Radioactivity

Radioactivity is the spontaneous emission of high-energy radiation from the nucleus of an unstable atom.

A radioactive substance is a substance whose nuclei are unstable and therefore emit radiation on their own.

Discovery of Radioactivity

Radioactivity was discovered in 1896 by A. H. Becquerel while studying uranium compounds.

He observed that a photographic plate darkened even when the uranium compound was wrapped and kept out of direct light, indicating that the substance emitted invisible radiation on its own.

Later, similar radioactive behaviour was found in thorium compounds, strengthening the understanding that some substances naturally emit energetic radiation.

Discovery Flow

1. Becquerel studied uranium salts while investigating fluorescence and phosphorescence.
2. A covered photographic plate was still exposed.
3. The effect could not be explained by ordinary visible light.
4. It was concluded that uranium spontaneously emitted penetrating radiation.
5. This phenomenon came to be known as radioactivity.

• Atoms of some elements have unstable nuclei.
• To become more stable, these nuclei undergo radioactive decay, emitting energy or particles.
• This process happens naturally and does not require heating, pressure, or any chemical reaction to start.

Nature shows three major types of radioactive decay: alpha decay, beta decay, and gamma decay.

Alpha Decay

In alpha decay, the nucleus emits an alpha particle, which is a helium nucleus \[\,^{4}_{2}\text {He}\]. 

• It carries a positive charge.
• It has a comparatively large mass.
• It has lower penetrating power than gamma rays.
• It is strongly ionising in nature.

Beta Decay

In beta decay, the nucleus emits an electron or a positron.

• Beta particles have a very small mass.
• Their penetrating power is greater than that of alpha particles but less than that of gamma rays.
• Their ionising power is lower than that of alpha particles.

Gamma Decay

In gamma decay, the nucleus emits high-energy electromagnetic radiation called gamma rays.

• Gamma rays have no mass and no charge.
• They have very high penetrating power.
• Their ionising power is lower than that of alpha particles.

Radioactivity has important applications in medicine, scientific research, and industry, though the present note focuses on the basic concept and types of decay.

A student may connect this topic with cancer treatment, medical imaging, and nuclear science as extended applications of radioactive phenomena.