What significant discovery did Henri Becquerel make in 1896?

Henri Becquerel's significant discovery in 1896 was radioactivity, which fundamentally changed our understanding of atomic physics. Becquerel discovered that certain materials, particularly uranium salts, emitted a type of energy or radiation without any external energy source acting upon them. This phenomenon indicated that the atom itself could be unstable and spontaneous radiation could be emitted, a concept that was revolutionary at the time.

Becquerel's work laid the foundation for future research in nuclear physics and chemistry, leading to further discoveries by scientists such as Marie Curie and Ernest Rutherford. Radioactivity also initiated discussions about the structure of the atom, eventually leading to the acceptance of nuclear physics as a discipline. His findings garnered him the Nobel Prize in Physics in 1903, highlighting the importance of radioactivity as a major scientific breakthrough.