Structure of the Atom
Karnataka Board · Class 9 · Science
Quick revision notes for Structure of the Atom — Karnataka Board Class 9 Science. Key concepts, formulas, and definitions for last-minute revision.
Key Topics to Revise
Discovery of Sub-atomic Particles
- Static electricity experiments showed that atoms contain charged particles
- J.J. Thomson discovered electrons (e⁻) - negatively charged particles with negligible mass
- E. Goldstein discovered canal rays (1886) which led to discovery of protons
Atomic Models
- Thomson's Model (Plum Pudding): Electrons embedded in a sphere of positive charge like seeds in watermelon
- Rutherford's α-particle scattering experiment used fast-moving α-particles on thin gold foil
- Observations: Most α-particles passed straight through, few deflected, very few bounced back 180°
Electronic Configuration and Shell Filling Rules
- Electrons fill shells in order: K, L, M, N (n = 1, 2, 3, 4)
- Maximum electrons: K=2, L=8, M=18, N=32
- Outermost shell can hold maximum 8 electrons (octet rule)
Valency
- Valency is the combining capacity of an atom
- Depends on number of electrons in outermost shell
- Atoms try to achieve stable octet (8 electrons) in outermost shell
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Sources & Official References
- Karnataka SSLC — kseeb.kar.nic.in
- Dept of Pre-University Education, Karnataka
- 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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