CBSE Class 9 Science Structure of the Atom MCQs Set 03

Question. Who used the term ATOM for the first time ?
(a) E. Rutherford
(b) James Chadwick
(c) John Dalton
(d) Neils Bohr
Answer: (c) John Dalton
In simple words: Dalton was the first modern scientist to suggest the concept of the atom as an indivisible unit of matter.
Exam Tip: Remember that while ancient philosophers proposed similar ideas, John Dalton is credited with introducing the formal scientific term and theory.

Question. The maximum number of electrons present in a shell of an atom is given by the formula
(a) \( 2n^2 \)
(b) \( (n - 1)^2 \)
(c) \( n^2 \)
(d) \( 3n^2 \)
Answer: (a) \( 2n^2 \)
In simple words: This formula tells us how many electrons can fit into any energy level, where n is the level's number.
Exam Tip: For any numerical shell n, always multiply the square of the shell number by 2 to get the maximum capacity.

Question. During a chemical reaction, Atomic Number of an element
(a) Increases
(b) Remains constant
(c) Decreases
(d) Increases then decreases
Answer: (b) Remains constant
In simple words: In a chemical reaction, only electrons are shared or transferred, while the protons in the nucleus stay exactly the same.
Exam Tip: Chemical reactions involve only valence electrons, so the atomic number (number of protons) never changes.

Question. Which of the following pairs are isobars?
(a) \( _{17}\text{Cl}^{35} \) & \( _{17}\text{Cl}^{37} \)
(b) \( _{18}\text{Ar}^{40} \) & \( _{20}\text{Ca}^{40} \)
(c) \( _{6}\text{C}^{12} \) & \( _{6}\text{C}^{14} \)
(d) None of the options
Answer: (b) \( _{18}\text{Ar}^{40} \) & \( _{20}\text{Ca}^{40} \)
In simple words: Isobars are elements with different atomic numbers but the same mass number (the top number).
Exam Tip: Look for pairs that have identical superscript numbers (mass numbers) but different subscript numbers (atomic numbers).

Question. Rutherford’s alpha (\(\alpha\)) particles scattering experiment resulted in the discovery of
(a) electron
(b) proton
(c) nucleus in the atom
(d) atomic mass
Answer: (c) nucleus in the atom
In simple words: Rutherford's experiment showed that atoms have a tiny, dense, positively charged center called the nucleus.
Exam Tip: Direct deflection of alpha particles at large angles was the key evidence that proved the existence of the nucleus.

Question. Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order
(i) Rutherford’s atomic model
(ii) Thomson’s atomic model
(iii) Bohr’s atomic model
(a) (i), (ii) and (iii)
(b) (ii), (iii) and (i)
(c) (ii), (i) and (iii)
(d) (iii), (ii) and (i)
Answer: (c) (ii), (i) and (iii)
In simple words: Thomson proposed his model first, followed by Rutherford's nuclear model, and then Bohr refined it with stable energy levels.
Exam Tip: Learn the historical timeline of atomic discoveries: Thomson (1904) - Rutherford (1911) - Bohr (1913).

Question. Hydrogen exists in three isotopic forms, \( _1\text{H}^1 \), \( _1\text{H}^2 \), \( _1\text{H}^3 \) known as protium, deuterium and tritium. Why are all the isotopes neutral in nature?
(a) Since neutrons are neutral in nature hence isotopes are electrically neutral.
(b) All the isotopes have one electron and one proton, hence they are neutral.
(c) All the isotopes have one proton and one neutron, hence they are neutral.
(d) Increasing number of protons in the isotopes make them neutral.
Answer: (b) All the isotopes have one electron and one proton, hence they are neutral.
In simple words: Every hydrogen isotope has exactly one positively charged proton and one negatively charged electron, which cancel each other out.
Exam Tip: Neutrality of any atom is always determined by having equal counts of protons and electrons, regardless of neutrons.

Question. The charge on an electron is equal to:
(a) \( 1.6 \times 10^{-19} \) C of -ve charge
(b) \( 2.6 \times 10^{-19} \) C of -ve charge
(c) \( 1.6 \times 10^{-22} \) C of -ve charge
(d) \( 1.6 \times 10^{-23} \) C of -ve charge
Answer: (a) \( 1.6 \times 10^{-19} \) C of -ve charge
In simple words: An electron carries a negative charge of \( 1.6 \times 10^{-19} \) Coulombs, which is the basic unit of electric charge.
Exam Tip: Memorize this fundamental constant value as it is frequently used in physical chemistry calculations.

Question. Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be …….. and …… respectively.
(a) 0 and 1 respectively
(b) 8 and 1 respectively
(c) 1 and 0 respectively
(d) 0 and 7 respectively
Answer: (a) 0 and 1 respectively
In simple words: Neon has a complete outer shell so it does not combine, while chlorine needs one electron to complete its shell.
Exam Tip: Elements with completely filled valence shells have a valency of zero.

Question. Isotopes of an element have
(a) the same physical properties
(b) different number of neutrons
(c) different number of protons
(d) different atomic numbers.
Answer: (b) different number of neutrons
In simple words: Isotopes of the same element are identical in proton count but differ in neutron count, making their mass numbers different.

Question: A student weighs 30kg. Suppose his body is entirely made of electrons. How any electrons are there in his body? Mass of an electron= 9.1X10-31kg
a) 3.29 X10³¹
b) 3.29 X10³⁰
c) 3.29 X10²³
d) 3.29 X10³²
Answer: a

Question: When a gold foil is bombarded by a beam of α particle, only a few of them get deflected whereas most go straight undeflected) This is because
a) The force of attraction exerted on α particles by the electrons is insufficient
b) The volume of nucleus is much smaller than that of the atom
c) The force of repulsion acting on the fast moving α particles is very small
d) The neutrons have no effect on α particles
Answer: d

Question: Na+ has 12 neutrons and 10 electrons. Which of the following statements is correct? 
a) Na+ has atomic number 10 and mass number 22.
b) Na+ has atomic number 11 and mass number 23.
c) Na+ has atomic number 10 and mass number 23.
d) Na+ has atomic number 11 and mass number 22.
Answer: b

Question: The highest value of e/m ratio for anode rays is observed when the discharge tube is filled with:
a) N₂
b) H₂
c) He
d) Ar
Answer: b

Question: The electronic configuration of Cl- ion is:
a) 2,8,7
b) 2,8,8
c) 2,8,6
d) 2,8,8,1
Answer: c

Question: Which of the following statements does not belong to Bohr’s model?
a) Energy of the electrons in the orbit is quantized
b) The electrons in the orbit nearest to the nucleus is the lowest energy
c) Electrons revolve around the nucleus in different orbits having fixed energies
d) The electrons radiate energy during revolution due to force of attraction between nucleus and electrons
Answer: b

Question: How many electrons, protons and neutrons are present in X+, if atomic number of X is 19 and its mass number is 39
a) E=19, P=19, N= 20
b) E=18, P=19, N= 20
c) E=18, P=19, N= 19
d) E=19, P=20, N= 20
Answer: a

Question: Cathode rays have :
a) Charge only
b) Mass only
c) Charge as well as mass
d) Neither charge nor mass
Answer: a

Question: Which of the following statement is correct about proton?
a) It is the nucleus of deuterium
b) It is an ionized hydrogen molecule
c) It is an ionized Hydrogen atom
d) It is an α particle with unit positive charge
Answer: c

Question: Which of the following does not have 8 valence electrons:
a) He
b) Ne
c) Ar
d) Cl-
Answer: a

Question: Which of the following are isotopes:1
a) E=19, P=19, N= 20
b) E=18, P=19, N= 20
c) E=18, P=19, N= 19
d) E=19, P=20, N= 20
Answer: a

Question: Which of the following are isobars:₁₈ Ar⁴⁰,₁₉K³⁹ ,₂₀Ca⁴⁰, ₁₉[K+]³⁹
a) ₁₉ K³⁹ , ₁₉₊[K+]³⁹
b) ₁₈ Ar⁴⁰,₁₉ K³⁹
c) ₁₈Ar⁴⁰,₂₀ Ca⁴⁰,
d) ₁₈ Ar⁴⁰,₁₉ K³⁹ ,₂₀Ca⁴⁰
Answer: c

Question: The number of valence electrons determines:
a) Physical properties of elements
b) Chemical properties of elements
c) Both physical and chemical properties of elements
d) Neither physical nor chemical properties of elements
Answer: b

Question: Which of the following does not have one electron in its valance shell
a) Na
b) Li
c) H
d) Ca
Answer: d

Assertion & Reasoning Based MCQs

The mass of an atom is due to the masses of protons and neutrons in the nucleus. The relative masses of protons and neutrons are almost equal to one. Therefore, the atomic mass of an element should be nearly a whole number. But in many cases the atomic masses are fractional. The main reason for these fractional atomic masses is that these elements occur in nature as a mixture of several isotopes. The atomic mass of an element is the average of the atomic masses of these isotopes in the ratio of their proportion of occurrence.

Question: An element occurs in two isotopic forms with atomic masses 10 and 11. What is the percentage abundance of two isotopes in the sample having atomic mass 10.80?
a) 20, 80
b) 50, 50
c) 25, 70
d) 60, 40 
Answer: a

Question: The fractional atomic masses of elements are due to the existence of
a) isotopes having different masses
b) diagonal relationship
c) equal number of electrons and protons
d) none of these. 
Answer: a

Question: Chlorine occurs in nature in the form of two isotopes with atomic masses 35 u and 37 u in the ratio of 3 : 1 respectively. Atomic mass of chlorine is
a) 35.5 u
b) 34.5 u
c) 35 u
d) 36 u 
Answer: a

a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
a) Both assertion and reason are true, and reason is correct explanation of the assertion.
b) Both assertion and reason are true, but reason is not the correct explanation of the assertion.
c) Assertion is true, but reason is false.
d) Assertion is false, but reason is true.

Question: Assertion : Thomson’s atomic model is known as ‘raisin pudding’ model.
Reason : The atom is visualized as a pudding of positive charge with electrons (raisins) embedded in it. 
Answer: a

Question: Assertion : Anions are larger in size than the parent atom.
Reason : In an anion, the number of protons in the nucleus is less than the number of electrons moving around it.
Answer: a

Question: Assertion : The number of electrons gained,lost or shared by the atom of an element so as to complete its octet is called the valency of the element.
Reason : Elements having the same number of valence electrons in their atoms possess different chemical properties. 
Answer: c

Question: Assertion : Cathode rays get deflected towards the positive plate of electric field)
Reason : Cathode rays consist of negatively charged particles known as electrons. 
Answer: a

Question: Assertion : For noble gases, valency is zero.
Reason : Noble gases have 8 valence electrons. 
Answer: a

Question: Assertion : Electrons moving in the same orbit will not lose or gain energy.
Reason : On jumping from higher to lower energy level, the electron will gain energy.
Answer: c

Question: Assertion : The distribution of electrons in different orbits or shells is governed by a scheme known as Bohr-Bury scheme.
Reason : Electrons are filled in the shells in a stepwise manner in increasing order of energy of the energy shell. 
Answer: b

Question: Assertion : Bohr’s orbits are called stationary orbits.
Reason : Electrons remain stationary in these orbits for sometime.
Answer: c

Question: Assertion : The size of the nucleus is very small as compared to the size of the atom.
Reason : The electrons revolve around the nucleus of the atom. 
Answer: b

Question: Assertion : In Rutherford’s gold foil experiment, very few a-particles are deflected back.
Reason : The size of the nucleus is very small as compared to the size of the atom
Answer: b

Case Based MCQs

The maximum number of the electrons which are permitted to be assigned to an energy shell of an atom is called the electron capacity of that shell. The distribution of electrons in different orbits or shell is governed by a scheme known as Bohr-Bury scheme. According to this scheme :
(i) The maximum number of the electrons that can be present in any shell is given by the formula 2n2 where, n is the number of energy level.
(ii) The maximum number of electrons that can be accommodated in the outermost shell is 8.
Electrons are filled in the shells in a stepwise manner in increasing order of energy of the energy shell.

Question: Which of the following configuration represent sodium?
a) 2, 8, 4
b) 2, 8, 5
c) 2, 3
d) 2, 8, 1
Answer: d

Question: Identify the element with the configuration K-2, L-8, M-3.
a) Aluminium
b) Magnesium
c) Sodium
d) Beryllium 
Answer: a

Question: What is the maximum electrons capacity of N shell?
a) 24
b) 8
c) 18
d) 32 
Answer: d

The table shows the number of sub-atomic particles in arbitrary elements, A to H

Question: Which of the given elements attains noble gas configuration by gaining an electron?
(i) A (ii) E
(iii) C (iv) H
a) (iii) only
b) (iv) only
c) (i) and (iv) only
d) (i) only C
Answer: c

Question: The pair of isotopes from the table is/are
(i) C and D       (ii) E and F
(iii) B and C     (iv) G and H
a) (ii) only
b) (iv) only
c) (ii) and (iv) only
d) (i), (ii), (iii) and (iv) 
Answer: c

Question: The atom _____ has nucleon number 13 and atom _____ has valency 3.
a) G and F
b) F and D
c) C and E
d) F and B 
Answer: b

Question: Identify pair of isobars from the table.
a) C and D
b) B and E
c) G and H
d) E and F 
Answer: a 

The given diagrams show the atomic structures of elements X and Y.

Question: Valency of elements X and Y are respectively,
a) 4 and 3
b) 2 and 5
c) 1 and 4
d) 3 and 4
Answer: a

Question: Element X and Y could be _____ and _____respectively.
a) Be and B
b) C and O
c) F and N
d) C and N
Answer: d

Question: Elements X and Y are
a) isotopes
b) isoelectronic
c) isobars
d) isomers. 
Answer: c

ASSERTION - REASON TYPE QUESTIONS

Directions: Each of these questions contains two statements Assertion (A) and Reason (R). Select one of the codes (a), (b), (c) and (d) given below:
(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 R is true.

Question. Assertion: Cathode rays get deflected towards the positive plate of electric field.
Reason: Cathode rays consist of negatively charged particles known as electrons.
Answer: (a) Both A and R are true and R is the correct explanation of A.
In simple words: Cathode rays are made of electrons, which carry a negative charge. Therefore, they are pulled towards a positive electrical plate.
Exam Tip: Deflection towards a positive plate is the classic way to prove a stream consists of negatively charged particles.

Question. Assertion: In Rutherford's gold foil experiment, very few \(\alpha\)- particles are deflected back.
Reason: Nucleus present inside the atom is heavy.
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
In simple words: Only a few alpha particles bounced straight back, which happens because the nucleus is tiny and positive, not just because it is heavy.
Exam Tip: Rebounding occurs because the massive positive charge is packed into a tiny volume, which is not fully explained by just saying the nucleus is heavy.

Question. Assertion: According to Bohr's model, the orbits of an atom are also called energy levels.
Reason: Orbits are the spheres containing definite value of energy.
Answer: (a) Both A and R are true and R is the correct explanation of A.
In simple words: Orbits are called energy levels because each shell represents a specific, constant amount of energy for the electrons inside it.
Exam Tip: Remember that Bohr's orbits are energy-quantized levels where electrons can revolve without losing any energy.

Question. Assertion: Atom is electrically neutral.
Reason: A neutral particle, neutron is present in the nucleus of atom.
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
In simple words: While atoms are neutral and neutrons are also neutral, the atom's neutrality is actually due to having equal numbers of protons and electrons.
Exam Tip: Atomic neutrality depends strictly on the balance between positive protons and negative electrons, not on the presence of neutrons.

Question. Assertion: Electronic configuration of neon is 2,8.
Reason: Atomic number of neon is 8
Answer: (c) A is true, but R is false.
In simple words: Neon has 10 electrons with a configuration of 2, 8, but its atomic number is 10, not 8.
Exam Tip: Verify that neon has 10 protons (atomic number 10) and 10 electrons in total.

Question. Assertion: According to Thomson's model of atom, an atom consists of a positively charged sphere
Reason: Electrons are embedded in it.
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
In simple words: Both statements are part of Thomson's model, but the presence of electrons does not explain why the sphere is positively charged.
Exam Tip: Thomson's model describes both the positive background and the embedded negative electrons to maintain overall neutrality.

Question. Assertion: According to Thomson's model, atom as a whole is electrically neutral.
Reason: The negative and positive charges are equal in magnitude
Answer: (a) Both A and R are true and R is the correct explanation of A.
In simple words: An atom is neutral because the total negative charge of its electrons matches the positive charge of the sphere.
Exam Tip: Equal amounts of positive and negative charges are essential for making any atomic model electrically neutral.

Question. Assertion: Rutherford model of atom most of the alpha particles passed straight through the gold foil
Reason: Most of the space inside the atom is empty
Answer: (a) Both A and R are true and R is the correct explanation of A.
In simple words: Alpha particles went right through the foil because there is nothing in their way, proving that atoms are mostly empty space.
Exam Tip: Non-deflection of the vast majority of alpha particles is direct evidence of a hollow atomic structure.

Question. Assertion: In gold foil experiment some of the alpha particles were deflected by the foil by some angles.
Reason: There is a positive charge space at the centre of the atom
Answer: (a) Both A and R are true and R is the correct explanation of A.
In simple words: Positively charged alpha particles get pushed away and deflected when they pass near the positive nucleus at the center.
Exam Tip: Like charges repel, so the deflection of positive alpha particles proved that the center of the atom also has a positive charge.

Question. Assertion: Size of the nucleus is very small compared to size of the atom.
Reason: Electrons revolve around the nucleus in well defined orbits
Answer: (b) Both A and R are true, but R is not the correct explanation of A.
In simple words: It is true that the nucleus is tiny and electrons orbit around it, but the electron paths do not explain why the nucleus is so small.
Exam Tip: The size comparison between the nucleus and the entire atom is a physical measurement, independent of how electrons orbit.