Models of the Atom

Models of the Atom

1. Thomson’s Model of the Atom (Plum Pudding Model, 1904)

J.J. Thomson proposed the Plum Pudding Model to explain the structure of an atom.

Key Features:

• The atom is a positively charged sphere with negatively charged electrons embedded in it, similar to raisins in a pudding.

• The total positive charge is equal to the total negative charge, making the atom neutral.

Diagram: Plum Pudding Model

(A labeled diagram depicting electrons embedded in a positively charged sphere.)

Limitations:

• This model could not explain the results of Rutherford’s gold foil experiment.

• It failed to describe the nucleus and electron arrangement correctly.

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2. Rutherford’s Alpha Particle Scattering Experiment (1911)

Ernest Rutherford conducted the Gold Foil Experiment to study atomic structure.

Experiment Setup:

• A thin gold foil (about 0.00004 cm thick) was bombarded with alpha particles (helium nuclei).

• A fluorescent screen around the foil detected the scattered particles.

Observations:

• Most alpha particles passed straight through the foil without deflection.

• Some particles were slightly deflected at small angles.

• A very few (1 in 10,000) bounced back at large angles.

Conclusions:

• Most of the atom is empty space.

• A small, dense, positively charged core (nucleus) exists at the center of the atom.

• Electrons revolve around the nucleus in orbits.

Diagram: Rutherford’s Gold Foil Experiment

(A labeled diagram showing the deflection of alpha particles by the nucleus.)

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Drawbacks of Rutherford’s Model

• Did not explain the stability of atoms:

  • According to classical physics, electrons should spiral into the nucleus due to continuous energy loss, but atoms are stable.

• Did not explain atomic spectra:

  • It failed to account for the discrete spectral lines observed in hydrogen and other elements.