Structure of The Atom Notes

• Neutron was discovered by James Chadwick in 1932.
• The reason for the late discovery of neutrons was its neutral nature.

• Chadwick bombarded a thin foil of beryllium with fast moving α-particles and observed that electrically neutral particles having mass slightly greater than that of protons were emitted.

₄Be⁹ + ₂He⁴ → ₆C¹² + ₀n¹

• An atom is composed of protons, neutrons and electrons. Protons and electrons mutually balance their charges.

Thomson’s Model of the Atom

Rutherford’s Model

Bohr’s Model

Thomson’s Model of the Atom:

• When J. J. Thomson proposed his model of the atom in 1903, then only electrons and protons were known to be present in the atom.
• Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge, were like dry fruits in a spherical Christmas pudding.

According to Thomson’s model of the atom: