What is inside the atom
Kerala Board · Class 9 · Science
Quick revision notes for What is inside the atom — Kerala Board Class 9 Science. Key concepts, formulas, and definitions for last-minute revision.
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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)
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
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
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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Sources & Official References
- Kerala Board of Public Examinations — keralapareekshabhavan.in
- National Education Policy 2020 — education.gov.in
Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.
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