Discoveries to Atomic Models | Thomson, Rutherford, and Millikan’s Work | Exam Essential Series

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Exam Relevance
This topic is essential for learners across all levels—from high school to doctoral research. At the school and pre-university level, it underpins exams such as CBSE/ICSE Boards, JEE Main & Advanced, NEET in India, as well as SAT Chemistry, AP Chemistry, GCSEs, Cambridge O-Levels and A-Levels, IB SL/HL, and HSC exams internationally. At the undergraduate and postgraduate level, it is central to competitive exams like IIT-JAM, GATE, CSIR-UGC NET, DBT-RA, and JEST in India, alongside GRE Physics, MCAT, and other international graduate entrance tests. At the research and PhD level, atomic structure and experimental models form a core part of qualifying and comprehensive examinations worldwide.

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1. Discovery of X-Rays and Radioactivity
Wilhelm Röentgen (1895):
• Produced when high-speed electrons strike a dense target.
• Not affected by electric or magnetic fields.
• Extremely penetrating (~0.1 nm wavelength).
• Later confirmed as electromagnetic radiation.
This discovery gave science a new tool to probe the unseen interior of matter.
Henri Becquerel (1896):
• α-rays: Heavy, positively charged (helium nuclei).
• β-rays: High-speed electrons.
• γ-rays: Neutral, highly penetrating radiation.
These findings exposed the complex internal architecture of atoms, showing they were far from indivisible.
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2. Thomson’s Model of the Atom (1898)
Building on his discovery of the electron, J.J. Thomson proposed the plum pudding model:
3. Rutherford’s α-Scattering Experiment (1909)
To test Thomson’s model, Rutherford (with Geiger and Marsden) bombarded thin gold foil with α-particles. The screen around it flashed when particles struck.
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4. Rutherford’s Nuclear Model (1911)
• Atoms are mostly empty space.
• Positive charge and most mass are concentrated in a tiny nucleus.
• Electrons orbit the nucleus like planets around the sun.
Analogy: If the atom is a 5 km stadium, the nucleus is a cricket ball at the center.
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5. Millikan’s Oil Drop Experiment (1909–1911)
Thomson had measured the charge-to-mass ratio (e/m) of the electron, but the absolute charge was unknown. Robert Millikan devised an ingenious method:
Closing Reflection
Together, these discoveries revolutionized atomic theory:
• Röentgen and Becquerel revealed the hidden world of radiation.
• Thomson proposed the first atomic model.
• Rutherford uncovered the nucleus.
• Millikan measured the quantized charge of the electron.

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