Key Discoveries in Atomic Theory and Radioactivity

John Dalton: The First Theory of Matter

John Dalton proposed the first theory of matter.

William Crookes: Perfecting Vacuum Tubes

William Crookes (1832-1919) perfected devices using a glass tube that had undergone a partial vacuum and contained two electrodes, one positive and one negative, connected to a source of high-voltage continuous current.

J.J. Thomson: Discovering the Electron

In 1897, J.J. Thomson proved that the cathode ray consisted of minute particles of negative electric charge that are attracted by the positive electrode. He achieved determining the relationship between charge and mass for these particles. He carefully measured how magnetic and electric fields deviated the electrons' trajectory. Independently of their origin, they always had the same charge-to-mass ratio (c/m).

Robert A. Millikan: Measuring Electron Charge

Robert A. Millikan determined the value of the electron's charge. His experiment involved suspending negatively charged oil droplets by applying an electric field to compensate for gravity.

Wilhelm Roentgen: Discovering X-rays

In 1895, German physicist Wilhelm Roentgen discovered X-rays.

Henri Becquerel: Natural Radioactivity

In 1896, French physicist Henri Becquerel discovered natural radioactivity.

Pierre and Marie Curie: Polonium and Radium

In 1898, Pierre and Marie Curie discovered two other elements that emit radiation. They named the first Polonium (Po) and the second Radium (Ra). Radium is the most active, emitting 1.4 million times more radiation than uranium.

Types of Radioactivity

• Alpha rays: Helium nuclei (2 protons and 2 neutrons) with a positive charge that are deflected under the action of an electric field. They are not very penetrating.
• Beta rays: Particles with a negative charge, traveling close to the speed of light. They are deflected under the action of an electric field and are nearly 100 times more penetrating than alpha rays.
• Gamma rays: Similar to X-rays, they are electromagnetic radiation of very high frequency and high energy. They are not deflected and are more penetrating than alpha and beta rays.

Natural Radioactivity

Natural radioactivity is the spontaneous emission of radiation from the atomic nucleus, resulting in the transmutation of radioactive atoms.

Eugen Goldstein: Positive Particle Radiation

Eugen Goldstein, through his gas-tube experiments, found that there was positive particle radiation.

Rutherford: The Elusive Particle

Rutherford proposed that a particle must exist, but it was not yet discovered.

James Chadwick: Identifying the Neutron

James Chadwick identified the subatomic particle known as the neutron.