Class 12 Physics — Chapter 4: ATOMS

What you'll learn

• ▸Thomson's Model of the Atom
• ▸Geiger-Marsden Alpha-Scattering Experiment
• ▸Observations and Conclusions from Alpha-Scattering
• ▸Rutherford's Nuclear Model of the Atom
• ▸Alpha-Particle Trajectory and Impact Parameter
• ▸Distance of Closest Approach
• ▸Electron Orbits and Classical Mechanics

Chapter Summary

--- PAGE 1 --- Chapter Twelve ATOMS 12.1 INTRODUCTION By the nineteenth century, enough evidence had accumulated in favour of atomic hypothesis of matter. In 1897, the experiments on electric discharge through gases carried out by the English physicist J. J. Thomson (1856 – 1940) revealed that atoms of different elements contain negatively charged constituents (electrons) that are identical for all atoms. However, atoms on a whole are electrically neutral. Therefore, an atom must also contain some positive charge to neutralise the negative charge of the electrons. But what is the arrangement of the positive charge and the electrons inside the atom? In other words, what is the structure of an atom? The first model of atom was proposed by J. J. Thomson in 1898. According to this model, the positive charge of the atom is uniformly distributed throughout the volume of the atom and the negatively charged electrons are embedded in it like seeds in a watermelon. This model was picturesqu…

Practice Questions from this Chapter

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1. Explain atomic models simply. Get Solution →
2. Show me an atom's size. Get Solution →
3. Describe alpha particle scattering. Get Solution →
4. Who proposed the first model of the atom in 1898, known as the 'plum pudding' model? Get Solution →
5. In the Geiger-Marsden experiment, the thin foil used as a target was made of which material? Get Solution →
6. According to Rutherford's nuclear model, where is the positive charge of an atom concentrated? Get Solution →
7. What are alpha-particles, as described in the chapter? Get Solution →
8. According to the text, the size of an atom is approximately how many times larger than the size of its nucleus? Get Solution →

Did you know?

• 💡 Most of an atom is empty space, like a tiny solar system.
• 💡 There are more atoms in a glass of water than glasses of water in all Earth's oceans.
• 💡 The electron was discovered before the atomic nucleus.
• 💡 Atoms were once thought to be unsplittable, but they can be broken apart.
• 💡 An atom's nucleus is like a pea in a football stadium-sized atom.

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