CUET UG Physics Booster Test 2-Atomic Structure and Masses -

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CUET UG Physics Booster Test 2-Atomic Structure and Masses

⏱️ 30 Minutes

❓ 20 Questions

📌 Answers are locked once submitted — results and explanations appear at the end.

CUET UG Physics Booster Test 2-Atomic Structure and Masses

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QUESTION 1 OF 20

If the mass of a hydrogen atom is mH and the mass of a single electron is me, the mass of a proton can be mathematically approximated as:

Answer Review

1 If the mass of a hydrogen atom is mH and the mass of a single electron is me, the mass of a proton can be mathematically approximated as:

A mp = mH − me

B mp = mH + me

C mp = me − mH

D mp = mH/me

💡 Short Explanation

�� Hydrogen atom contains one proton and one electron. �� Electron mass contributes to atomic mass. �� Proton mass is obtained by subtraction.

✅ Correct Answer Explanation

A hydrogen atom consists of one proton and one electron. Neglecting binding energy, the mass of the hydrogen atom is approximately equal to the sum of the proton mass and electron mass. mH = mp + me Therefore, mp = mH − me Hence, option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Electron mass must be subtracted, not added.
�� Option C → Gives a negative value.
�� Option D → Mass is not determined by division.

🎯 Strategy

Used

Substitution

Application:

Apply the mass relationship for a hydrogen atom.

Final Logic:

Hydrogen atom mass = proton mass + electron mass.

🧠 Memory Trick

Hydrogen = Proton + Electron

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Proton Mass

2 When determining the nature of the neutral radiation emitted from beryllium, Chadwick applied the principles of conservation of energy and momentum, concluding that

A The radiation consisted of high-energy photons exclusively.

B The neutral radiation must consist of a new type of neutral particles because photon energy would have to be much higher than available.

C electrons were ejected directly from the nucleus.

D The mass of the new particle was equivalent to an alpha particle.

💡 Short Explanation

�� Chadwick analyzed collision data. �� Photon explanation failed. �� Discovery led to the neutron.

✅ Correct Answer Explanation

Chadwick observed that the neutral radiation emitted from beryllium could not be explained as gamma rays because the required photon energies would be unrealistically high. Applying conservation of energy and momentum, he proposed the existence of a new neutral particle having mass comparable to that of the proton. This particle was later identified as the neutron. Therefore, option B is correct.

❓ Why Other Options Are Incorrect

�� Option A → High-energy photons could not explain the observations.
�� Option C → Electrons were not emitted from the nucleus in this experiment.
�� Option D → Neutron mass is close to proton mass, not alpha-particle mass.

🎯 Strategy

Used

Elimination

Application:

Reject explanations inconsistent with conservation laws.

Final Logic:

Only the neutron hypothesis explained the experimental results.

🧠 Memory Trick

Photon Failed → Neutron Found

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Discovery of Neutron

3 What is the precise energy equivalent of one atomic mass unit (1 u) expressed in MeV?

A 931.5 MeV

B 1.4924 × 10⁻¹⁰ MeV

C 1.00727 MeV

D 127.5 MeV

💡 Short Explanation

�� Mass and energy are equivalent. �� Einstein's equation applies. �� 1 u corresponds to 931.5 MeV.

✅ Correct Answer Explanation

Using Einstein's mass-energy relation: E = mc² The energy equivalent of 1 atomic mass unit is: 1 u = 931.5 MeV This value is widely used in nuclear physics for mass defect and binding energy calculations. Therefore, option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Incorrect magnitude.
�� Option C → Close to proton mass in u, not energy equivalent.
�� Option D → Much smaller than the accepted value.

🎯 Strategy

Used

Definition Recall

Application:

Recall the standard conversion factor.

Final Logic:

1 u always corresponds to 931.5 MeV.

🧠 Memory Trick

1 u = 931.5 MeV

📚 Class XII – NCERT Physics Part II – Chapter: Nuclei – Topic: Mass-Energy Equivalence

4 Statements about mass spectrometry:
1. Accurate mass spectrometry shows that atomic masses of elements are often not integral multiples of the hydrogen atom's mass.
2. This observation indicates that practically every element consists of a mixture of isotopes with different masses.
3. The observation proves that the mass of a proton changes from element to element.
4. The observation proves that atoms do not contain a central nucleus.

A (1), (2)

B (2), (3)

C (3), (4)

D (4)

💡 Short Explanation

�� Atomic masses are often fractional. �� Isotopes explain average masses. �� Proton mass remains constant.

✅ Correct Answer Explanation

Statement 1 is correct because mass spectrometry shows that many atomic masses are not whole-number multiples of hydrogen mass. Statement 2 is correct because most elements occur as mixtures of isotopes having different masses, leading to weighted average atomic masses. Statement 3 is incorrect because proton mass does not vary from one element to another. Statement 4 is incorrect because the existence of a nucleus is well established and unrelated to this observation. Therefore, statements 1 and 2 are correct.

❓ Why Other Options Are Incorrect

�� Option B → Includes statement 3, which is incorrect.
�� Option C → Includes statements 3 and 4, both incorrect.
�� Option D → Statement 4 is incorrect.

🎯 Strategy

Used

Elimination

Application:

Test each statement against isotope theory.

Final Logic:

Only statements 1 and 2 correctly explain the observation.

🧠 Memory Trick

Fractional Mass → Isotopes

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Mass Spectrometry and Isotopes

5 Nuclides with same Z but different N, Nuclides with same A:

A Isotopes, Isobars

B Isobars, Isotopes

C Isotones, Isobars

D Isotopes, Isotones

💡 Short Explanation

�� Isotopes have the same atomic number. �� Isobars have the same mass number. �� Neutron numbers differ in isotopes.

✅ Correct Answer Explanation

The first term refers to nuclides having the same atomic number (Z) but different neutron numbers (N). Such nuclides are called isotopes. The second term refers to nuclides having the same mass number (A). Such nuclides are called isobars. Therefore, the correct pair is "Isotopes, Isobars". Hence, option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → The terms isotopes and isobars are interchanged.
�� Option C → Isotones have the same number of neutrons, not the same atomic number.
�� Option D → Isobars are incorrectly replaced by isotones.

🎯 Strategy

Used

Option Grouping

Application:

Identify the definition of the first term (same Z, different N) and then the second term (same A).

Final Logic:

Same Z → Isotopes; Same A → Isobars.

🧠 Memory Trick

Same A = Isobars

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Isotopes and Isobars

6 Match List I (Isotope of Chlorine) with List II (Abundance/Mass values)

List I	List II
1. Chlorine-35 isotope mass	a. 75.4 %
2. Chlorine-37 isotope mass	b. 24.6 %
3. Chlorine-35 relative abundance	c. 34.98 u
4. Chlorine-37 relative abundance	d. 36.98 u

A (1)-c, (2)-d, (3)-a, (4)-b

B (1)-d, (2)-c, (3)-b, (4)-a

C (1)-a, (2)-b, (3)-c, (4)-d

D (1)-c, (2)-d, (3)-b, (4)-a

💡 Short Explanation

�� Chlorine has two major isotopes. �� ³⁵Cl is more abundant. �� Isotopic masses differ.

✅ Correct Answer Explanation

1 → c : Chlorine-35 has a mass of 34.98 u. 2 → d : Chlorine-37 has a mass of 36.98 u. 3 → a : Chlorine-35 abundance is 75.4%. 4 → b : Chlorine-37 abundance is 24.6%. Thus, option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Masses and abundances are interchanged.
�� Option C → Mass values incorrectly matched as abundances.
�� Option D → Relative abundances are reversed.

🎯 Strategy

Used

Option Grouping

Application:

Match isotope masses and abundances separately.

Final Logic:

34.98 → 75.4%, 36.98 → 24.6%.

🧠 Memory Trick

35 → 75%, 37 → 25%

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Chlorine Isotopes

7 Statements regarding Deuterium:
1. It is a stable isotope of hydrogen containing one proton and one neutron.
2. Its nucleus is known universally as the proton.
3. It is represented by the symbol ²H or D.
4. Its absolute mass is exactly 1.0078 u.

A (1), (3)

B (1), (2), (3)

C (2), (4)

D (1), (3), (4)

💡 Short Explanation

�� Deuterium contains one neutron. �� It is stable. �� Symbol is D or ²H.

✅ Correct Answer Explanation

Statement 1 is correct because deuterium contains one proton and one neutron. Statement 2 is incorrect because the nucleus of deuterium is called a deuteron, not a proton. Statement 3 is correct because deuterium is represented as D or ²H. Statement 4 is incorrect because 1.0078 u corresponds approximately to protium, not deuterium. Therefore, statements 1 and 3 are correct.

❓ Why Other Options Are Incorrect

�� Option B → Includes statement 2, which is incorrect.
�� Option C → Both statements are incorrect.
�� Option D → Includes statement 4, which is incorrect.

🎯 Strategy

Used

Elimination

Application:

Verify each statement using deuterium properties.

Final Logic:

Only statements 1 and 3 are correct.

🧠 Memory Trick

D = Deuteron = One Neutron

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Deuterium

8 Incorrect statement about Tritium:

A It contains one proton and two neutrons.

B Its chemical properties drastically differ from normal hydrogen due to its higher mass.

C It is physically unstable and does not occur naturally.

D It is produced artificially in laboratories.

💡 Short Explanation

�� Isotopes have similar chemical properties. �� Tritium is radioactive. �� Tritium contains two neutrons.

✅ Correct Answer Explanation

Tritium (³H) contains one proton and two neutrons and is radioactive. Since isotopes possess the same electronic configuration, their chemical properties remain nearly identical. Therefore, the statement that tritium has drastically different chemical properties is incorrect. Hence, option B is correct.

❓ Why Other Options Are Incorrect

�� Option A → Correct description of tritium.
�� Option C → Tritium is unstable and occurs only in trace amounts naturally.
�� Option D → Tritium is produced artificially.

🎯 Strategy

Used

Definition Recall

Application:

Recall that chemical properties depend mainly on electronic configuration.

Final Logic:

Same electrons produce nearly identical chemistry.

🧠 Memory Trick

Same Electrons → Same Chemistry

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Tritium

9 Statements about the standard for atomic mass:
1. 1 u is exactly 1/12th the mass of a Carbon-12 atom.
2. 1 u equates to 1.660539 × 10⁻²⁷ kg.
3. The mass of a Carbon-12 atom is exactly 12 u.
4. 1 u is the exact mass of one free electron.

A (1), (2), (3)

B (1), (3), (4)

C (2), (3), (4)

D (1), (2)

💡 Short Explanation

�� Carbon-12 defines the atomic mass scale. �� 1 u is a standard unit. �� Electron mass is much smaller.

✅ Correct Answer Explanation

Statement 1 is correct because 1 u is defined as one-twelfth of the mass of a carbon-12 atom. Statement 2 is correct because 1 u = 1.660539 × 10⁻²⁷ kg. Statement 3 is correct because carbon-12 is assigned a mass of exactly 12 u. Statement 4 is incorrect because the electron mass is approximately 0.0005486 u. Therefore, statements 1, 2 and 3 are correct.

❓ Why Other Options Are Incorrect

�� Option B → Includes statement 4, which is incorrect.
�� Option C → Includes statement 4, which is incorrect.
�� Option D → Omits statement 3, which is correct.

🎯 Strategy

Used

Elimination

Application:

Check each statement against the definition of atomic mass unit.

Final Logic:

Only statements 1, 2 and 3 are correct.

🧠 Memory Trick

Carbon-12 → Divide by 12

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Atomic Mass Unit

10 If an atom is completely ionized by removing all its electrons, the net electrical charge of the remaining ion is:

A +Ze

B −Ze

C 0

D +e/Z

💡 Short Explanation

�� Electrons carry negative charge. �� Removing all electrons leaves the nucleus. �� Nuclear charge equals +Ze.

✅ Correct Answer Explanation

A neutral atom contains Z protons and Z electrons. When all electrons are removed, only the positively charged nucleus remains. Total charge = +Ze Therefore, option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Nucleus is positively charged.
�� Option C → Charge is not neutral after electron removal.
�� Option D → Incorrect mathematical expression.

🎯 Strategy

Used

Substitution

Application:

Charge = Number of protons × elementary charge.

Final Logic:

Removing all electrons leaves a charge of +Ze.

🧠 Memory Trick

No Electrons → +Ze

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Nuclear Charge and Ionization

11 In any given nuclide denoted by ᴬZX, the total number of neutrons can be determined by

A multiplying A and Z.

B subtracting the atomic number Z from the mass number A.

C adding the mass number to the chemical symbol.

D dividing the mass number by the atomic number.

💡 Short Explanation

�� Mass number equals protons plus neutrons. �� Atomic number equals protons. �� Neutrons are obtained by subtraction.

✅ Correct Answer Explanation

For a nuclide ᴬZX: A = Z + N where A is the mass number, Z is the atomic number and N is the neutron number. Therefore, N = A − Z Hence, option B is correct.

❓ Why Other Options Are Incorrect

�� Option A → Multiplication has no relation to neutron count.
�� Option C → Chemical symbol does not affect neutron number.
�� Option D → Division does not yield neutron number.

🎯 Strategy

Used

Substitution

Application:

Use the relation A = Z + N.

Final Logic:

Neutrons = Mass Number − Atomic Number.

🧠 Memory Trick

N = A − Z

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Atomic Number, Mass Number and Neutron Number

12 Consider a gold nucleus denoted by ¹⁹⁷₇₉Au. What is the exact number of nucleons present in this nucleus?

A 197

B 79

C 118

D 276

💡 Short Explanation

�� Nucleons include protons and neutrons. �� Mass number equals total nucleons. �� Gold has mass number 197.

✅ Correct Answer Explanation

Nucleons are protons and neutrons present in the nucleus. For ¹⁹⁷₇₉Au: Mass number (A) = 197 The mass number directly represents the total number of nucleons. Therefore, the gold nucleus contains 197 nucleons. Hence, option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → 79 is the atomic number (protons only).
�� Option C → 118 is the neutron number (197 − 79).
�� Option D → Sum of unrelated quantities.

🎯 Strategy

Used

Definition Recall

Application:

Recall that mass number equals total nucleons.

Final Logic:

Number of nucleons = Mass number = 197.

🧠 Memory Trick

A = All Nucleons

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Nucleons and Mass Number

13 Statements regarding the historical hypothesis that electrons could reside inside the nucleus:
1. It was confirmed by Chadwick's beryllium experiments.
2. It was ruled out later using strong arguments based on quantum theory.
3. It explains the presence of gamma decay perfectly.
4. It accounts for the difference between atomic mass and mass number.

A (1)

B (2)

C (3)

D (4)

💡 Short Explanation

�� Nuclear electrons hypothesis was proposed historically. �� Quantum mechanics disproved it. �� Electrons are extra-nuclear particles.

✅ Correct Answer Explanation

Statement 1 is incorrect because Chadwick's experiments led to the discovery of the neutron, not confirmation of nuclear electrons. Statement 2 is correct because quantum mechanical arguments showed that confining electrons within the tiny nucleus would require unrealistically high energies. Statement 3 is incorrect because gamma decay is explained by nuclear energy transitions. Statement 4 is incorrect because the difference between atomic mass and mass number is unrelated to the nuclear-electron hypothesis. Therefore, only statement 2 is correct.

❓ Why Other Options Are Incorrect

�� Option A → Statement 1 is incorrect.
�� Option C → Statement 3 is incorrect.
�� Option D → Statement 4 is incorrect.

🎯 Strategy

Used

Elimination

Application:

Evaluate each statement using modern nuclear theory.

Final Logic:

Only statement 2 is scientifically correct.

🧠 Memory Trick

Quantum Theory Removed Nuclear Electrons

📚 Class XII – NCERT Physics Part II – Chapter: Nuclei – Topic: Nuclear Composition

14 Charge of the nucleus, Total charge of atomic electrons:

A +Ze, −Ze

B −Ze, +Ze

C +Ze, +Ze

D −Ze, −Ze

💡 Short Explanation

�� Protons create nuclear charge. �� Electrons carry negative charge. �� Neutral atoms have balanced charges.

✅ Correct Answer Explanation

A nucleus contains Z protons. Charge of nucleus = +Ze A neutral atom contains Z electrons. Total electronic charge = −Ze Therefore, option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Charges are reversed.
�� Option C → Electrons are not positively charged.
�� Option D → Nucleus is not negatively charged.

🎯 Strategy

Used

Definition Recall

Application:

Recall charges of protons and electrons.

Final Logic:

Nucleus = +Ze and electrons = −Ze.

🧠 Memory Trick

Protons Plus, Electrons Minus

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Electrical Neutrality of Atom

15 Match List I (Variables) with List II (Values for Chlorine Isotopes)

List I	List II
1. Mass of lighter isotope	a. 24.6 %
2. Mass of heavier isotope	b. 35.47 u
3. Average atomic mass	c. 34.98 u
4. Percentage of heavier isotope	d. 36.98 u

A (1)-c, (2)-d, (3)-b, (4)-a

B (1)-d, (2)-c, (3)-a, (4)-b

C (1)-c, (2)-b, (3)-d, (4)-a

D (1)-b, (2)-d, (3)-c, (4)-a

💡 Short Explanation

�� Chlorine has two major isotopes. �� Average mass is 35.47 u. �� Heavier isotope abundance is 24.6%.

✅ Correct Answer Explanation

1 → c : Lighter isotope mass = 34.98 u 2 → d : Heavier isotope mass = 36.98 u 3 → b : Average atomic mass = 35.47 u 4 → a : Heavier isotope abundance = 24.6% Therefore, option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Mass values are interchanged.
�� Option C → Average mass is incorrectly assigned.
�� Option D → Lighter isotope mass is incorrectly assigned.

🎯 Strategy

Used

Option Grouping

Application:

Separate isotope masses, average mass and abundance values.

Final Logic:

34.98 u, 36.98 u, 35.47 u and 24.6% match uniquely.

🧠 Memory Trick

35 → 75%, 37 → 25%

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Chlorine Isotopes

16 Statements regarding weighted average atomic mass calculation:
1. It requires the exact masses of all isotopes present.
2. It requires the relative abundances of all isotopes.
3. It merely takes the simple arithmetic mean of the isotopic masses, ignoring abundance.
4. It works out to be exactly 35.47 u for chlorine.

A (1), (2), (4)

B (1), (2), (3)

C (2), (3), (4)

D (1), (3)

💡 Short Explanation

�� Weighted averages require masses. �� Relative abundances are essential. �� Simple arithmetic mean is incorrect.

✅ Correct Answer Explanation

Statement 1 is correct because isotopic masses are needed. Statement 2 is correct because relative abundances must be included. Statement 3 is incorrect because weighted average atomic mass cannot be obtained by taking a simple arithmetic mean. Statement 4 is correct because chlorine has an average atomic mass of approximately 35.47 u. Therefore, statements 1, 2 and 4 are correct.

❓ Why Other Options Are Incorrect

�� Option B → Includes statement 3, which is incorrect.
�� Option C → Includes statement 3, which is incorrect.
�� Option D → Includes statement 3 and excludes statement 2.

🎯 Strategy

Used

Elimination

Application:

Check which statements are necessary for weighted average calculations.

Final Logic:

Masses + abundances are required; simple average is wrong.

🧠 Memory Trick

Weighted = Mass × Abundance

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Average Atomic Mass

17 Incorrect statement regarding free particles:

A A free proton is fundamentally stable.

B A free neutron is fundamentally unstable.

C A free proton decays into a neutron, electron, and antineutrino.

D Neutrons are considered stable only when situated inside a nucleus.

💡 Short Explanation

�� Proton is stable. �� Free neutron decays. �� Proton decay is not observed.

✅ Correct Answer Explanation

A free proton is considered stable. A free neutron is unstable and undergoes beta decay: n → p + e⁻ + ν̅ The statement that a free proton decays into a neutron, electron and antineutrino is incorrect. Therefore, option C is correct.

❓ Why Other Options Are Incorrect

�� Option A → Correct statement.
�� Option B → Correct statement.
�� Option D → Generally true in the NCERT context; many neutrons are stable within nuclei.

🎯 Strategy

Used

Elimination

Application:

Compare known decay processes of neutrons and protons.

Final Logic:

Neutron decays; proton does not.

🧠 Memory Trick

Neutron Decays, Proton Stays

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Free Proton and Free Neutron

18 Statements about the decay of a free neutron:
1. It has a mean life of about 1000 s.
2. It decays into a proton.
3. It emits an electron and an antineutrino during decay.
4. It emits a photon and an alpha particle.

A (1), (2), (3)

B (1), (3), (4)

C (2), (3), (4)

D (1), (2)

💡 Short Explanation

�� Free neutron is unstable. �� Beta decay occurs. �� Mean life is about 1000 s.

✅ Correct Answer Explanation

Statement 1 is correct because a free neutron has a mean life of approximately 1000 s. Statement 2 is correct because neutron decay produces a proton. Statement 3 is correct because an electron and antineutrino are emitted. Statement 4 is incorrect because neutron decay does not produce an alpha particle. Therefore, statements 1, 2 and 3 are correct.

❓ Why Other Options Are Incorrect

�� Option B → Includes statement 4, which is incorrect.
�� Option C → Includes statement 4, which is incorrect.
�� Option D → Omits statement 3, which is correct.

🎯 Strategy

Used

Elimination

Application:

Apply the neutron decay equation.

Final Logic:

n → p + e⁻ + ν̅

🧠 Memory Trick

Neutron → Proton + Electron + Antineutrino

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Neutron Decay

19 In the standard notation ᴬZX, the number of neutrons is mathematically represented as:

A A − Z

B A + Z

C Z − A

D A/Z

💡 Short Explanation

�� Mass number counts nucleons. �� Atomic number counts protons. �� Neutrons are found by subtraction.

✅ Correct Answer Explanation

For a nuclide: A = Z + N Therefore, N = A − Z Hence, option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Gives a larger unrelated value.
�� Option C → Produces incorrect sign.
�� Option D → No physical meaning.

🎯 Strategy

Used

Substitution

Application:

Use the relation A = Z + N.

Final Logic:

Neutrons = Mass Number − Atomic Number.

🧠 Memory Trick

N = A − Z

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Neutron Number

20 Since the atoms of isotopes have identical electronic structure, they

A have identical chemical behaviour and are placed in the same location in the periodic table.

B differ wildly in chemical behaviour but share the exact same physical mass.

C form chemical bonds with completely different sets of valency rules.

D decay at exactly the same rate due to the presence of Z protons.

💡 Short Explanation

�� Chemical properties depend on electrons. �� Isotopes have identical electronic configuration. �� Periodic table position depends on atomic number.

✅ Correct Answer Explanation

Isotopes have the same atomic number and therefore the same electronic configuration. Since chemical behaviour depends mainly on electronic structure, isotopes show nearly identical chemical properties and occupy the same position in the periodic table. Therefore, option A is correct.

❓ Why Other Options Are Incorrect

�� Option B → Physical masses differ and chemical behaviour is similar.
�� Option C → Valency rules remain the same.
�� Option D → Radioactive decay depends on nuclear stability, not merely atomic number.

🎯 Strategy

Used

Definition Recall

Application:

Relate electronic structure to chemical behaviour.

Final Logic:

Same electronic configuration gives similar chemistry.

🧠 Memory Trick

Same Electrons = Same Chemistry

📚 Class XI – NCERT Chemistry Part I – Chapter: Structure of Atom – Topic: Chemical Behaviour of Isotopes

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