The nucleons are
I. Protons and electrons
II. Neutrons and electrons
III. Protons and neutrons
IV. None of these
2. The isotope deuterium of hydrogen has
I. No neutrons and one proton
II. One neutrons and two protons
III. One electron and two neutron
IV. One proton and one neutron
3. The electrons present in the outermost shell are called
I. Valency electrons
II. Octate electrons
III. Duplet electrons
IV. Valence electrons
4. An alpha particle contains
I. 4 positive charge and 2 mass unit
II. 2 positive charge and 4 mass unit
III. 2 positive charge and 2 mass unit
IV. 4 positive charge and 4 mass unit
5. The atomic number of sodium is 11 and its mass number is 23. It has
I. 11 neutrons and 12 protons
II. 12 protons and 11 electrons
III. 11 electrons and 12 neutrons
IV. 12 electrons and 11 neutrons
6. The electronic configuration of chlorine is
I. 2,7
II. 2,8,8,7
III. 2,8,7
IV. 2,7,8
7. The isotope used to remove the brain tumours and treatment of cancer is
I. U-235
II. Na-24
III. Iodine
IV. C0-60
8. In an alpha scattering experiment, few alpha particles rebounded because
I. Most of the space in the atom is occupied
II. Positive charge of the atoms very little space
III. The mass of the atom is concentrated in the centre
IV. All the positive charge and mass of the atom is concentrated in small volume
ANSWERS
1. III
2. IV
3. IV
4. II
5. III
6. III
7. IV
8. II
MCQ Questions for Class 9 Science Chapter
4 Structure of the Atom
Q.1. The nucleons are
(a) Protons and electrons
(b) Neutrons and electrons
(c) Protons and neutrons
(d) None of these
Answer
Q.2. Which isotope is used in the nuclear power plants to generate electricity?
(a) Uranium 235
(b) Iodine 131
(c) Cobalt 60
(d) Uranium 238
Answer
Q.3. Who discovered the electron?
(a) Rutherford
(b) Chadwick
(c) Thomson
(d) Goldstein
Answer
Q.4. What was the source of alpha particles in Rutherford scattering experiment?
(a) Hydrogen nucleus
(b) Argon nucleus
(c) Helium nucleus
(d) None of these
Answer
Q.5. Why was the Thomson’s Model of an atom failed?
i. It could not explain the screening of negative charges from that of positive
ii. It did not tell about the presence of electrons
iii. It did not give an idea about the discrete energy levels
iv. It explained the atom as a whole to be electrically neutral.
Choose the correct option from the following:
(a) Only (iii)
(b) Both (i) & (iii)
(c) Only (i)
(d) Both (ii) & (iv)
Answer
Q.6. What property of an element determines its chemical behaviour?
(a) Size of an element
(b) Valency of an element
(c) Molar mass of the element
(d) None of these
Answer
Q.7. Which of the following does not match the characteristics of an Isotope?
(a) Isotopes of some elements are radioactive
(b) Isotopes are the atoms of different elements
(c) Isotopes differ in number of neutrons
(d) Isotopes have similar chemical properties
Answer
Q.8. Which of the two will be chemically more reactive, Sulphur(S) with atomic
number 16 or Chlorine (Cl) with atomic number 17?
(a) Chlorine
(b) Sulphur
(c) Both are equally reactive
(d) Can’t say
Answer
Q.9. Which of the following statements is incorrect about the structure of an atom?
i. The whole mass of an atom is concentrated in the nucleus
ii. The atom is an indivisible particle
iii. The atom as a whole is neutral
iv. All the atoms are stable in their basic state
Choose the right option among the following:
(a) (i) and (iii)
(b) only (ii)
(c) (ii) and (iv)
(d) none of these
Answer
Q.10. Which of the following elements does not exhibit the electrovalencey?
(a) Sodium
(b) Calcium
(c) Carbon
(d) Chlorine
Answer
Q.11. Which scientist gave the concept of fixed energy levels around the nucleus?
(a) Ernest Rutherford
(b) Neils Bohar
(c) J.J.Thomsan
(d) None of these
Answer
Q.12. What prevents an atom from being collapsed?
(a) The nuclear forces
(b) Movement of electrons in discrete energy levels
(c) The electron-electron repulsions
(d) All of these
Answer
Q.13. Which of the following is an incorrect statement in reference with observation
in Rutherford’s α-particle scattering experiment?
(a) Some of the α-particles rebound after hitting the gold foil
(b) Some of the particles deflected from their path
(c) Some of the particles not pass through the gold foil
(d) Most of the particles pass straight through the gold foil
Answer
Q.14. Which of the following pairs are isobars?
(a) 17Cl35 & 17Cl37
(b) 18Ar40 & 20Ca40
(c) 6C12 & 6C14
(d) None of these
Answer
Q.15. Which radioactive element is used in the treatment of cancer?
(a) Iodine-131
(b) Uranium-234
(c) Plutonium-239
(d) Cobalt-60
Answer
Q.16. Why do most of the elements try to participate in the chemical combinations?
i. To gain more electrons
ii. To achieve Inert Gas configuration
iii. To complete their octet
iv. To complete their inner shells
Choose the correct option among the following
(a) Both (i) & (iii)
(b) Both (ii) & (iii)
(c) Only (ii)
(d) Both (i) & (iv)
Answer
Q.17. The isotope deuterium of hydrogen has
(a) No neutrons and one proton
(b) One neutrons and two protons
(c) One electron and two neutron
(d) One proton and one neutron
Answer
Q.18. The electrons present in the outermost shell are called
(a) Valency electrons
(b) Octate electrons
(c) Duplet electrons
(d) Valence electrons
Answer
Q.19. The atomic number of sodium is 11 and its mass number is 23. It has
(a) 11 neutrons and 12 protons
(b) 12 protons and 11 electrons
(c) 11 electrons and 12 neutrons
(d) 12 electrons and 11 neutrons
Answer
Q.20. An alpha particle contains
(a) 4 positive charge and 2 mass unit
(b) 2 positive charge and 4 mass unit
(c) 2 positive charge and 2 mass unit
(d) 4 positive charge and 4 mass unit
Answer
Q.21. The electronic configuration of chlorine is
(a) 2,7
(b) 2,8,8,7
(c) 2,8,7
(d) 2,7,8
Answer
Q.22. In an alpha scattering experiment, few alpha particles rebounded
because
(a) Most of the space in the atom is occupied
(b) Positive charge of the atoms very little space
(c) The mass of the atom is concentrated in the centre
(d) All the positive charge and mass of the atom is concentrated in small volume
Answer
Q.23. The isotope used to remove the brain tumours and treatment of cancer is
(a) U-235
(b) Na-24
(c) Iodine
(d) C0-60
Answer
You may also check the MCQs on other chapters from the links provided below:
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MCQs on CBSE Class 9 Science Chapter 1
MCQs on CBSE Class 9 Science Chapter 2
MCQs on CBSE Class 9 Science Chapter 3
MCQs on CBSE Class 9 Science Chapter 5
MCQs on CBSE Class 9 Science Chapter 6
MCQs on CBSE Class 9 Science Chapter 7
MCQs on CBSE Class 9 Science Chapter 8
MCQs on CBSE Class 9 Science Chapter 9
MCQs on CBSE Class 9 Science Chapter 10
MCQs on CBSE Class 9 Science Chapter 11
MCQs on CBSE Class 9 Science Chapter 12
MCQs on CBSE Class 9 Science Chapter 13
MCQs on CBSE Class 9 Science Chapter 14
MCQs on CBSE Class 9 Science Chapter 15
Also Check:
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NCERT Solutions for Class 9 Science
NCERT Solutions for Class 9 Maths
Lakhmir Singh Class 9 Science Solutions
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Class 9 Science MCQs From Chapter 3 Atoms
and Molecules
1. Which of the following correctly represents 360g of water?
(i) 2 moles of water
(ii) 20 moles of water
(iii) 6.022 × 1023 molecules of water
(iv) 1.2044 × 1025 molecules of water
(a) (i)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Answer. (d) (ii) and (iv)
2. Which of the following statements is not true about an atom?
(a) Atoms are not able to exist independently.
(b) Atoms are the basic units from which molecules and ions are
formed.
(c) Atoms are always neutral in nature.
(d) Atoms aggregate in large numbers to form the matter that we can see, feel
or touch.
Answer. (d) Atoms aggregate in large numbers to form the matter that
we can see, feel or touch.
3. 1 u or 1 amu means
(a) 1/12th mass of C-12 atoms
(b) Mass of C-12 atom
(c) Mass of O-16 atom
(d) Mass of Hydrogen molecule
Answer. (a) 1/12th mass of C-12 atoms
4. Which of the following contains maximum number of molecules?
(a) 19 CO2
(b) 1 g N2
(c) 1 g H2
(d) 1 g CH4
Answer. (c) 1 g H2
5. A sample of NH3 molecule irrespective of source contains 82.35% Nitrogen
and 17.65% of Hydrogen by mass. This data supports:
(a) Law of Conservation of Mass
(b) Las of Multiple Proportions
(c) Law of Definite Proportions
(d) Avogadro’s Law
Answer. (c) Law of Definite Proportions
6. An element X is divalent and another element Y is tetravalent. The
compound formed by these two elements will be:
(a) XY
(b) XY2
(c) X2Y
(d) XY4
Answer. (b) XY2
7. The molecular formula of potassium nitrate is ________.
(a) KNO3
(b) KNO
(c) KNO2
(d) KON
Answer. (a) KNO3
8. 3.42 g of sucrose are dissolved in 18 g of water in a beaker. The numbers
of oxygen atoms in the solution are:
(a) 6.68 ×1023
(b) 6.09 ×1022
(c) 6.022 ×1023
(d) 6.022 ×1021
Answer. (a) 6.68 × 1023
9. Molecular mass is defined as the:
(a) Mass of one molecule of any substance compared with the mass of one
atom of C – 12
(b) Mass of one atom compared with the mass of one atom of hydrogen
(c) Mass of one atom compared with the mass of one molecule
(d) None of the above
Answer. (a) Mass of one molecule of any substance compared with the
mass of one atom of C – 12
10. A change in the physical state can be brought about
(a) only when energy is given to the system
(b) only when energy is taken out from the system
(c) When energy is either given to, or taken out from the system
(d) Without any energy change
Answer. (c) When energy is either given to, or taken out from the system
11. The atomic mass of sodium is 23. The number of moles in 46g of sodium
is ________.
(a) 4
(b) 2
(c) 0
(d) ½
Answer. (b) 2
12. Which of the following represents a correct chemical formula?
(a) CaCl
(b) BiPO4
(c) NaSO4
(d) NaS
Answer. (b) BiPO4
13. What is the formula mass unit of ZnO?
(a) 18 u
(b) 81 u
(c) 88 u
(d) 188 u
Answer. (b) 81 u
14. How many atoms of oxygen are present in 300 grams of CaCO3?
(a) 54.207 × 1023
(b) 6.207 × 1023
(c) 12.207 × 1023
(d) 22.2 × 1023
Answer. (a) 54.207 × 1023
15. Which of the following represents the correct relation between
Avogadro’s number (No), number of particles (N) and moles (n)?
(a) n = N / No
(b) n = No / N
(c) n = N No
(d) all are correct
Answer. (a) n = N / No
Identify the incorrect statement.
(a) According to the alpha particle scattering experiment, very few alpha particles retrace their own path
(b) According to Niels Bohr’s atomic model, the energy of the second orbit is more than the energy of the first
orbit.
(c) According to Thomson’s atomic model, electrons are embedded in uniform positive charge.
(d) According to Rutherford’s atomic model, electrons revolve around the nucleus in circular as well as
elliptical paths
Q.2. A tripositive ion has 23 electrons and 30 neutrons. What is the atomic mass of the element?
(a) 56
(b) 53
(c) 50
(d) 55
Q.3. What would be the radius of the nucleus if 10-8 cm is the radius of the atom
(a) 10-8 cm
(b) 10-13 cm
(c) 10-5 cm
(d) 105 cm
Q.4. Identify the element(s) whose A-Z value is 14.
(i) Aluminium
(ii) Silicon
(iii) Phosphorus
(iv) Sulphur
(a) Only i
(b) i and ii
(c) i, ii and iv
(d) iii and iv
Q.5. Which of the following experiments led to the discovery of neutrons?
(a) Discharge tube experiment
(b) Alpha ray scattering experiment
(c) Chadwick’s experiment
(d) Oil drop experiment
Q.6. Match column A with column B.
Column A
Column B
Atomic number
Valency
(A) 12
(i) 3
(B) 17
(ii) 0
(C) 10
(iii) 2
(D) 15
(iv) 1
(a) A → ii, B → iv, C → iii, D → i
(b) A → iii, B → iv, C → ii, D → i
(c) A → iii, B → iv, C → i, D → ii
(d) A → iii, B → ii, C → i, D → iv
Q.7. Out of 1000 atoms of chlorine, 750 atoms have atomic mass 35u and 250 atoms have atomic mass
37u. What is the average atomic mass of chlorine atom?
(a) 35.5u
(b) 36u
(c) 36.5u
(d) 35.7u
Q.8. What is the total mass of neutrons present in Fe-56 atom? (Atomic number of iron is 26)
(a) 30 u
(b) 56 u
(c) 26 u
(d) 40 u
Q.9. Identify the decreasing order of specific charges of the particles: Electron(e), proton(p), neutron(n)
and α particle.
(a) e, p, n, α
(b) p, e, n, α
(c) e, p, α, n
(d) n, α, p, e
Q.10. Which of the following set of elements ratio of atomic numbers is 1:2:3:4?
(a) H, He, Li, B
(b) He, Be, C, Ne
(c) Be, O, Mg, Ca
(d) B, Ne, P, Ca
Q.11. In the valence shell, which of the following pairs contains duplet and octet configuration
respectively?
(a) Ne, Ar
(b) Ca+2 , O-2
(c) Li+1 , N-3
(d) Mg+2 , Be+2
Q.12. The maximum number of electrons that a valence shell of an atom can possess is _____
(a) 32
(b) 18
(c) 8
(d) 2
Q.13. Which of the following has a similar electronic configuration as that of argon?
(i) Potassium ion
(ii) Sulphide ion
(iii) oxide
(iv) chloride
(a) i and iii
(b) i, ii and iv
(c) ii and iv
(d) i, iii and
Q.14. The correct electronic configuration of potassium is _______?
(a) 2, 8, 4
(b) 2, 8, 8, 6
(c) 2, 8, 8, 18
(d) 2, 8, 8, 1
Q.15. Identify the correct statement(s);
(i) The penultimate shell of calcium is the M shell.
(ii) The charge of the stable ion with Z=16 is-2.
(iii) The valence shell of aluminium is N shell.
(iv) The maximum capacity of accommodating electrons of O shell is 50.
(a) i, ii, iv
(b) ii, iii, iv
(c) i, iii and iv
(d) i, ii, iii
Answer Key for Structure of the Atom Class 9 MCQs
Q.
Ans
Q.
Ans
1
(d)
9
(c)
2
(a)
10
(d)
3
(b)
11
(c)
4
(b)
12
(c)
5
(c)
13
(b)
6
(b)
14
(d)
7
(a)
15
(a)
8
(a)
Atoms and Molecules MCQ Question Answers Class
9 Science Chapter 3
CBSE Class 9 Science Chapter 3 “Atoms and Molecules
Multiple Choice Question Answers”
Atoms and Molecules MCQ – Here is a compilation of Free MCQs of Class 9 Science Book Chapter
3 Atoms and Molecules with Answers. Students can practice free MCQs as have been added by CBSE in
the new Exam pattern. At the end of Multiple- Choice Questions, the answer key also has been provided for
your reference. Also, Take Free Online MCQ Test for Class 9.
Click Here for Atoms and Molecules Notes, Explanation, Question Answers
Click here for Atoms and Molecules Video Explanation
Q.1. Laws which explain the formation of many oxides by nitrogen is
(a) Law of conservation of mass
(b) Law of multiple proportions
(c) Law of definite proportions
(d) Avogadro’s law
Q.2. Which of the following is not equal in mass to the 116th part of an oxygen atom?
(a) 14th of helium atom
(b) 112th of carbon atom
(c) 128th of nitrogen atom
(d) None of these
Q.3. Name the salts which contain the same number of ions per molecule as magnesium nitride.
(i) Aluminium phosphate
(ii) Potassium sulphite
(iii) Barium phosphate
(iv) Ferric chromate
(a) i, iii, iv
(b) ii, iv
(c) i, iii
(d) iii, iv
Q.4. Which pairs of elements have gram molecular mass same as gram atomic mass?
(a) Argon, Nitrogen
(b) Fluorine, Magnesium
(c) Phosphorus, Gold
(d) Silicon, Neon
Q.5. What would be the gram molecular mass of solid sulphur?
(a) 256 g
(b) 128 g
(c) 64 g
(d) 32 g
Q.6. Identify the number of ions in 1 mole of ferric sulphate.
(a) 3 x 1024
(b) 9 x 1024
(c) 1.02 x 1025
(d) 1.2 x 1024
Q.7. How many moles are present in 560 g of iron?
(a) 1 mole
(b) 10 moles
(c) 100 moles
(d) 0.1 mole
Q.8. The central atom does not acquire stable octet configuration in which of the following molecules
(a) Carbon dioxide
(b) Methane
(c) Boron trichloride
(d) Ammonia
Q.9. In which of the following compounds, formula unit mass is considered instead of molecular mass?
(a) Carbon tetrachloride
(b) Phosphorus pentoxide
(c) Barium bromide
(d) Sulphur hexafluoride
Q.10. From which oxyacid two acidic radicals can be derived.
(a) Phosphoric acid
(b) Sulphurous acid
(c) Nitric acid
(d) Chlorous acid
Q.11. A metal can form two Sulphides. In one Sulphide, the metal and Sulphur atoms are in the ratio of
2:1. In the other, the metal and Sulphur atoms are in the ratio 2:3. The metal which complies with the
above compounds is.
(a) Gold
(b) Copper
(c) Iron
(d) Lead
Q.12. According to the law of definite proportions, ________.
(a) matter remains constant
(b) matter can neither be created nor destroyed matter remains constant
(c) a chemical compound is always made up of the same element combined together in the same fixed
proportion matter remains constant
(d) All of these
Q.13. Three-fourth mass of a substance contains the same number of molecules as in a certain mass of
Sulphur dioxide. Identify the substance among the following.
(a) Methane
(b) Ozone
(c) Sulphur trioxide
(d) Oxygen
Q.14. When alpha-particles are sent through a thin metal foil, most of them go straight through the foil
because, _______.
(a) alpha-particles are much smaller than electrons.
(b) alpha-particles are positively charged
(c) most part of the atom is empty space
(d) alpha-particles move with a low velocity
Q.15. Which among the following formulae is wrong?
(a) MClO3
(b) M(ClO3)2
(c) M(ClO3) 3
(d) M2ClO3
Answer Key for Class 9 Science Chapter 3 Atoms and Molecules
MCQs
Q.
Ans
Q.
Ans
1
(b)
9
(c)
2
(c)
10
(b)
3
(d)
11
(c)
4
(d)
12
(c)
5
(a)
13
(b)
6
(a)
14
(c)
7
(a)
15
(d)
8
(c)
8186-7530
3732-3885
8122-0094
Alpha radiation:
Alpha radiation consists of two protons and two neutrons; since they have no electrons,
they carry a positive charge. Due to their size and charge, alpha particles are barely able
to penetrate skin and can be stopped completely by a sheet of paper.
Beta radiation:
Beta radiation consists of fast-moving electrons ejected from the nucleus of an atom.
Beta radiation has a negative charge and is about 1/7000th the size of an alpha particle,
so it is more penetrating. However, it can still be stopped by a small amount of shielding,
such as a sheet of plastic.
Gamma radiation:
Gamma radiation is a very penetrating type of radiation. It is usually emitted immediately
after the ejection of an alpha or beta particle from the nucleus of an atom. Because it has
no mass or charge, it can pass through the human body, but it is absorbed by denser
materials, such as concrete or lead.
X-rays:
X-rays are a form of radiation similar to gamma radiation, but they are produced mainly
by artificial means rather than from radioactive substances.
Units of Radioactivity
Curie and Rutherford are the units of radioactivity.
1C = 3.7 × 104 Rd is the relationship between Curie and Rutherford.
Alpha Decay
Alpha decay is a type of radioactive decay where the unstable atomic nuclei emit a helium nucleus (alpha
particle) and in the process transforms into another more stable element. The particle ejected out, the alpha
particle, consists of four nucleons, and they are two neutrons and two protons. Alpha radiation reduces the
ratio of protons to neutrons in the parent nucleus, bringing it to a more stable configuration. An alpha particle
is identical to the nucleus of a helium atom. The first observations and investigation into alpha decay were
made by Ernest Rutherford, who used alpha particles in his gold foil scattering experiment. As the alpha
particle; made of two protons and neutron; exits the nucleus, the atomic number of the radioactive sample
changes. The element left behind after alpha decay is two atomic numbers lesser and four mass numbers lower.
For example, Uranium 238 92 decays to form Thorium23490.
92238U→90234Th+24He
Thus, the transformation of a nucleus in an alpha decay can be written as;
��X→�−2�−4Y+24He
Occurrence of alpha decay
Alpha decay occurs only in the heaviest of the elements. The element’s nucleus should be large or unstable
enough to undergo spontaneous fission-type changes. It is the most common form of decay in such elements.
The alpha particles emitted out of the nucleus usually have an energy level of around 5 MeV and have a speed
of around 5% of light. It is important to note that alpha particles possess a charge of +2 due to the absence of
electrons. Due to this charge and owing to its heavy mass, an alpha particle reacts with the surroundings
vigorously to lose all of its energy almost immediately. Their forward motion can be stopped by a few
centimeters of air.
Owing to their heaviness and their charge, this kind of radioactive decay reacts most violently with the human
body. They have a high ionizing power due to which they can wreak havoc with a tissue. An overdose of alpha
radiation results in the formation of blisters and burns on the victim’s bodies.
Uses of Radioactivity
Some radioactivity uses are provided in the points below.

Americium-241 is an alpha emitter and is used for domestic smoke detectors in the United States.

The alpha particles given out by the Americium sample ionize the air in the chamber of the smoke
detector leading to a small current in the chamber.

When smoke enters the chamber, it causes a drop in current causing the alarm to go off. Although
Alpha particles have a very short range, they are devastating when in close contact.

Alpha emitters, when swallowed, come in close contact with tissue and are deadly in such
circumstances and is therefore used in assassination attempts by radiation poisoning