Who discovered the nucleus of an atom? 

Ernest Rutherford

The correct answer is option 2. Ernest Rutherford.

Ernest Rutherford's discovery of the atomic nucleus is a pivotal moment in the history of atomic theory and our understanding of the structure of atoms.

In Rutherford's famous alpha-particle scattering experiment, conducted in 1909, alpha particles (positively charged helium nuclei) were directed at a thin gold foil. The expectation at the time, based on the prevailing "plum pudding" model proposed by J.J. Thomson, was that the positively charged alpha particles would be minimally deflected as they passed through the uniformly distributed positive charge of the atom.

However, the experimental results were unexpected and groundbreaking. While most alpha particles did indeed pass straight through the foil with minimal deflection, some were scattered at wide angles, and a few even bounced directly back. This indicated that the positive charge of the atom was not uniformly distributed as previously thought. Instead, it suggested that the majority of the atom's mass and positive charge were concentrated in a small, dense region at the center of the atom, which Rutherford termed the "nucleus."

This discovery revolutionized our understanding of atomic structure. It led to the rejection of the "plum pudding" model and the development of the nuclear model of the atom, where electrons orbit a central nucleus containing protons and neutrons. Rutherford's work laid the foundation for subsequent developments in atomic theory, including Niels Bohr's model of the atom with quantized electron energy levels and the eventual development of quantum mechanics.

Overall, Rutherford's discovery of the atomic nucleus was a critical milestone in the advancement of atomic theory, providing invaluable insights into the fundamental nature of matter and paving the way for modern atomic physics.