an atom? 

(a)Atoms cannot exist on their own.

(b)Atoms are the primary unit that is the basis of molecules and ions. 

(c)Atoms are neutral.

(d) Atoms are gathered in huge numbers to create the material we can feel, touch, or see.


(d) This observation is not valid.


The correct statement is that the molecules and ions assemble in large numbers to create the substance. It is impossible to see individual molecules/ions through our eyes. However, we can see the many substances which form an extensive collection of molecules and ions. This means that alternative (d) is not correct.

The atoms of most elements are very reactive and are not present in the state of freedom. The atoms in noble gases are chemically non-reactive and are present in the free state. Atoms are usually found in two types.

(i) molecular and (ii) Ions

So atoms are the primary element from which molecules and ions form.

When atoms create the form of molecules or even ions, they are stable due to their stable arrangement for electrons in the noble gas. Thus, they are naturally neutral.

Q22. A change in the system’s physical state can come about

(a) only if energy is supplied to the system.

(b) only the moment that energy is removed from the system.

(c) the energy source is given to or withdrawn from the system.

(d) without energy changes


(c) the energy source is given to or withdrawn from the system.


Changes in physical properties can be triggered by the transfer of energy to or removed from the system. It’s because energy changes can alter the force of attraction between particles, which assists change the physical state (i.e. gas, liquid, solid).

Q23. Is the following symbol for elements wrong? Provide the correct symbols.

(a) Cobalt CO 

(b) Carbon c

(c) Aluminium AL 

(d) Helium He

(e) Sodium So


(a) Cobalt CO is an incorrect symbol. The correct symbol should be Co.

(b) Carbon c is not the correct symbol. Its sound symbolism is C.

(c) Aluminium AL is an incorrect symbol. The correct symbol should be Al.

(e) So is an incorrect symbol. The appropriate symbolism is Na. (It comes from the Latin name “Natrium”).

(d)  ‘He’ is the correct symbol of Helium.

Q24. Which of the following correctly represents 360 grams of water?

  1. i) 2 moles of H2O
  2. ii) 20 moles of water 

iii) 6.022 x 10 (23) molecules of water 

  1. iv) 1.2044 x 10 (25) molecules of water 


  1. a) (i)
  2. b) (i) and (iv)
  3. c) (ii) and (iii)
  4. d) (ii) and (iv) 

Answer: Option d 

Explanation: The two moles of water molecule H2O is 36 grams. Thus, the 20 moles of water molecules (H20) will be 360 grams. 

If the mass of 6.022 x 10 molecules of water molecules is 18 grams, then the mass of 1.2044 x 10 is 20 x 18 grams = 360 grams. Therefore, the options representing 360 grams of water are option (ii) and option (iv). 

Therefore, the options representing 360 grams of water are option (ii) and option (iv). So, the answer is an option (d). 

Q25. It is possible to change the physical state of your body:

(a) Only when energy has been given to the system.

(b) When energy is removed from the system.

(c) Energy is given or taken from the system.

(d) Without any energy changes

Answer: The correct option is  (c) energy is given or taken from the system.


Energy can be given or taken out of a system to change its physical state. A solid becomes a liquid and absorbs energy. A liquid becomes a solid and releases energy. The correct answer is option c.

Q26. Classify each of the following based on their atomicity:

  1. a) F2

Answer: The atomicity of F2 is two atoms. 

  1. b) NO2

Answer: The atomicity of NO2 is three atoms. 

  1. c) N2O

Answer: The atomicity of N2O is three atoms. 

  1. d) C2H6

Answer: The atomicity of C2H6 is eight atoms. 

  1. e) P4 

Answer: The atomicity of P4 is four atoms. 

Q27. A fine white powder is provided, which can be either sugar or salt. It is difficult to identify without tasting it. Which two experiments can be conducted to determine if the finely coloured powder is sugar or salt?


  1. Heating: 

Sugar melts into liquid when heated. This is because sucrose has a melting point and a decomposition temperature between 190 and 192 degrees Celsius. This will make sugar light brown. Further heat will cause sugar to turn black. Salt has a melting temperature of 841 degrees Celsius and 1545.8 degrees Fahrenheit. It will not change if it isn’t heated to that temperature.

  1. Electric conductivity: 

We can test for electric conductivity by soaking the substance in water. This will allow us to determine if it is sugar or salt. It conducts electricity if it is salt. Salt (NaCl), which has both positive sodium ions and harmful chloride, conducts electricity. However, sugar doesn’t conduct electricity because it only has positive ions.


(a) Name an element used to determine the atomic mass scale?

(b) What atom of this element is used to accomplish this purpose?

(c) What is the value of this reference atom’s mass unit?


(a) Carbon is used in the atomic mass scale as a standard.

(b) The atomic mass scale is handy for the carbon-12 atom.

(c) Carbon-12 has been given precisely 12 units of mass 

Q29. Answer the following questions: 

(a) Two symbols that have been derived using the English names of the elements.

(b) Two symbols that have been derived using the Latin names of the elements.


(a) Two example symbols that have been derived using the “English names of their respective elements”: Sulphur – S and Aluminium – Al.

(b) The symbols derived from Latin names of the elements are Iron-Ferrum (Fe)-Cuprum.

Q30. Answer the following questions: 

(a) What form of oxygen gas can be found in nature?

(b) What form of noble gases can you find in nature?


(a) Oxygen naturally as diatomic molecules. To create a molecule of O2, two oxygen atoms are chemically mixed.

(b) Noble gases are found in the earth’s atmosphere. They are monatomic gaseous.

Key Topics Covered in Class 9 Science Chapter 3

Students must recall the wide range of identities they have read so far. In addition, the chapter requires proper comprehension of the Science concepts. 

Law of Chemical Combination

Chemistry is the study and application of chemical reactions to change matter from one form of matter to another. These chemical changes are often caused by combining two types of matter. Specific rules govern the formation of different compounds from different elements. These rules are what we refer to as the laws of chemical combination. Atoms and molecules are crucial for forming tiny sand particles and substantial black holes. An atom is a unit of matter that makes up all of the things we see. It is tiny, measuring less than 0.1-0.5 nanometers.

Chemical Reactions

  • Reactants are compounds that occur when two or more molecules react in a chemical reaction to create new compounds.
  • A chemical change must take place in a chemical reaction. This is usually observed with physical changes such as precipitation, heat generation, and colour change.

Conservation of mass law:

The law of conservation states that matter cannot be created or destroyed through chemical reactions. It is protected. The mass of reactants will equal that of the products. This law, in simple terms, states that matter cannot be created or destroyed. The total mass, the sum of the reacting mixture and products, remains constant. Antoine Lavoisier created this law in 1789, based upon data he gathered after studying many combustion reactions.

Law of constant proportions

The law of constant proportion states that a pure chemical compound contains all elements in the same amount. Water from an ocean or river will contain the same amount of hydrogen and oxygen. Besides, Joseph Proust, a French Chemist, stated that the weight of an element in a compound will always be the same. Simply put, it is possible to say that regardless of the compound’s source, origin, or quantity, its percent composition in elements by weight will always be the same.

What is an Atom:

The formation of small particles is a function of atoms. These are the basic atoms as described:

An atom is the most basic structure of an element. It cannot be changed by chemical means.

  • The three-part atomic symbol for:
  • The symbol X represents an element.
  • The number of protons is equal to A, the atomic number.
  • An element’s number of protons and neutrons is called the mass number Z.

The atom is the most fundamental and building block of matter. Anything with a mass, or anything that occupies space, is made up of atoms. Although its original name meant a particle that could not be divided, the minor possible thing, we now know that every atom is made up of smaller particles. These particles are sometimes called subatomic particles because they make up atoms. There are three subatomic particles, protons, neutrons, and electrons.

Dalton’s Atomic Theory

These are the critical elements of Dalton’s Atomic Theory as follows: 

  • Every property of an element’s atoms is identical, down to their mass. This holds even if all atoms of an element have the same mass and chemical properties. Atoms from different elements will have different masses or chemical properties.
  • You can make compounds by combining atoms of different elements in fixed proportions.
  • It is impossible to create or destroy atoms. In chemical reactions, atoms can’t be rearranged to create new products.
  • The relative numbers and types of atoms in a compound are constants.

The Postulates of Dalton’s Atomic Theory

  • All matter is composed of tiny, invisible particles called Atoms.
  • Each atom of an element is identical in mass, size, as well as other properties. However, different elements have different properties. They can exhibit different properties and differ in their mass and sizes.
  • Atoms cannot be destroyed or created. Atoms can’t be broken down into smaller particles.
  • Compounds can be formed when atoms from different elements are combined in whole fixed numbers.
  • Chemical reactions can be used to reorganise, combine, or separate atoms.

Limitations of Dalton’s Atomic Theory

  • This theory does not account for subatomic particles. Dalton’s Atomic Theory stated that atoms are indivisible. This postulate was disproved by the discovery of subatomic particles (such as protons, electrons, and neutrons).
  • It doesn’t account for isotopes. Dalton’s Atomic Theory states that all elements have the same masses and densities. As shown by hydrogen, tritium and deuterium, different elements can have different atomic masses.
  • This theory does not account for the isobars. This theory states that two elements’ masses must be different. Two elements can have the same mass number. These atoms are known as isobars (Examples: 40Ar, and 40Ca).
  • Elements do not have to be combined in whole-number combinations to make compounds. Certain complex organic compounds are not made up of simple proportions of constituent atoms. Example: sugar/sucrose (C11H22O11).
  • The theory doesn’t account for allotropes. Dalton’s Atomic Theory cannot explain the differences in properties of graphite and diamond, which both contain carbon.

Atomic Mass

Atomic mass and atomic unit

These are the critical features of atomic mass:

  • The atomic mass is the sum of all electrons, neutrons, and protons in an atom or group.
  • This is often expressed as an international agreement that uses the unified atomic weight unit (AMU).
  • It is 1/12th the mass of the carbon-12-atom in its ground state.

Molecular mass

  • The molecular mass is the sum of all elements within a molecule.
  • The atomic mass of an element is multiplied by the number of atoms within a molecule. This then gives rise to the mass of all elements within the molecule.
  • All matter is composed of atoms.
  • Elements are substances that only contain one type of atom.
  • The central nucleus of an atom contains protons as well as neutrons.
  • The nucleus is in motion, and electrons orbit around it.
  • Protons have a relative mass of 1 and a positive charge.
  • Neutrons are neutral and have a relative weight of 1.
  • Negligible mass and a positive charge are the two main characteristics of electrons.

Mole Concept

The mole concept can be used to express the amount of a substance. Any measurement can be broken down into the numerical magnitude and the units in which it is expressed. If a ball’s mass is 2 kilograms, the magnitude is “2”, and the unit is “kilogram”.

It is well-known that even one gram (or one molecular gram) of pure elements contains many atoms. Therefore the mole concept is so popular. This concept focuses primarily on the unit called a mole, which is a count that includes many particles.

Some of the key points: 

  • A mole is a substance’s total number of entities, such as atoms, molecules, or ions. A mole, in other words, is a mixture of atoms and molecules. A mole is any substance with a dimension of 6.22×10 23 moles.
  • One way to express reactants and products is with the mole concept.
  • Avogadro’s number is approximately 6.022×10. The Avogadro number is the number of particles in a substance’s Mol (or mole). These particles can be electrons, molecules or atoms.

Molar mass:

A substance is something that has mass and occupies space. The sum of all the atoms in a mole or molecule is called the molar mass/molecular weight. Grams/moles are the units of molar mass. Molar mass refers to the smallest unit of a compound with a one-twelfth mass of one carbon. This is 12 atoms. 

The concept of Molar Mass can be used to determine how many grams of substance are required once we have determined the number of moles needed. The molecular mass, also known as molar mass, is the sum in grams of all the moles that make up a molecule. Grams per mole is the unit used to measure.

Atomic Valency

Atomicity and Molecules: A molecule refers to the smallest unit of a compound that has chemical properties. The number of elements within a single molecule is called an element’s atomicity. For example, hydrogen, oxygen, chlorine, and iodine have two atoms in their molecules. The atomicity of hydrogen, oxygen, bromine, chlorine, and oxygen is also two.

Structure of an Atom:

  • An Atom is composed of three parts: an electron, proton, and neutron.
  • The nucleus is an atom’s centre. The nucleus contains the entire mass of an atom.
  • To arrange electrons within an atom, the orbitals and shells are used.

Valency: These electrons can be found in the outermost orbits of an atom. They are known as valence electrons. An atom’s ability to acquire, lose, or share valence electrons to complete its octet determines its valency.

Writing Chemical Formulae


  • A compound is a chemical reaction that combines two or more elements into a fixed mass ratio.
  • A compound is a substance which contains more than one type of element in a fixed percentage of its atoms.


  • An ion is an atom, molecule or other substance that has lost one or more of its valence electrons. This gives it either a net positive or negative charge.
  • An anion is a negatively charged particle, while a cation can be positively charged.

The chemical formula for ionic compounds:

  • Ionic compounds are chemical compounds which contain ions and are held together by special bonds known as ionic bonds.
  • An Ionic compound contains equal amounts of positive and negative charges.
  • Example: Calcium chloride is one example of an ionic link. It is formed from the oppositely charged chloride ions and calcium ions.

Benefits of Solving Important Questions Class 9 Science Chapter 3

Students may face difficulty comprehending the concepts while learning new topics and recalling previous related topics in Science. Besides, one way to approach this is to practise important questions in Class 9 Science Chapter 3. These questions help students in learning the essential concepts of the laws. Therefore, there are various advantages to solving these important questions of Class 9 Science Chapter 3- Atoms and Molecules. Students will get an overall idea about the types of questions that may appear in the examination. 

 The importance of studying through these important questions is as follows: 

  • Students will get good practice with different questions, and they can prepare for the Science paper. The more you practice, the better you will get.
  • Our Extramarks subject matter experts prepare the questions exclusively for the exams following the NCERT books and the guidelines laid down by CBSE. 
  • The essential questions in Class 9 Science Chapter 3 help students understand different types of questions with different chapter weightage. It helps them to prepare in a planned, systematic and organised manner.
  • The list of essential questions is a compilation of the NCERT book, CBSE’s previous year’s question papers, and CBSE’s additional questions. Students will be well prepared and confident during the exams.
  • Students will get enough practice through these revision materials and will be able to face any question without much difficulty.  They will get detailed and authentic solutions without having to look anywhere else. Extramarks provides a one-stop solution to all your problems.
  • The solutions provide detail of the practicalities required in clearing the question paper. The students will get to know the type of questions which may appear in the exam, the difficulty level, and the duration of each question. So students can make use of these sample papers and revision notes according to their needs and preparation level.
  • Our complete list of essential questions for Class 9 Science Chapter 3 consists of objective type, short answers, and long answer questions. It can help them understand the important points easily and make them revise quickly.
  • Answers have detailed step-by-step explanations with the appropriate reasons given in Science Class 9 Chapter 3 important questions.  
  • The comprehensive guide for important questions puts together all relevant questions and  encourages students to prepare well ahead of the annual exams. 
  • Students will get an idea of the concepts and definitions they need to learn and revise from Chapter 3. 

Extramarks is one of the leading online learning platforms which offers a repository of  study materials to the students from classes 1 to 12. We also offer additional course materials,  which students can access with the links given below: 

Q.1 Assertion (A): Seeds consist of stored food.
Reason (R): Stored food is used during germination for the initial growth of an embryo.

A. Both A and R are true and R is the correct explanation of A.

B. Both A and R are true but R is not the correct explanation of A.

C. A is true but R is false.

D. A is false but the R is true.


Both A and R are true and R is the correct explanation of A.

Q.2 Observe the fruit in the given image. Name the group of plants to which it belongs.

A. Angiosperms

B. Gymnosperms

C. Pteridophytes

D. Bryophytes



Q.3 Pick the odd one out and justify your choice by giving reasons.
Coelenterates, Echinoderms, Vertebrates, Molluscs


Vertebrates is the odd one out because they have vertebral column while the rest are invertebrates and do not possess vertebral column.

Q.4 Enlist the characteristic features of gymnosperms.


The following are the characteristic features of gymnosperms:

  • They bear naked seeds.
  • They are usually perennial, evergreen and woody.

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FAQs (Frequently Asked Questions)

1. Are there any numerical problems in important questions in Class 9 Science Chapter 3?

Yes, there are a few multiple-choice questions as numerical. Students should practise the numerical problems in Chapter 3 of Class 9 Science.. For better exam results, students should practise all the numerical problems in Chapter 3 of 9 Science. They can also practise numerical questions from the NCERT books. 

2. What is an atom?

The most crucial element in the fundamental unit of matter is the atom. It is everywhere. Atoms are the most fundamental structure and cannot be broken, and they are also a part of our body. To understand the concept of atoms, students should carefully read NCERT Chapter 3. To learn more about atoms, they can refer to Extramarks’ Important Questions for CBSE Class9 Science Atoms and Molecules.

3. Which are the essential topics covered in the list of important questions in Class 9 Science Chapter 3?

Students will study the definitions of atoms and molecules in Class 9 Science Chapter 3. Students will also learn about different theories about the discovery of atoms and formation. In addition, students will learn how to create a chemical formula for any compound. Further, they will understand the laws of constant proportion and other laws relating to chemical reactions. Chapter 3 of Class 9 Science also includes essential topics such as molecule mass and mole.