Nuclear chemistry is the branch of chemistry that deals with the study of the nucleus of the atom and the changes that occur within it. It focuses on nuclear reactions, radioactive decay, and the energy transformations that accompany these processes.

Unlike ordinary chemical reactions, which involve the rearrangement of electrons, nuclear reactions involve changes in the composition of the atomic nucleus. Such changes can result in the transformation of one element into another.

Radioactivity is the spontaneous disintegration of unstable atomic nuclei accompanied by the emission of radiation such as alpha (α), beta (β), and gamma (γ) rays. This process occurs naturally in certain elements and results in the formation of new elements or isotopes.

Radioactive decay continues until a stable, non-radioactive isotope is formed. The rate of this decay is characteristic of each radioactive element and is measured by its half-life.

The discovery of radioactivity dates back to the late 19th century. In 1896, Henri Becquerel discovered that uranium salts emitted invisible rays that could darken photographic plates even without exposure to sunlight.

Following Becquerel’s discovery, Marie and Pierre Curie conducted extensive research and discovered two new radioactive elements — polonium and radium. Their work established the foundation of nuclear chemistry and revealed that radioactivity was an atomic property, not a chemical one.

Henri Becquerel’s discovery was accidental. While studying phosphorescent materials, he placed a photographic plate wrapped in black paper with uranium salt crystals on it. Later, he found that the plate had darkened, proving that uranium emitted some form of penetrating radiation on its own — without any external energy source like sunlight.

This observation marked the first evidence of spontaneous nuclear emission. Later, the Curies extended this research, showing that radioactivity was a general property of certain elements and that the intensity of radiation depended on the amount of radioactive material present.

Radioactive substances exhibit certain common characteristics which distinguish them from non-radioactive substances. These properties include:

• They emit alpha (α), beta (β), and gamma (γ) radiations spontaneously.
• The rate of decay is not affected by temperature, pressure, or chemical state of the element.
• They can cause ionization of gases through which they pass.
• They can blacken photographic plates and produce visible tracks on them.
• They can cause fluorescence in certain substances like zinc sulfide.
• They undergo transmutation, i.e., they change into new elements after radioactive decay.
• Each radioactive substance has a specific half-life that determines how quickly it decays.