Class IX SEBA Science – Chapter 4: Structure of the Atom
Class IX SEBA Science – Chapter 4: Structure of the Atom
⚛️ 1. Charged Particles in Matter
- Atoms consist of positively charged protons and negatively charged electrons.
- Experiments with cathode rays and canal rays led to the discovery of electrons (J.J. Thomson) and protons (Goldstein).
🔬 2. The Structure of an Atom
- An atom has a central nucleus (containing protons and neutrons) and electrons revolving in orbits around it.
- Major atomic models:
- Thomson’s Model
- Rutherford’s Model
- Bohr’s Model
💡 3. Thomson’s Model of Atom
- Proposed the “plum pudding” model.
- Atom is a sphere of positive charge with negatively charged electrons embedded in it.
- Could not explain atomic stability or experimental results.
🧪 4. Rutherford’s Model of Atom
- Performed the gold foil experiment.
- Observations:
- Most α-particles passed straight.
- Some deflected; few bounced back.
- Conclusions:
- Atom has a dense, positively charged nucleus.
- Atom is mostly empty space.
- Electrons revolve around the nucleus.
- Drawback: Couldn’t explain why electrons don’t fall into the nucleus.
🪐 5. Bohr’s Model of Atom
- Electrons revolve in fixed orbits or energy levels without radiating energy.
- Energy levels are labeled as K, L, M, N… (starting from the nucleus).
- Electrons jump levels by absorbing or emitting energy.
🧬 6. Neutrons
- Discovered by James Chadwick.
- Neutral particles present in the nucleus.
- Mass nearly equal to that of a proton.
⚖️ 7. How Are Electrons Distributed in Different Orbits?
- Distribution is governed by Bohr-Bury rules:
- Maximum electrons in a shell = 2n² (n = orbit number)
- K → 2, L → 8, M → 18…
- Outer shell holds max 8 electrons.
- Next shell starts only after previous shell is filled.
- Maximum electrons in a shell = 2n² (n = orbit number)
🧮 8. Valency
- Valency is the combining capacity of an atom.
- Atoms try to complete their outer shell (usually 8 electrons).
- Valency = Number of electrons gained/lost/shared to complete the octet.
🔢 9. Atomic Number and Mass Number
- Atomic number (Z) = Number of protons = Number of electrons.
- Mass number (A) = Number of protons + Number of neutrons.
- Neutrons = A – Z
🔁 10. Isotopes and Isobars
- Isotopes: Atoms of the same element with same atomic number but different mass numbers.
Example: H-1, H-2 (Deuterium), H-3 (Tritium) - Isobars: Atoms of different elements with same mass number but different atomic numbers.
Example: ⁴⁰Ar and ⁴⁰Ca
📘 Exercise Questions and Answers
1. Compare the properties of electrons, protons, and neutrons.
- Electron → Negative charge, negligible mass
- Proton → Positive charge, mass = 1 u
- Neutron → No charge, mass ≈ 1 u
2. What are the limitations of Rutherford’s model?
- It could not explain the stability of the atom.
- Electrons revolving should emit energy and spiral into the nucleus.
3. Describe Bohr’s model of the atom.
- Electrons revolve in fixed orbits without losing energy.
- Energy is emitted/absorbed only when electrons jump orbits.
4. Compare all the proposed models of an atom.
- Thomson: Sphere of positive charge with electrons embedded.
- Rutherford: Nucleus with electrons revolving in orbits; unstable.
- Bohr: Electrons in stable, fixed orbits.
5. Define valency.
- The number of electrons lost, gained or shared to complete outer shell.
6. Define atomic number, mass number, and isotopes.
- Atomic number (Z): Number of protons
- Mass number (A): Protons + Neutrons
- Isotopes: Same atomic number, different mass number
7. How will you find the number of protons, electrons, and neutrons in ¹⁷Cl³⁵?
- Protons = 17
- Electrons = 17
- Neutrons = 35 – 17 = 18
✍️ Extra Important Questions with Answers
1. Who discovered the electron and how?
J.J. Thomson discovered electrons using the cathode ray tube experiment.
2. What are nucleons?
Protons and neutrons together are called nucleons.
3. Why are atoms electrically neutral?
Because the number of protons = number of electrons, their charges cancel.
4. Why are isotopes chemically similar?
Because they have the same number of electrons and hence similar chemical behavior.
5. Give two uses of isotopes.
- Cobalt-60 → Used in cancer treatment
- Iodine-131 → Used in treating thyroid disorders
proceed to Chapter 5: The Fundamental Unit of Life?
