Class 9 Science Chapter 3 Important Questions of Atoms and Molecules. You can use Class 9 Science Chapter 3 Extra Questions for the revision of chapter during the exams or tests. These sets of important extra questions provide a perfect preparation of exams. There are all types of questions like very short questions, short questions and long questions with answers. Answers of each question is just confined to NCERT Textbook only. UP Board and MP Board students are also using NCERT Books for their course. So, they also can use these questions for the revision and exam preparations. There are one word questions which are generally asked in one marks, short questions for 2 and 3 marks, long questions for 5 marks. All the categories of questions are included here in the sets of important extra questions.

Class 9 Science Chapter 3 Important Extra Questions

Class: 9Science
Contents:Important Questions with Answers
Chapter: 3Atoms and Molecules

9th Science Chapter 3 Extra Question Answers

Important extra questions of class 9 science chapter 3 Atoms and Molecules are given below with explanation and answers. All the questions are answered are taken from NCERT Textbook point to point according to the requirement. Prepare your school tests through these important questions and score good marks in exams.

Class 9 Science Chapter 3 Important Extra Questions Set – 1

Name two scientist who established the law of chemical combination?

Antoine L. Lavoisier and Joseph L. Proust.

Give an example of a triatomic molecule of an element.


Define atomicity.

It is the number of atoms present in one molecule of a substance.

Write the atomicity of the following molecules: (i) Sulphur (ii) Phosphorus.

(i) 8
(ii) 4

What is an ion? Give one example.

The negatively and positively charged particle or molecules are called ions.
For example: Cl⁻, Br⁻, H⁺, Pb⁺ etc.

Actual Mass and Gram Molecules Mass

Actual mass of a molecule is obtained by driving the molar mass by Avogadro’s number whereas gram molecular mass represent the molecular mass excepted in grams, i.e., it is the mass of 1 mole of molecular, i.e., Avogadro’s number of molecules.

Class 9 Science Chapter 3 Important Extra Questions Set – 2

Give one word for the following: (i) A group of atoms carrying a charge (ii) Positively charged ion.

(i) Ion
(ii) Cation

The atomic number of three elements A, B and C are 9, 10 and 13 respectively. Which of them will form a cation?

Electronic configuration of A: 2, 7
Electronic configuration of B: 2, 8
Electronic configuration of C: 2,8,3

What is wrong in saying one mole of nitrogen?

The statement does not clarify whether we are talking about atoms or molecules of nitrogen. We should say “one mole of nitrogen atoms” or “one mole of nitrogen molecules”.

“Dalton’s atomic theory is contradicted by the formula of sucrose(C12H22O11). Justify the statement.

Dalton’s atomic theory state that atoms of different elements combine together in simply whole number ratio. In the formula of C12H22O11, the carbon, hydrogen and oxygen combine in whole number ratio but the ratio is not simple.

How many times heavier is one atom of carbon than one atom of oxygen?

Atomic mass of carbon = 12u
Atomic mass of oxygen = 16u
Therefore one atom of carbon is 12u/16u = 3/4 times heavier than one atoms of oxygen.

Conservation of mass applies to physical changes

Law of conservation of mass state that mass can neither be created nor destroyed in a chemical reaction. However, this law applies to physical changes also. For example, when ice melts into water the mass of ice equals to the mass of water, i.e. the mass is conserved. This verifies the law of conservation of mass.

Class 9 Science Chapter 3 Important Extra Questions Set – 3

Which of the following symbols of elements are incorrect? Give their correct symbols. (a) Cobalt CO (b) Carbon c (c) Aluminium AL (d) Helium He (e) Sodium So

(a) Incorrect, the correct symbol of cobalt is Co.
(b) Incorrect, the correct symbol of carbon is C.
(c) Incorrect, the correct symbol of Aluminium is Al
(d) Correct (He)
(e) Incorrect the correct symbol of sodium is Na.

Which of the following are tri-atomic and tetra-atomic molecule? CH₃Cl, CaCl₂, NH₃, PCl₃, P₂O₅, H₂O, C₂H₅OH

(i) Tri-atomic molecules are CaCl₂, H₂O
(ii) Tetra-atomic molecules are NH₃, PCl₃.

Does solubility of a substance change with temperature? Explain with the help of an example.

Yes, it is a temperature depend property. The solubility generally increases with increase in temperature.
For example, you can dissolve more sugar in hot water than in cold water.

Calculate the formula mass of sodium carbonate (Na₂CO₃.10H₂O).

Formula mass of sodium carbonate
(2× atomic mass of Na) + (1 atomic mass of C) + (3 × atomic mass of O) + 10 [(2 × atomic mass of H) + (1 × atomic mass of O)]
= 2 ×23 + 1 × 12 + 3 × 16 + 10 [(2 × 1) + (1 × 16)]
= 46 + 12 + 48 + 180
= 286u

Define law of conservation of mass and law of constant proportion?

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. Law of constant proportion state that in a chemical substance the elements are always present in definite proportions by mass.

Postulates of John Dalton Atomic Structure

    • All matter is made of very tiny particles called atoms.
    • Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
    • Atoms of a given element are identical in mass and chemical properties.
    • Atoms of different elements have different masses and chemical properties.
    • Atoms combine in the ratio of small whole numbers to form compounds.
    • The relative number and kinds of atoms are constant in a given compound.

Atomic Mass

According to remarkable concept of Dalton’s atomic theory proposed was that of the atomic mass. According to him, each element had a characteristic atomic mass. The theory could explain the law of constant proportions so well that scientists were prompted to measure the atomic mass of an atom. Since determining the mass of an individual atom was a relatively difficult task, relative atomic masses were determined using the laws of chemical combinations and the compounds formed. The mass of an atoms of an element is known as Atomic mass. in 1961 for a universally accepted atomic mass unit, carbon-12 isotope was chosen as the standard reference for measuring atomic masses. One atomic mass unit is a mass unit equal to exactly one- twelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12.

Class 9 Science Chapter 3 Important Extra Questions Set – 4

How big are atoms?

Atoms are very small. They are smaller than anything. That we can imagine or compare with.

What is the IUPAC system? Give example.

In the beginning, the names of elements were derived from the name of the place where they were found for the first time. For example, the name copper was taken from Cyprus. Some names were taken from specific colours. For example, gold was taken from the English word meaning yellow. Now-a-days, IUPAC (International Union of Pure and Applied Chemistry) approves names of elements. Many of the symbols are the first one or two letters of the element’s name in English. The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase).
For example
(i) hydrogen, H
(ii) Aluminium, Al and not AL
(iii) cobalt, Co and not CO.
Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name. Examples are: (i) chlorine, Cl, (ii) zinc, Zn etc.

How relative atomic mass of an atom is fixed?

The relative atomic mass of an element is found with respect to an atom of carbon = 12.

How does atom exist?

Atoms of most elements are not able to exist independently. Atoms form molecules and ions. These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.

What is a molecule of an atom?

A molecule is in general a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces. A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance. Atoms of the same element or of different elements can join together to form molecules.

Class 9 Science Chapter 3 Important Extra Questions Set – 5

What is the molecules of an atom?

Atoms of different elements join together in definite proportions to form molecules of compounds.

What are molecules of compound?

The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element. But this is not the case with most of the non- metals. For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule, O₂. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone. The number of atoms constituting a molecule is known as its atomicity. Molecules of metals and some other elements, such as carbon, do not have a simple structure but consist of a very large and indefinite number of atoms bonded together.

What are tetratomic molecules? Give example.

A molecules which consist of atoms is known as tetratomic molecules. For example, O₃, P₃

What is atomic city give example?

The number of atoms constituting a molecule is known as atomic city. Example atomicity of oxygen is 2, nitrogen is 2 and Argon is 1.

What are polyatomic ions?

A group of atoms carrying a charge is known as polyatomic ion.
For example PO₄⁻³, CO₃⁻² , NO₃⁻¹ etc.

What are ions?

Answer: The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an “anion” and the positively charged ion, a “cation”. Take, for example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na+) and negatively charged chloride ions (Cl–). Ions may consist of a single charged atom or a group of atoms that have a net charge on them.

Avogadro Number or Avogadro Constant

The number of particle (atoms, molecules ions) present in 1 mole of any substance is fixed with a value of 6.022 × 10^23 . This no is called Avogadro number or Avogadro constant. It was given by an Italian scientist Amedio Avogadro.
Amedio Avogadro: 1 mole (of anything) = 6.022 × 10^23 in number, as, 1 dozen = 12 nos. 1 gross = 144 nos.
Besides being related to a number, a mole has one more advantage over a dozen or a gross. This advantage is that mass of 1 mole of a particular substance is also fixed. The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams. The atomic mass of an element gives us the mass of one atom of that element in atomic mass units (u). To get the mass of1 mole of atom of that element, that is, molar mass, we have to take the same numerical value but change the units from “u” to “g”. Molar mass of atoms is also known as gram atomic mass. For example, atomic mass of hydrogen=1u. So, gram atomic mass of hydrogen = 1 g. 1 u hydrogen has only 1 atom of hydrogen1 g hydrogen has 1 mole atoms, that is, 6.022 × 10^23 atoms of hydrogen. Similarly, 16 u oxygen has only 1 atom of oxygen, 16 g oxygen has 1 mole atoms, that is, 6.022 × 10^23 atoms of oxygen.

Class 9 Science Chapter 3 Important Extra Questions Set – 6

Define valency? With example.

The combining power (or capacity) of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. The valency of the atom of an element can be thought of as hands or arms of that atom. For example: The Valency of calcium is 2. The valency of chlorine is 1.

What are the rules that you have to follow while writing a chemical formula?

The rules that you have to follow while writing a chemical formula are as follows:
(i) The valences or charges on the ion must balance.
(ii) When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.
(iii) In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

What is molecular mass? Give example.

The sum of the atomic mass of all the atoms in a molecule of the substance is called molecular mass. It is expressed in atomic mass unit. For example, the molecular mass of H₂O = 18.

What is formula unit mass?

The sum of the atomic masses of all atoms in a formula unit of a compound is called formula unit mass. It is used for those substance whose constituent particles are ions. Example Formula unit mass of NaCl = 58.5.

Define one mole? Give example.

One mole of any species (atoms, ions, molecules) is that quantity in number having a mass equal to its atomic or molecular mass of atoms. Take an example of the reaction of hydrogen and oxygen to form water: 2H₂+ O₂ → 2H₂O.
The above reaction indicates that
(i) two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water, or
(ii) 4 u of hydrogen molecules combine with 32 u of oxygen molecules to form36 u of water molecules.
We can infer from the above equation that the quantity of a substance can be characterised by its mass or the number of molecules. But, a chemical reaction equation indicates directly the number of atoms or molecules taking part in the reaction. Therefore, it is more convenient to refer to the quantity of a substance in terms of the number of its molecules or atoms, rather than their masses.

Class 9 Science Chapter 3 Important Extra Questions Set – 7

How many (a) molecules (b) hydrogen atom (c) oxygen atoms are there in 0.5 mole of water?

(a) 1 mole of water contains 6.022 × 10²³ molecules.
0.5 mole of water contains (6.022 × 10²³)/(2 ) molecules
= 3.011 × 10²³ molecules
(b) 1 molecules of water contain 2 atoms of hydrogen
1 mole of water contains 2 × 6.022 × 10²³ atoms of hydrogen.
0.5 mole of water contains (6.022 × 10²³)/(2 ) atoms of hydrogen.
= 6.022 × 10²³ atoms of hydrogen.
(c) 1 molecule of water contains 1 atom of oxygen
1 mole of water contains 6.022 × 10²³ atoms of hydrogen.
0.5 mole of water contains (6.022 × 10²³)/(2 ) atoms of hydrogen.
= 3.011 × 10²³ atoms of molecules.

Calculate the number of moles present in: (i) 3.011 ×10^23 number of oxygen atoms. (ii) 60g of calcium [Given the atomic mass of Ca = 40u Avogadro No. = = 3.011 × 10²³ ]

(i) 1 mole of oxygen contains 6.022 × 10²³ atoms
6.022 × 10²³ atoms of oxygen = 1 mole
1 atom of oxygen = 1/(6.022 × 10²³ mol
3.011 × 10²³ atoms of oxygen
= (1 × 3.011 × 10²³/(6.022 × 10²³
= 0.5 mole
(ii) Atomic mass of Ca = 40u
40g of calcium = 1 mole
60g of calcium = 60/40 mol = 1.5mol

Calculate the mass per cent of each element of sodium chloride in one mole of it.

Molecular mass of NaCl = ( 1 ×23+1 ×35.5 ) u = 58.5 u
Atomic mass of sodium = 23 u
Mass per cent of Na = (Atomic mass of Na)/(Molecular mass of NaCl)×100
= 23/58.5× 100
= 39.32%

Mass % of Na = 39.32%
Atomic mass of chlorine = 35.5u
Mass % of Cl
= (Atomic mass of Na)/(Molecular mass of NaCl)×100
= (35.5 )/58.5× 100
= 60.68%

Calculate the number of particles in each of the following: (a) 46g of Na atom (b) 8g of O₂ molecules (c) 0.1 moles of carbon atom.

(a) No. of moles of sodium = 46/23 = 2 moles
We know that one mole of sodium contains 6.022 × 10²³ atoms.
2 moles of sodium contain = 2 ×6.022 × 10²³ atoms.
= 1.204 × 10²⁴ atoms
(b) 1 mole of oxygen = 32g
32 g of O₂ contain 6.022 × 10²³ molecules
Therefore 8g of O2 contains = (6.022 × 10²³)/32 × 8 molecules
= 1.51 × 10²³ molecules
(c) 1 mole of carbon atoms contains 6.022 × 10²³ atoms.
0.1 mole of carbon atoms contains = 6.022 × 10²³ × 0.1 atoms
= 6.022 × 10²² atoms

Raunak took 5 moles of carbon atoms in a container and krish also took 5 moles of sodium atoms another container of same weight. (a) Whose container is heavier. (b) Whose container has more number of atoms?

(a) Mass of sodium atoms carried by Krish = (5×23)g = 115g
Mass of carbon atoms carried by the Raunak = (5 ×12) g = 60g
Thus Krish container is heavier.
(b) Both the bags have same number of atoms as they have same number of moles of atoms.

Numerical Problem

Compute the number of ions present in 5.85 g of sodium chloride.


5.85 g of NaCl = 0.1 moles
Or 0.1 moles of NaCl particle.
Each of NaCl particle.
Each NaCl particle is equivalent to 2 ions, that’s why one Na+1 and one Cl-
Total moles of ions = 0.1 × 2 = 0.2 moles
Number of ions = 0.2 × 6.022 × 〖10〗^23
= 1.2042 × 〖10〗^23 ions

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Last Edited: June 10, 2023