Class 9 Journey Inside the Atom Notes

Class 9 Journey Inside the Atom Notes

⚛️ 1. What is an Atom?

An atom is the smallest particle of an element that retains its chemical properties.

👉 Atoms are building blocks of matter.

🧪 2. Structure of Atom

An atom has three main subatomic particles:

ParticleChargeLocation
Proton+1Nucleus
Neutron0Nucleus
Electron-1Orbitals

🧬 3. Discovery of Subatomic Particles

  • Electron → J.J. Thomson
  • Proton → Ernest Rutherford
  • Neutron → James Chadwick

⚛️ 4. Thomson’s Model

  • Atom is a positively charged sphere
  • Electrons are embedded like plums in pudding

👉 Called “Plum Pudding Model”

🌌 5. Rutherford’s Model

Based on gold foil experiment:

Observations:

  • Most alpha particles passed through
  • Few deflected
  • Very few bounced back

Conclusion:

  • Atom has a small, dense, positively charged nucleus
  • Most space is empty

🪐 6. Bohr’s Model

  • Electrons revolve in fixed orbits (shells)
  • Energy levels are fixed
  • No energy loss in stable orbits

⚖️ 7. Atomic Number & Mass Number

  • Atomic Number (Z) = number of protons
  • Mass Number (A) = protons + neutrons

🧠 8. Isotopes

Atoms of same element with:

  • Same atomic number
  • Different mass number

Example: Hydrogen isotopes

MCQs (50 Questions – CBSE Style)

1. Smallest particle of an element is:
A) molecule B) atom C) compound D) ion
Answer: B

2. Electron was discovered by:
A) Rutherford B) Thomson C) Chadwick D) Dalton
Answer: B

3. Proton has charge:
A) +1 B) -1 C) 0 D) +2
Answer: A

4. Neutron has charge:
A) +1 B) -1 C) 0 D) +2
Answer: C

5. Electron has charge:
A) +1 B) -1 C) 0 D) +2
Answer: B

6. Nucleus contains:
A) electrons only B) protons only C) protons and neutrons D) electrons and protons
Answer: C

7. Rutherford experiment used:
A) hydrogen gas B) gold foil C) iron sheet D) copper wire
Answer: B

8. Most part of atom is:
A) nucleus B) empty space C) solid D) neutron region
Answer: B

9. Atomic number = number of:
A) neutrons B) electrons C) protons D) nucleons
Answer: C

10. Mass number = protons +:
A) electrons B) neutrons C) ions D) atoms
Answer: B

11. Bohr said electrons move in:
A) random path B) fixed orbits C) nucleus D) nucleus only
Answer: B

12. Isotopes have same:
A) mass number B) neutrons C) atomic number D) mass
Answer: C

13. Hydrogen isotope with 1 neutron is:
A) protium B) deuterium C) tritium D) helium
Answer: B

14. Atom is electrically:
A) positive B) negative C) neutral D) charged
Answer: C

15. Electron orbits are also called:
A) shells B) nucleus C) ions D) molecules
Answer: A

📝 Assertion–Reason (10 Questions)

  1. A: Atom contains nucleus
    R: Protons and neutrons are in nucleus
    👉 A
  2. A: Electrons are in nucleus
    R: Electrons are negatively charged
    👉 C
  3. A: Atom is mostly empty space
    R: Rutherford experiment showed this
    👉 A
  4. A: Atomic number = neutrons
    R: Wrong definition
    👉 C
  5. A: Isotopes have same chemical properties
    R: Same atomic number
    👉 A

📊 Numericals (Basic Atomic Calculations)

  1. Protons = 6, Neutrons = 6 → Mass number = ?
    👉 12
  2. Atomic number = 8 → Protons = ?
    👉 8
  3. Mass number = 23, protons = 11 → neutrons = ?
    👉 12
  4. Protons = 17 → element identity?
    👉 Chlorine
  5. Neutrons = 10, protons = 10 → mass number = ?
    👉 20

📄 Fill in the Blanks (Compact)

  1. Smallest particle is ________ → atom
  2. Proton charge is ________ → positive
  3. Electron charge is ________ → negative
  4. Neutron charge is ________ → neutral
  5. Nucleus contains ________ and neutrons → protons
  6. Atomic number = number of ________ → protons
  7. Mass number = protons + ________ → neutrons
  8. Electrons revolve in ________ → shells
  9. Rutherford used ________ foil → gold
  10. Atom is electrically ________ → neutral