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What is inside the atom

Telangana Board · Class 9 · Physical Science

Quick revision notes for What is inside the atom — Telangana Board Class 9 Physical Science. Key concepts, formulas, and definitions for last-minute revision.

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Key Topics to Revise

1

Discovery of Sub-atomic Particles

  • Michael Faraday's electrolysis experiments showed that atoms could gain or lose charges, contradicting Dalton's theory of indivisible atoms
  • J.J. Thomson discovered electrons in 1897 using cathode ray tube experiments - first sub-atomic particle discovered
  • Electrons are negatively charged particles with negligible mass (1/1836th of proton mass)
2

Atomic Models - Evolution of Understanding

  • Thomson's 'Plum Pudding' Model (1898): Atom as sphere of positive charge with electrons embedded like plums in pudding
  • Rutherford's Gold Foil Experiment (1909): α-particles fired at thin gold foil to study atomic structure
  • Rutherford's observations: Most α-particles passed through, few deflected at small angles, very few reflected back
3

Electronic Configuration and Bohr-Bury Rules

  • Electrons are arranged in shells K, L, M, N corresponding to n = 1, 2, 3, 4
  • Maximum electrons in shell = 2n² where n is shell number
  • K shell (n=1): maximum 2 electrons, L shell (n=2): maximum 8 electrons, M shell (n=3): maximum 18 electrons
4

Atomic Number, Mass Number, and Isotopes

  • Atomic number (Z) = Number of protons in nucleus = Number of electrons in neutral atom
  • Mass number (A) = Number of protons + Number of neutrons = Z + N
  • Standard notation: ᴬZ X where A is mass number, Z is atomic number, X is element symbol

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Full Notes

Key Concepts

Atoms contain three types of particlesJBased on alpha particle scattering experimentsNiels Bohr proposed that electrons moveElectrons are arranged in shells following

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Sources & Official References

Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.

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