📚 Structure of the Atom
Question Paper & Answer Key with Explanations
Total Marks: 80 | Time: 2 Hours
Instructions:
- All questions are compulsory
- Section A: 1-4 marks each (20 marks)
- Section B: 2-4 marks each (30 marks)
- Section C: 4-5 marks each (20 marks)
- Section D: 2-4 marks each (10 marks)
📊 Marks Distribution
Section A (Very Short Answer)
20 marks
Section B (Short Answer)
30 marks
Section C (Long Answer)
20 marks
Section D (Application/Numerical)
10 marks
Total
80 marks
SECTION A: Very Short Answer Questions (1 mark each)
Q1. 1 Mark
What are canal rays?
Q2. 1 Mark
Name the scientist who discovered the electron.
Q3. 1 Mark
What is the charge on a proton?
Q4. 1 Mark
Define atomic number.
Q5. 1 Mark
How many neutrons does a helium atom (mass number 4) have if it has 2 protons?
Q6. 1 Mark
What is the mass number of an atom with 6 protons and 7 neutrons?
Q7. 1 Mark
Define isotopes with one example.
Q8. 1 Mark
What is valency?
Q9. 1 Mark
Who discovered the neutron and in which year?
Q10. 1 Mark
State the octet rule.
Total Section A: 10 marks
SECTION B: Short Answer Questions (2-4 marks each)
Q11. 2 Marks
Compare the properties of electrons and protons.
Q12. 2 Marks
What were the main limitations of Thomson's "Plum Pudding" model of the atom?
Q13. 3 Marks
Explain Rutherford's alpha-particle scattering experiment and the conclusions he drew.
Q14. 2 Marks
What is the main drawback of Rutherford's atomic model?
Q15. 3 Marks
Explain how Bohr's model solved the stability problem of Rutherford's model.
Q16. 2 Marks
What are discrete orbits? Why did Bohr propose them?
Q17. 3 Marks
Write the distribution of electrons in carbon (atomic number 6) and sodium (atomic number 11) atoms.
Q18. 2 Marks
How many maximum electrons can be accommodated in K, L, and M shells?
Q19. 3 Marks
Explain how valency is determined for sodium and chlorine.
Q20. 2 Marks
Define isobars. Give one example.
Total Section B: 24 marks
SECTION C: Long Answer Questions (4-5 marks each)
Q21. 5 Marks
Compare Thomson's, Rutherford's, and Bohr's models of the atom. What was the contribution of each scientist?
Q22. 4 Marks
Describe the rules for distribution of electrons in different shells. Apply these rules to write the electron configuration of the first three elements.
Q23. 5 Marks
Explain the concept of isotopes. How do isotopes of chlorine exist in nature? Why are they chemically similar but physically different?
Q24. 4 Marks
Write the difference between atomic number and mass number. Give examples of two elements showing the relationship between them.
Q25. 5 Marks
Explain why atoms have a tendency to achieve an octet. Using the octet rule, explain the valency of oxygen, fluorine, and magnesium.
Total Section C: 23 marks
SECTION D: Application & Numerical Questions (2-4 marks each)
Q26. 3 Marks
Chlorine occurs in nature in two isotopic forms: 3517Cl (75%) and 3717Cl (25%). Calculate the average atomic mass of chlorine.
Q27. 2 Marks
An atom has atomic number 8 and mass number 16. How many protons, neutrons, and electrons does it have?
Q28. 3 Marks
For the symbol 147N, write the number of protons, neutrons, electrons, and determine its valency.
Q29. 2 Marks
If an element has electronic configuration 2, 8, 1, what is its atomic number? Name the element and give its valency.
Total Section D: 10 marks
📋 ANSWER KEY & EXPLANATIONS
Complete Solutions with Detailed Explanations
SECTION A: ANSWER KEY
Q1. Answer 1 Mark
Answer:
Canal rays are positively charged radiations discovered by E. Goldstein in a gas discharge tube, which eventually led to the discovery of protons.
Explanation:
Canal rays are charged particles that move in the opposite direction to cathode rays. They helped scientists discover that atoms contain positive charges, leading to the identification of protons as sub-atomic particles.
Q2. Answer 1 Mark
Answer:
J.J. Thomson discovered the electron in 1897.
Explanation:
Thomson used a cathode ray tube experiment to identify the electron as the first sub-atomic particle, proving that atoms are divisible and contain negatively charged particles.
Q3. Answer 1 Mark
Answer:
The charge on a proton is +1 (positive one unit).
Explanation:
Protons are positively charged particles found in the nucleus. The proton's charge is equal in magnitude but opposite in sign to the electron's charge (−1), which is why atoms are electrically neutral when the number of protons equals the number of electrons.
Q4. Answer 1 Mark
Answer:
Atomic number (Z) is the total number of protons present in the nucleus of an atom. It defines the element.
Explanation:
The atomic number uniquely identifies each element. All atoms of the same element have the same atomic number. For example, all hydrogen atoms have Z=1 (1 proton), all carbon atoms have Z=6 (6 protons), etc.
Q5. Answer 1 Mark
Answer:
Number of neutrons = Mass number − Protons = 4 − 2 = 2 neutrons
Explanation:
The mass number includes both protons and neutrons. Subtracting the number of protons from the mass number gives us the number of neutrons. This formula is fundamental: Mass Number (A) = Protons (Z) + Neutrons (N)
Q6. Answer 1 Mark
Answer:
Mass number = Protons + Neutrons = 6 + 7 = 13
Explanation:
Mass number (A) is the sum of all nucleons (protons and neutrons) in the nucleus. Since the atom has 6 protons, it is carbon (C), and with 7 neutrons, the complete notation would be 136C (Carbon-13).
Q7. Answer 1 Mark
Answer:
Isotopes are atoms of the same element having the same atomic number but different mass numbers.
Example: 126C and 146C (both are carbon with 6 protons but different numbers of neutrons)
Example: 126C and 146C (both are carbon with 6 protons but different numbers of neutrons)
Explanation:
Since isotopes have the same number of protons, they are the same element chemically. However, due to different numbers of neutrons, they have different mass numbers and thus different physical properties like density and radioactivity.
Q8. Answer 1 Mark
Answer:
Valency is the combining capacity of an atom, determined by the number of electrons an atom must gain, lose, or share to achieve a stable octet.
Explanation:
Valency depends on the number of valence electrons (electrons in the outermost shell). Atoms try to achieve 8 electrons in their outermost shell to become stable. The number of electrons gained, lost, or shared to achieve this stable configuration is the valency.
Q9. Answer 1 Mark
Answer:
J. Chadwick discovered the neutron in 1932.
Explanation:
Chadwick's discovery of the neutral neutron completed the picture of atomic structure. The neutron has negligible charge and a mass nearly equal to that of a proton, and it is present in the nucleus of all atoms except hydrogen.
Q10. Answer 1 Mark
Answer:
The octet rule states that atoms tend to achieve stability by completing their outermost electron shell with 8 electrons (or 2 for the first shell).
Explanation:
The octet rule explains why atoms react and form bonds. Noble gases like neon (2,8) and argon (2,8,8) already have complete octets and are highly stable, showing little chemical activity. Other atoms gain, lose, or share electrons to achieve this stable configuration.
SECTION B: ANSWER KEY
Q11. Answer 2 Marks
Answer:
| Property | Electron | Proton |
|---|---|---|
| Charge | −1 (negative) | +1 (positive) |
| Location | Orbiting nucleus | Inside nucleus |
| Mass | 1/2000 of proton | 1 unit (reference) |
| Discovered by | J.J. Thomson | E. Goldstein |
Explanation:
Electrons and protons are complementary particles with opposite charges. Electrons orbit the nucleus and are much lighter, while protons reside in the nucleus and carry most of the atomic mass. Their equal magnitudes of charge (but opposite signs) ensure electrical neutrality in atoms.
Q12. Answer 2 Marks
Answer:
Main limitations of Thomson's "Plum Pudding" Model:
- Could not explain experimental results: Results of later experiments could not be explained by this model.
- Incorrect structure: The model suggested the positive charge was spread throughout the atom, which was later disproven.
- Failed to account for nucleus: It did not predict the existence of a concentrated nucleus with a very high charge density.
Explanation:
Thomson's model was the first attempt to explain atomic structure and correctly identified that atoms are electrically neutral and contain electrons. However, Rutherford's alpha particle scattering experiment revealed that this model was fundamentally incorrect, as it could not account for the strong deflection of some alpha particles.
Q13. Answer 3 Marks
Answer:
Rutherford's Alpha-Particle Scattering Experiment:
- Setup: Fast-moving alpha particles were directed at a thin gold foil (~1000 atoms thick)
- Observations:
- Most particles passed straight through
- Some were deflected by small angles
- 1 in 12,000 particles bounced back
- Conclusions:
- Most space inside atom is empty
- Positive charge concentrated in small nucleus
- Nucleus has 105 times smaller radius than atom
Explanation:
This groundbreaking experiment revolutionized atomic physics. The fact that most particles passed through indicated that atoms are mostly empty space. The few particles that bounced back suggested a small, dense, positively charged nucleus at the atom's center. This disproved Thomson's model and led to the nuclear model of the atom.
Q14. Answer 2 Marks
Answer:
Main Drawback: According to classical physics, revolving electrons undergo acceleration and should continuously radiate energy, causing them to lose energy and spiral into the nucleus. This would make atoms highly unstable, but we know atoms are stable.
Explanation:
Rutherford's model correctly identified the nucleus but failed to explain atomic stability. The model predicted that atoms should collapse within fractions of a second, which is obviously not what we observe in nature. This instability problem was solved by Niels Bohr's introduction of discrete orbits.
Q15. Answer 3 Marks
Answer:
How Bohr's Model Solved the Stability Problem:
- Discrete Orbits: Bohr proposed that electrons can only revolve in certain special orbits (discrete orbits or shells) with specific energy levels.
- No Energy Radiation: While revolving in these discrete orbits, electrons do not radiate energy, maintaining their orbit.
- Stability Explanation: Since electrons don't lose energy while in discrete orbits, they don't fall into the nucleus, explaining why atoms are stable.
Explanation:
Bohr's postulates were revolutionary because they introduced quantum concepts (discreteness of energy) to explain atomic structure. This model successfully explained the stability of atoms and the spectral lines of hydrogen. It bridged classical physics (Rutherford) and quantum mechanics, making it a major advancement in atomic theory.
Q16. Answer 2 Marks
Answer:
Discrete Orbits: Discrete orbits are specific, allowed paths around the nucleus where electrons can revolve without radiating energy. Each orbit corresponds to a specific energy level.
Why Bohr Proposed Them: To explain why Rutherford's model predicted atom instability. Bohr proposed discrete orbits to maintain atomic stability while keeping the nuclear model intact.
Why Bohr Proposed Them: To explain why Rutherford's model predicted atom instability. Bohr proposed discrete orbits to maintain atomic stability while keeping the nuclear model intact.
Explanation:
The concept of discrete orbits introduced quantization to atomic models. Instead of electrons being able to orbit at any distance with any energy, Bohr suggested that only certain orbits are allowed. This explains why atoms don't collapse and why electrons maintain stable orbits.
Q17. Answer 3 Marks
Answer:
Carbon (Atomic number 6):
Sodium (Atomic number 11):
Electronic configuration: 2, 4
Distribution: K shell (2 electrons), L shell (4 electrons)
Sodium (Atomic number 11):
Electronic configuration: 2, 8, 1
Distribution: K shell (2 electrons), L shell (8 electrons), M shell (1 electron)
Explanation:
The Bohr-Bury scheme states that the maximum number of electrons in a shell is 2n², so K shell = 2, L shell = 8, M shell = 18, etc. Electrons fill inner shells completely before entering outer shells. Carbon has 6 electrons distributed as 2,4 while sodium has 11 electrons distributed as 2,8,1. The 1 electron in sodium's outermost shell is its valence electron, giving sodium a valency of 1.
Q18. Answer 2 Marks
Answer:
Maximum electrons using formula 2n²:
K shell (n=1): 2×1² = 2 electrons
L shell (n=2): 2×2² = 8 electrons
M shell (n=3): 2×3² = 18 electrons
Explanation:
The formula 2n² gives the maximum electrons in each shell based on the shell number (n). However, there's an additional rule: the outermost shell can hold maximum 8 electrons only. So M shell can hold up to 8 when it's the outermost shell, but can hold up to 18 when there's another shell beyond it.
Q19. Answer 3 Marks
Answer:
Sodium (Z = 11):
Chlorine (Z = 17):
- Electronic configuration: 2, 8, 1
- Valence electrons: 1 in outermost shell
- Action: Loses 1 electron to achieve octet
- Valency: 1
Chlorine (Z = 17):
- Electronic configuration: 2, 8, 7
- Valence electrons: 7 in outermost shell
- Action: Gains 1 electron (easier than losing 7) to achieve octet
- Valency: 1
Explanation:
Valency is determined by the easiest way for an atom to achieve an octet. Sodium with 1 valence electron easily loses it. Chlorine with 7 valence electrons would more easily gain 1 electron than lose 7. Both achieve a stable octet (Sodium becomes Na⁺ with 2,8 configuration; Chlorine becomes Cl⁻ with 2,8,8 configuration) and both have valency 1.
Q20. Answer 2 Marks
Answer:
Definition: Isobars are atoms of different elements having the same mass number but different atomic numbers.
Example: Calcium (4020Ca) and Argon (4018Ar) are isobars.
Example: Calcium (4020Ca) and Argon (4018Ar) are isobars.
- Both have mass number = 40
- But Ca has Z=20 and Ar has Z=18 (different elements)
Explanation:
Isobars are different from isotopes. While isotopes are atoms of the same element with different mass numbers, isobars are atoms of different elements with the same mass number. They have different chemical properties because they have different numbers of electrons (different atomic numbers).
SECTION C: ANSWER KEY
Q21. Answer 5 Marks
Answer:
1. Thomson's "Plum Pudding" Model (1898):
2. Rutherford's Nuclear Model (1911):
3. Bohr's Model (1913):
- Atom is a sphere of positive charge with electrons embedded in it (like raisins in pudding)
- Contribution: First atomic model; explained electrical neutrality; identified electron as sub-atomic particle
- Limitation: Could not explain experimental results; no nucleus prediction
2. Rutherford's Nuclear Model (1911):
- Based on alpha-particle scattering experiment; proposed concentrated nucleus with orbiting electrons
- Contribution: Discovered nucleus; explained alpha-particle scattering; proved most of atom is empty space
- Limitation: Couldn't explain atomic stability (electrons should radiate energy and fall)
3. Bohr's Model (1913):
- Proposed discrete orbits where electrons revolve without radiating energy
- Contribution: Solved stability problem; explained discrete energy levels; successfully predicted hydrogen spectrum
- Limitation: Limited to hydrogen; doesn't explain complex atoms perfectly
Explanation:
Each model improved upon its predecessor. Thomson established that atoms are divisible. Rutherford discovered the nucleus and explained atomic structure. Bohr's addition of discrete orbits completed the classical atomic model and successfully explained atomic stability and spectra. These three scientists built the foundation for modern atomic physics through incremental discoveries and refinements.
Q22. Answer 4 Marks
Answer:
Rules for Electron Distribution (Bohr-Bury Scheme):
Application to First Three Elements:
- Maximum electrons formula: Maximum electrons in nth shell = 2n² (where n = 1, 2, 3...)
- Outermost shell limit: Maximum 8 electrons in outermost shell
- Step-wise filling: Inner shells must be completely filled before electrons enter outer shells
Application to First Three Elements:
- Hydrogen (Z=1): Configuration = 1
K shell: 1 electron
- Helium (Z=2): Configuration = 2
K shell: 2 electrons (K shell full, maximum = 2×1² = 2)
- Lithium (Z=3): Configuration = 2, 1
K shell: 2 electrons (full), L shell: 1 electron
Explanation:
The Bohr-Bury scheme provides a systematic way to determine electron configurations. The formula 2n² ensures that each shell has specific capacity. The step-wise filling rule means we must fill K shell (max 2) before L shell (max 8). This explains why lithium has its 3rd electron in L shell rather than all 3 in K shell.
Q23. Answer 5 Marks
Answer:
Concept of Isotopes: Isotopes are atoms of the same element having the same atomic number but different mass numbers. They contain the same number of protons but different numbers of neutrons.
Chlorine Isotopes in Nature:
Why Chemically Similar but Physically Different:
Chlorine Isotopes in Nature:
- 3517Cl: 17 protons, 18 neutrons, mass number = 35 (75% abundant)
- 3717Cl: 17 protons, 20 neutrons, mass number = 37 (25% abundant)
- Both are chlorine atoms with Z = 17, but different mass numbers
Why Chemically Similar but Physically Different:
- Chemically Similar: Chemical properties depend on electrons and electronic configuration. Both isotopes have 17 electrons, so same electron configuration (2, 8, 7), leading to same valency and chemical behavior.
- Physically Different: Physical properties depend on mass and nuclear properties. Different neutron numbers cause different atomic masses, affecting density, boiling point, and radioactivity.
Explanation:
Isotopes are a key concept in modern chemistry and physics. The existence of isotopes in nature is why we use average atomic masses in chemistry. Both chlorine isotopes react identically with other elements to form compounds like NaCl, but they have different densities and mass-dependent properties. Some isotopes are radioactive and used in medical applications.
Q24. Answer 4 Marks
Answer:
Differences:
Examples:
| Atomic Number (Z) | Mass Number (A) |
|---|---|
| Total number of protons in nucleus | Sum of protons and neutrons (nucleons) |
| Defines the element | Defines the isotope |
| Same for all atoms of an element | Different for different isotopes |
| Denoted by Z | Denoted by A |
Examples:
- Carbon: 126C (Z=6, A=12) and 146C (Z=6, A=14)
Both have atomic number 6 (same element) but different mass numbers
- Oxygen: 168O (Z=8, A=16) and 188O (Z=8, A=18)
Both have atomic number 8 but different mass numbers due to different neutrons
Explanation:
Understanding atomic and mass numbers is crucial for identifying elements and their isotopes. Atomic number uniquely identifies an element - it never changes for that element. Mass number changes with the number of neutrons, giving us different isotopes of the same element. The notation AZX clearly shows both numbers for any atom.
Q25. Answer 5 Marks
Answer:
Why Atoms Achieve Octet:
Valency Using Octet Rule:
- Atoms with 8 electrons in outermost shell (or 2 for first shell) are extremely stable
- Noble gases like Ne (2, 8), Ar (2, 8, 8) have full octets and show no chemical activity
- Other atoms react to achieve this stable octet configuration by gaining, losing, or sharing electrons
Valency Using Octet Rule:
- Oxygen (Z=8): Configuration = 2, 6
Needs 2 more electrons to achieve octet → Gains 2 electrons → Valency = 2
- Fluorine (Z=9): Configuration = 2, 7
Needs 1 more electron to achieve octet (easier than losing 7) → Gains 1 electron → Valency = 1
- Magnesium (Z=12): Configuration = 2, 8, 2
Easier to lose 2 electrons than gain 6 → Loses 2 electrons → Valency = 2
Explanation:
The octet rule is fundamental to understanding chemical bonding. Atoms achieve octet by the easiest method - either gaining, losing, or sharing electrons. The direction is determined by what requires fewer electrons to be moved. This concept explains why sodium (1 valence electron) readily loses it to form Na⁺, while chlorine (7 valence electrons) readily gains one to form Cl⁻. Both end up with stable octets.
SECTION D: ANSWER KEY (Application & Numerical)
Q26. Answer 3 Marks
Answer:
Calculation:
Answer: Average atomic mass of chlorine = 35.5 u
Average atomic mass = (Mass₁ × % abundance₁) + (Mass₂ × % abundance₂) / 100
= (35 × 75) + (37 × 25) / 100
= (2625) + (925) / 100
= 3550 / 100
= 35.5 u
Answer: Average atomic mass of chlorine = 35.5 u
Explanation:
The average atomic mass accounts for the natural abundance of isotopes. Since chlorine naturally contains 75% of 35Cl and 25% of 37Cl, we calculate the weighted average. This is why the atomic mass in the periodic table (≈35.5) is not a whole number. This same principle applies to all elements that have naturally occurring isotopes.
Q27. Answer 2 Marks
Answer:
Given: Atomic number (Z) = 8, Mass number (A) = 16
Calculations:
Answer: Protons = 8, Neutrons = 8, Electrons = 8 (Oxygen-16)
Calculations:
Number of protons: = Atomic number = 8
Number of electrons: = Atomic number = 8
Number of neutrons: = Mass number − Protons = 16 − 8 = 8
Explanation:
In a neutral atom, the number of electrons equals the number of protons. Using the formulas: Z = number of protons, A = Z + N (neutrons), we can determine all sub-atomic particles. This atom is oxygen-16, the most common isotope of oxygen with 8 neutrons.
Q28. Answer 3 Marks
Answer:
For 147N:
- Number of protons: = Atomic number (Z) = 7
- Number of neutrons: = Mass number (A) − Protons = 14 − 7 = 7
- Number of electrons: = Atomic number = 7 (neutral atom)
- Electronic configuration: 2, 5
- Valence electrons: 5 in outermost shell
- Valency determination:
Needs 3 more electrons to achieve octet (8 − 5 = 3) → Valency = 3
Explanation:
Nitrogen is a non-metal that typically shows a valency of 3 or 5 depending on the compound it forms. With 5 valence electrons, it needs 3 more to complete an octet, hence valency 3. In some compounds, it shows valency 5 by sharing electrons in different ways. The most common valency for nitrogen is 3 (as in NH₃, ammonia).
Q29. Answer 2 Marks
Answer:
Given electronic configuration: 2, 8, 1
Calculations:
Explanation: Sodium with 11 electrons has electronic configuration 2, 8, 1. It has only 1 valence electron, which it easily loses to form Na⁺ ion with stable octet configuration of 2, 8.
Calculations:
- Atomic number: Total electrons = 2 + 8 + 1 = 11
- Element name: Sodium (Na) (Z = 11)
- Valence electrons: 1 electron in outermost shell
- Valency: 1 (loses 1 electron to achieve octet)
Explanation: Sodium with 11 electrons has electronic configuration 2, 8, 1. It has only 1 valence electron, which it easily loses to form Na⁺ ion with stable octet configuration of 2, 8.
Explanation:
This question tests the ability to work backwards from electron configuration. By counting total electrons, we determine the atomic number and thus identify the element. Sodium is highly reactive because it readily loses its single valence electron to achieve the stable octet of neon. This is why sodium compounds are so common in chemistry.
📌 IMPORTANT FORMULAS & KEY CONCEPTS SUMMARY
Sub-atomic Particles
Electron (e⁻): Charge = −1, Location = Orbits nucleus, Mass = negligible
Proton (p⁺): Charge = +1, Location = Nucleus, Mass = 1 unit
Neutron (n): Charge = 0, Location = Nucleus, Mass = 1 unit
Atomic Structure Formulas
Atomic Number (Z) = Number of Protons
Mass Number (A) = Protons + Neutrons
Number of Neutrons = A − Z
Maximum electrons in shell = 2n² (where n = shell number)
Bohr-Bury Rules for Electron Distribution
- Maximum electrons in K shell (n=1) = 2 × 1² = 2
- Maximum electrons in L shell (n=2) = 2 × 2² = 8
- Maximum electrons in M shell (n=3) = 2 × 3² = 18
- Outermost shell can hold maximum = 8 electrons
- Shells fill in step-wise manner (K → L → M → N)
Valency & Octet Rule
Octet = 8 electrons in outermost shell (or 2 for K shell)
Valency = Electrons to gain/lose/share to achieve octet
For elements with >4 valence electrons: Valency = 8 − (number of valence electrons)
Classification
Isotopes: Same element (same Z), different mass numbers (different A)
Isobars: Different elements (different Z), same mass number (same A)
Atomic Models in Order: Thomson → Rutherford → Bohr
✓ COMMON MISTAKES TO AVOID
❌ Mistake 1: Confusing Atomic Number with Mass Number
✓ Remember: Atomic number = Protons (defines element), Mass number = Protons + Neutrons
❌ Mistake 2: Not filling shells step-wise
✓ Remember: K shell must be full (2) before L shell gets any electrons
❌ Mistake 3: Confusing Isotopes with Isobars
✓ Isotopes = Same element, Isobars = Different elements
❌ Mistake 4: Incorrect valency calculation
✓ Choose the easier path: gain electrons if <4 valence, lose if exactly 1-3 valence
❌ Mistake 5: Ignoring the octet rule
✓ Most atoms try to achieve 8 electrons in outermost shell to become stable
Total Marks: 80 | Time: 2 Hours
This question paper comprehensively covers all topics from Chapter 4: Structure of the Atom
Good luck with your studies! 📚
