Joseph John Thomson, an English scientist, provided the first hint that an atom is made of even...

JOSEPH JOHN THOMSON

Joseph John Thomson, an English scientist, provided the first hint that an atom is made of even smaller particles.

He observed that there are NEGATIVELY CHARGED and POSITIVELY CHARGED particles in the atom.

J.J. THOMSON’SATOMIC MODEL

In 1987, he proposed a model of the atom that is sometimes called the plum pudding model (something like a raisin bread).

In this model the electrons (raisins) and protons (pudding) are uniformly mixed throughout the atom.

The 1904 Thomson model was disproved

by the 1911 experiment, which was interpreted by Ernest

Rutherford

ERNESTRUTHERFORD

In 1911, British physicist Ernest Rutherford presented a new theory about the structure of the atom.

He reasoned that all of an atom’s positively charged particles were contained at the center, which he then called NUCLEUS.

RUTHERFORD’S ATOMIC MODEL

Ernest Rutherford publishes his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons.

The electrostatic force of attraction between electrons and nucleus was likened to the gravitational force of attraction between the revolving planets and the Sun.

This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space.

The scattering of alpha particles was due to the strong [electrostatic repulsion]which the alpha particles experienced when they approached the positive nucleus. The closer the approach, the greater the scattering.

Explanation of Alpha-Scattering Results

Plum-pudding atom

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Alpha particles

Nuclear atom

Nucleus

Thomson’s model Rutherford’s model

Results of foil experiment if plum-pudding had been correct.

Electrons scatteredthroughout positive

charges

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

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Interpreting the Observed Deflections

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

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gold foil

deflected particle

undeflected particles

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.beam ofalpha particles

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In a ¼ sheet of paper:

How did Rutherford's model of the atom differ from Thomson's model?