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These are complete exercise solutions for Samacheer Kalvi 9th Science — Atomic Structure. Solve each question yourself first, then verify your method and answer below.
Key Concepts (Quick Recap)
- Atom: The smallest particle of an element that retains its chemical properties.
- Subatomic particles: Electron (−1, negligible mass, orbits nucleus), Proton (+1, 1 u, in nucleus), Neutron (0, 1 u, in nucleus).
- Atomic number (Z): Number of protons in the nucleus. In a neutral atom, Z = number of protons = number of electrons.
- Mass number (A): A = number of protons + number of neutrons.
- Electronic configuration: Arrangement of electrons in shells (K=2, L=8, M=18, N=32). Maximum electrons per shell = 2n².
- Valency: The combining capacity of an atom, determined by the number of valence electrons (outermost shell).
- Isotopes: Same atomic number, different mass number (same element, different neutrons). e.g., ¹H, ²H, ³H.
- Isobars: Same mass number, different atomic number (different elements). e.g., ⁴⁰Ar and ⁴⁰Ca.
- Thomson's model: Plum pudding — electrons embedded in a sphere of positive charge.
- Rutherford's model: Nuclear atom — small dense nucleus with electrons orbiting like planets.
- Bohr's model: Electrons revolve in fixed orbits (energy levels) without radiating energy.
I. Choose the Correct Answer
Q1. The positively charged particle in an atom is: (a) Electron (b) Proton ✓ (c) Neutron (d) Meson
Answer(b) ✓ — Protons carry a positive charge (+1) and are located in the nucleus.
Q2. The nucleus of an atom contains: (a) Only protons (b) Only neutrons (c) Protons and neutrons ✓ (d) Protons and electrons
Answer(c) ✓ — The nucleus contains protons (positively charged) and neutrons (neutral). Electrons orbit outside the nucleus.
Q3. The maximum number of electrons that can be accommodated in the L shell is: (a) 2 (b) 8 ✓ (c) 18 (d) 32
Answer(b) ✓ — Maximum electrons in a shell = 2n². For L shell (n=2): 2(2)² = 8.
Q4. Isotopes have: (a) Same mass number (b) Same number of neutrons (c) Same atomic number ✓ (d) Different atomic number
Answer(c) ✓ — Isotopes are atoms of the same element, so they have the same atomic number (same number of protons) but different mass numbers (different neutrons).
Q5. Isobars have: (a) Same atomic number (b) Same number of protons (c) Same number of electrons (d) Same mass number ✓
Answer(d) ✓ — Isobars are atoms of different elements with the same mass number but different atomic numbers. e.g., ⁴⁰Ar (Z=18) and ⁴⁰Ca (Z=20).
Q6. The valency of an element with electronic configuration 2, 8, 7 is: (a) 1 ✓ (b) 7 (c) 2 (d) 8
Answer(a) ✓ — The element has 7 valence electrons. It needs 1 more electron to complete its octet, so its valency is 1.
Q7. Who proposed the nuclear model of the atom? (a) J.J. Thomson (b) Niels Bohr (c) Ernest Rutherford ✓ (d) John Dalton
Answer(c) ✓ — Rutherford proposed the nuclear model based on his gold foil experiment (1911).
Q8. The element with electronic configuration 2, 8, 1 is: (a) Neon (b) Sodium ✓ (c) Magnesium (d) Fluorine
Answer(b) ✓ — Sodium (Na) has atomic number 11. Electronic configuration: K(2), L(8), M(1).
Q9. The number of neutrons in ²³Na¹¹ is: (a) 11 (b) 12 ✓ (c) 23 (d) 34
Answer(b) ✓ — Number of neutrons = Mass number − Atomic number = 23 − 11 = 12.
Q10. Cathode rays are: (a) Protons (b) Neutrons (c) Electrons ✓ (d) Alpha particles
Answer(c) ✓ — Cathode rays are streams of electrons discovered by J.J. Thomson in 1897.
II. Fill in the Blanks
1. The smallest particle of an element that retains its chemical properties is called an atom. 2. The atomic number of an element is equal to the number of protons in the nucleus. 3. The mass number of an atom is the sum of protons and neutrons. 4. The maximum number of electrons in the K shell is 2. 5. The maximum number of electrons that can be accommodated in the third shell (M shell) is 18. 6. Isotopes of an element have the same atomic number but different mass numbers. 7. Isobars have the same mass number but different atomic numbers. 8. The negatively charged particle in an atom is the electron. 9. The nucleus of an atom is composed of protons and neutrons. 10. The electronic configuration of carbon (Z=6) is 2, 4.
III. True or False
1. An atom is electrically neutral.
AnswerTrue — In a neutral atom, the number of protons equals the number of electrons, so the net charge is zero.
2. Electrons are present inside the nucleus.
AnswerFalse — Correction: Electrons orbit outside the nucleus in defined shells/orbits. The nucleus contains protons and neutrons.
3. Isotopes have different chemical properties.
AnswerFalse — Correction: Isotopes have the same chemical properties because they have the same electronic configuration (same number of protons and electrons).
4. The mass of an atom is mainly concentrated in the nucleus.
AnswerTrue — Protons and neutrons (in the nucleus) account for almost all the mass of an atom; electrons have negligible mass.
5. Isobars belong to the same element.
AnswerFalse — Correction: Isobars belong to different elements because they have different atomic numbers (different number of protons).
6. The K shell can hold a maximum of 8 electrons.
AnswerFalse — Correction: The K shell (n=1) can hold a maximum of 2 electrons (2n² = 2×1² = 2).
7. Rutherford's gold foil experiment led to the discovery of the nucleus.
AnswerTrue — Most alpha particles passed through, some deflected, and very few bounced back, leading Rutherford to conclude that atoms have a small, dense, positively charged nucleus.
8. The number of valence electrons determines the chemical properties of an element.
AnswerTrue — Elements with the same number of valence electrons show similar chemical properties.
IV. Match the Following
Set 1: Scientists and Models
| Column I | Column II |
|---|
| 1. J.J. Thomson | (b) Plum pudding model |
| 2. Rutherford | (a) Nuclear model |
| 3. Niels Bohr | (c) Fixed orbits model |
| 4. Chadwick | (d) Discovery of neutron |
| 5. Goldstein | (e) Discovery of canal rays (protons) |
Set 2: Particles and Properties
| Column I | Column II |
|---|
| 1. Electron | (c) Negatively charged, negligible mass |
| 2. Proton | (a) Positively charged, 1 u mass |
| 3. Neutron | (d) No charge, 1 u mass |
| 4. Alpha particle | (b) 2 protons + 2 neutrons |
| 5. Nucleus | (e) Contains protons and neutrons |
V. Very Short Answers
Q1. Define atomic number.
Answer
Atomic number (Z) is the number of protons in the nucleus of an atom. In a neutral atom, atomic number = number of protons = number of electrons.
Q2. Define mass number.
Answer
Mass number (A) is the sum of the number of protons and neutrons in the nucleus of an atom. A = Number of protons + Number of neutrons.
Q3. What are isotopes? Give one example.
Answer
Isotopes are atoms of the same element having the same atomic number but different mass numbers (i.e., same number of protons, different number of neutrons). Example: Hydrogen has three isotopes — Protium (¹H), Deuterium (²H), Tritium (³H).
Q4. What are isobars? Give one example.
Answer
Isobars are atoms of different elements having the same mass number but different atomic numbers. Example: ⁴⁰₁₈Ar (argon) and ⁴⁰₂₀Ca (calcium) — both have mass number 40 but different atomic numbers.
Q5. Write the electronic configuration of chlorine (Z=17).
Answer
Chlorine has 17 electrons. Electronic configuration: K(2), L(8), M(7) = 2, 8, 7
Q6. What is valency?
Answer
Valency is the combining capacity of an atom, determined by the number of electrons it can lose, gain, or share to achieve a stable electronic configuration (octet in the outermost shell).
Q7. State the maximum number of electrons in a shell.
Answer
The maximum number of electrons in the nth shell = 2n².
- K shell (n=1): 2
- L shell (n=2): 8
- M shell (n=3): 18
- N shell (n=4): 32
VI. Short Answers
Q1. Differentiate between isotopes and isobars.
Answer
| Isotopes | Isobars |
|---|
| Same atomic number (Z) | Different atomic number (Z) |
| Different mass number (A) | Same mass number (A) |
| Same element | Different elements |
| Same number of protons | Different number of protons |
| Different number of neutrons | Different number of neutrons |
| Same chemical properties | Different chemical properties |
| Different physical properties | Different physical properties |
| e.g., ¹H, ²H, ³H | e.g., ⁴⁰Ar, ⁴⁰Ca |
Q2. Describe Thomson's model of the atom.
Answer
J.J. Thomson proposed the "Plum Pudding" model (1897): 1. An atom is a sphere of positive charge. 2. Electrons are embedded within this sphere like plums in a pudding. 3. The total positive charge equals the total negative charge, making the atom neutral. 4. The atom is a homogeneous mixture of positive and negative charges.
Limitations: Could not explain Rutherford's gold foil experiment results (alpha particle scattering).
Q3. Describe Rutherford's gold foil experiment and its conclusions.
Answer
Experiment: Rutherford directed a beam of alpha particles at a thin gold foil. A fluorescent screen surrounded the foil to detect scattered particles.
Observations: 1. Most alpha particles passed straight through the foil (atom is mostly empty space). 2. Some particles were deflected at small angles (positive charge repels positive). 3. Very few particles bounced back (hitting a small, dense, positive nucleus).
Conclusions: 1. An atom has a small, dense, positively charged nucleus at its centre. 2. The nucleus contains protons; electrons orbit around it. 3. Most of the atom is empty space. 4. The size of the nucleus is very small compared to the atom.
Q4. What are the applications of isotopes?
Answer
1. Medical: Cobalt-60 for cancer treatment, Iodine-131 for thyroid disorders, Technetium-99m for imaging. 2. Agricultural: Phosphorus-32 as a tracer to study fertiliser uptake by plants. 3. Industrial: Gamma rays from Cobalt-60 for detecting flaws in metal structures. 4. Archaeological: Carbon-14 dating to determine the age of fossils and archaeological artefacts. 5. Energy: Uranium-235 and Plutonium-239 as fuel in nuclear reactors.
Q5. Write the electronic configuration and valency of the following elements:
Answer
| Element | Atomic Number | Electronic Configuration | Valency |
|---|
| Hydrogen | 1 | 1 | 1 |
| Helium | 2 | 2 | 0 (stable) |
| Carbon | 6 | 2, 4 | 4 |
| Nitrogen | 7 | 2, 5 | 3 |
| Oxygen | 8 | 2, 6 | 2 |
| Sodium | 11 | 2, 8, 1 | 1 |
| Magnesium | 12 | 2, 8, 2 | 2 |
| Chlorine | 17 | 2, 8, 7 | 1 |
| Calcium | 20 | 2, 8, 8, 2 | 2 |
Q6. Explain Bohr's model of the atom.
Answer
Niels Bohr (1913) proposed modifications to Rutherford's model: 1. Electrons revolve in certain fixed circular paths called orbits or energy levels (K, L, M, N). 2. Each orbit has a fixed energy; electrons do not radiate energy while revolving in a fixed orbit. 3. An electron can jump from a lower energy orbit to a higher energy orbit by absorbing energy. 4. When an electron jumps from a higher energy orbit to a lower one, it emits energy as radiation. 5. The energy of an orbit increases with its distance from the nucleus: K < L < M < N.
Q7. Differentiate between protons, neutrons, and electrons.
Answer
| Property | Proton | Neutron | Electron |
|---|
| Charge | +1.6 × 10⁻¹⁹ C | 0 | −1.6 × 10⁻¹⁹ C |
| Mass | 1.672 × 10⁻²⁴ g (≈1 u) | 1.675 × 10⁻²⁴ g (≈1 u) | 9.1 × 10⁻²⁸ g (≈1/1837 u) |
| Location | In nucleus | In nucleus | Outside nucleus |
| Discovered by | Goldstein (1886) | Chadwick (1932) | J.J. Thomson (1897) |
| Symbol | p⁺ | n⁰ | e⁻ |
VII. Detailed Answers
Q1. Describe the discovery of subatomic particles.
Answer
Discovery of the Electron (J.J. Thomson, 1897):
- Thomson conducted cathode ray tube experiments.
- Cathode rays were found to be negatively charged particles that deflected towards the positive plate.
- These particles were named electrons (e⁻).
- Properties: negatively charged, very light (1/1837 of a proton), present in all atoms.
Discovery of the Proton (Goldstein, 1886):
- Goldstein used a perforated cathode in a discharge tube and observed rays moving in the opposite direction to cathode rays.
- These were called canal rays (anode rays), later identified as protons.
- Protons are positively charged particles, 1837 times heavier than electrons.
Discovery of the Neutron (Chadwick, 1932):
- Rutherford had predicted the existence of neutral particles in the nucleus.
- Chadwick bombarded beryllium with alpha particles and observed a neutral radiation.
- These neutral particles were named neutrons.
- Neutrons have mass approximately equal to protons but carry no charge.
Q2. Compare Thomson's, Rutherford's, and Bohr's atomic models.
Answer
| Feature | Thomson's Model | Rutherford's Model | Bohr's Model |
|---|
| Year | 1897 | 1911 | 1913 |
| Structure | Sphere of positive charge with electrons embedded | Small dense nucleus with electrons orbiting | Nucleus with electrons in fixed energy levels |
| Nucleus | No distinct nucleus | Small, dense, positive nucleus | Small, dense, positive nucleus |
| Electrons | Embedded in positive sphere | Orbit nucleus (any path) | Revolve in fixed orbits/energy levels |
| Stability | Inherently unstable (should collapse) | Cannot explain stability or spectra | Stable orbits (electrons don't radiate energy) |
| Limitations | Could not explain scattering experiment | Could not explain stability or spectra | Could not explain spectra of multi-electron atoms |
Q3. Explain how the electronic configuration determines the chemical properties of elements.
Answer
The chemical properties of an element are determined by the number of electrons in its outermost shell (valence electrons).
1. Elements with 1, 2, or 3 valence electrons tend to lose them and form positive ions (cations). These are metals with low valency.
- Na (2,8,1) → loses 1 electron → Na⁺ (valency = 1)
- Mg (2,8,2) → loses 2 electrons → Mg²⁺ (valency = 2)
2. Elements with 5, 6, or 7 valence electrons tend to gain electrons to complete their octet and form negative ions (anions). These are non-metals.
- Cl (2,8,7) → gains 1 electron → Cl⁻ (valency = 1)
- O (2,6) → gains 2 electrons → O²⁻ (valency = 2)
3. Elements with 8 valence electrons (octet) are stable and chemically inert (noble gases).
- Ne (2,8) → stable, valency = 0
- Ar (2,8,8) → stable, valency = 0
4. Elements with 4 valence electrons tend to share electrons (covalent bonding).
- C (2,4) → shares 4 electrons → valency = 4
Noble gas configuration (octet) is the driving force behind chemical reactions — atoms react to achieve a stable electronic configuration.
Common Mistakes
1. Confusing atomic number and mass number — Atomic number (Z) = protons only; Mass number (A) = protons + neutrons. 2. Saying electrons are inside the nucleus — Electrons orbit outside the nucleus in defined shells. Only protons and neutrons are in the nucleus. 3. Confusing isotopes and isobars — Isotopes: same element, same Z, different A. Isobars: different elements, different Z, same A. 4. Wrong shell capacity formula — Maximum electrons = 2n² (where n = shell number). K=2, L=8, M=18, N=32. 5. Forgetting that isotopes have the same chemical properties — Chemical properties depend on electronic configuration (number of electrons), which is the same for isotopes. 6. Confusing Thomson's and Rutherford's models — Thomson: "plum pudding" (positive sphere with embedded electrons); Rutherford: "nuclear model" (small dense nucleus with orbiting electrons). 7. Wrong valency determination — For elements with 5, 6, or 7 valence electrons, valency = 8 − valence electrons (not the number of valence electrons). e.g., Cl has 7 valence electrons; valency = 8 − 7 = 1.
Exam Checklist
- [ ] Three subatomic particles: electron, proton, neutron — charges, masses, locations
- [ ] Atomic number (Z) and mass number (A) definitions and relationship
- [ ] Electronic configuration: 2n² rule (K=2, L=8, M=18, N=32)
- [ ] Valency: number of electrons lost, gained, or shared
- [ ] Isotopes: same Z, different A — examples and applications
- [ ] Isobars: same A, different Z — examples
- [ ] Thomson's model (plum pudding) — features and limitations
- [ ] Rutherford's gold foil experiment — observations and conclusions
- [ ] Bohr's model — fixed orbits, energy levels, stability
- [ ] Electronic configurations of elements H to Ca (1–20)
- [ ] Common mistakes: Z vs A, isotopes vs isobars, shell capacity formula
Internal Links
- [Atomic Structure Guide](/school/tamil-nadu-state-board/class/9/science/atomic-structure)
- [All Class 9 Science Solutions](/school/tamil-nadu-state-board/class/9/science/solutions)
- [Class 9 Science Subject Guide](/school/tamil-nadu-state-board/class/9/science)
