Natural radioactivity, the spontaneous emission of radiation from certain unstable nuclei, was first discovered by which scientist?

Introduction / Context:
Radioactivity is the spontaneous emission of particles or electromagnetic radiation from the unstable nucleus of an atom. The discovery of natural radioactivity at the end of the nineteenth century opened a new era in physics, chemistry, and medicine. Several famous scientists worked in this area, but one physicist is credited with the first discovery of natural radioactivity. This question asks you to identify that scientist.

Given Data / Assumptions:

• The phenomenon in question is natural radioactivity, not artificial transmutation.
• The scientists listed are Marie Curie, Ernest Rutherford, Henri Becquerel, and Enrico Fermi.
• We are interested in who first observed and reported spontaneous emission from uranium salts.
• We assume standard historical accounts of the discovery.

Concept / Approach:
Henri Becquerel, a French physicist, discovered natural radioactivity in 1896 while studying phosphorescence in uranium salts. He found that certain salts emitted penetrating radiation that could fog photographic plates even in the absence of sunlight. Marie and Pierre Curie later extended this work by isolating new radioactive elements such as polonium and radium, and Rutherford made major contributions to understanding radioactive decay and the structure of the atom. Enrico Fermi worked much later on neutron induced radioactivity and nuclear reactors. Therefore, the credit for the first discovery of natural radioactivity goes to Henri Becquerel.

Step-by-Step Solution:
Step 1: Recall that Becquerel was investigating potential connections between X rays and phosphorescence using uranium salts placed on photographic plates wrapped in black paper. Step 2: He observed that even when the salts were kept in the dark, the photographic plates became fogged, indicating that the salts themselves were emitting some invisible penetrating radiation. Step 3: This spontaneous emission did not depend on external stimulation like sunlight, proving that it was a property of the uranium atoms. Step 4: This phenomenon was later named radioactivity, and similar properties were found in other elements by the Curies. Step 5: Considering the timeline, Becquerel discovery in 1896 predates the detailed work of Marie Curie, Rutherford, and Fermi, so he is recognised as the discoverer of natural radioactivity.

Verification / Alternative check:
Historical timelines confirm that in 1895, Roentgen discovered X rays, and in 1896, Becquerel discovered uranium rays, now known as radioactivity. In the late 1890s and early 1900s, Marie Curie and Pierre Curie isolated radium and polonium and coined the term radioactivity. Rutherford later distinguished between alpha, beta, and gamma radiation and proposed the nuclear model of the atom. Enrico Fermi work on artificial radioactivity and chain reactions took place in the 1930s and 1940s. This chronology supports the conclusion that Becquerel made the first discovery of natural radioactivity.

Why Other Options Are Wrong:
Marie Curie: She greatly advanced the study of radioactivity and discovered new radioactive elements but did not make the initial discovery. Ernest Rutherford: He made fundamental contributions to understanding radioactive decay and nuclear structure but came after Becquerel. Enrico Fermi: Known for his work on neutron induced radioactivity and nuclear reactors, he worked decades after the discovery of natural radioactivity.

Common Pitfalls:
Because Marie Curie is strongly associated with radioactivity and even gave the phenomenon its name, students sometimes mistakenly credit her with the original discovery. The key to avoiding this error is to separate the initial observation of the effect, made by Becquerel, from the later detailed investigations and applications developed by the Curies, Rutherford, and others.

Final Answer:
Natural radioactivity was first discovered by Henri Becquerel.