1. Atomic Theory Gets Experimental Support

• By the 19th century, many experiments supported the atomic theory of matter.
• Scientists became convinced that all matter is made up of atoms.

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2. Discovery of Electron (1897) – J. J. Thomson

J. J. Thomson performed experiments on electric discharge through gases.

Observations:

• Atoms contain negatively charged particles called electrons.
• Electrons are identical in all elements.
• Since atoms are electrically neutral, there must also be positive charge inside the atom.

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3. Thomson’s Atomic Model (1898)

J. J. Thomson proposed the first atomic model.

Main Features

• Positive charge is uniformly spread throughout the atom.
• Electrons are embedded in the positive sphere.
• Similar to:
  • Plum pudding
  • Watermelon with seeds

Limitation

• Later experiments proved this model incorrect.

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Continuous Spectrum vs Line Spectrum

A. Continuous Spectrum

Produced by:

• Solids
• Liquids
• Dense gases

Characteristics

• Contains all wavelengths.
• Intensity varies with wavelength.
• Caused by interaction among neighbouring atoms and molecules.

Examples

• Hot iron
• Sunlight
• Incandescent bulb

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B. Line Spectrum

Produced by:

• Rarefied gases
• Electrically excited gases

Examples

• Neon sign
• Mercury vapour lamp
• Hydrogen discharge tube

Characteristics

• Contains only certain discrete wavelengths.
• Appears as bright coloured lines.
• Produced by individual atoms.

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Characteristic Spectrum of Elements

Every element has its own unique spectrum.

For example:

• Hydrogen always produces the same set of spectral lines.
• No two elements have identical spectra.

Importance

This suggested that:

• Every atom has a unique internal structure.
• Spectrum depends on the arrangement of particles inside the atom.

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Balmer’s Contribution (1885)

Johann Jakob Balmer gave an empirical formula for the wavelengths of visible lines in the hydrogen spectrum.

Importance

• First successful mathematical relation for atomic spectra.
• Helped later scientists develop atomic models.

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Rutherford’s Alpha Scattering Experiment

Ernest Rutherford studied α-particles emitted by radioactive substances.

Experiment Proposed

• Proposed in 1906
• Performed around 1911 by:
  • Hans Geiger
  • Ernest Marsden

Objective

To determine the internal structure of the atom.

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Rutherford’s Nuclear (Planetary) Model

Main Features

• Almost all positive charge is concentrated in a tiny nucleus.
• Nearly all the mass is inside the nucleus.
• Electrons revolve around the nucleus like planets around the Sun.
• Most of the atom is empty space.

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Limitation of Rutherford’s Model

It could not explain:

• Why atoms emit only specific wavelengths.
• Why hydrogen gives a line spectrum instead of a continuous spectrum.
• Why revolving electrons do not lose energy and fall into the nucleus (according to classical physics).

These problems led to the development of Bohr’s atomic model.

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Important Years

Year	Scientist	Discovery
1897	J. J. Thomson	Electron discovered
1898	J. J. Thomson	Plum pudding model
1885	Johann Jakob Balmer	Balmer formula
1906	Ernest Rutherford	Proposed α-scattering experiment
1911	Geiger & Marsden	Performed α-scattering experiment
1911	Ernest Rutherford	Nuclear model of atom

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JEE/NEET One-Liners

• Atom is electrically neutral.
• Electron was discovered by J. J. Thomson (1897).
• Thomson’s model is called the Plum Pudding Model.
• Continuous spectrum contains all wavelengths.
• Line spectrum contains discrete wavelengths.
• Every element has a characteristic spectrum.
• Balmer gave the formula for the visible hydrogen spectrum.
• Rutherford’s experiment used α-particles.
• Positive charge is concentrated in the nucleus.
• Rutherford’s model failed to explain the line spectrum and atomic stability.

Memory Trick

T → B → R → B

• T = Thomson discovers electron
• B = Balmer explains hydrogen spectrum
• R = Rutherford discovers nucleus
• B = Bohr explains line spectrum & atomic stability