3. Rutherford’s Nuclear Model of the Atom

Concept Explanation:

Ernest Rutherford proposed the nuclear model of the atom in 1911 after conducting the famous gold foil experiment. In this experiment, alpha particles were directed at a thin sheet of gold foil, and most of them passed through, but some were deflected at large angles. Rutherford concluded that the atom is mostly empty space, with a small, dense, positively charged nucleus at its center, where most of the atom’s mass is concentrated. The electrons orbit the nucleus at relatively large distances.

This model replaced the earlier "plum pudding" model proposed by J.J. Thomson, where the atom was thought to consist of a diffuse positive charge with negatively charged electrons embedded within it.

Example Problem:

Explain why Rutherford's gold foil experiment disproved the "plum pudding" model of the atom.

Solution:

According to the "plum pudding" model, alpha particles should have passed through the gold foil with minimal deflection, as the positive charge was thought to be spread out evenly across the atom. However, Rutherford observed that while most alpha particles passed through, some were deflected at large angles, and a few even bounced back. This could only be explained if the atom had a small, dense, positively charged nucleus that repelled the positively charged alpha particles. Therefore, Rutherford's experiment showed that the atom's positive charge and most of its mass are concentrated in a central nucleus, disproving the "plum pudding" model.