Radioactivity can be broadly classified into two main types — Natural Radioactivity and Artificial Radioactivity. The classification is based on how the radioactive process occurs — whether it happens spontaneously in nature or is induced artificially in the laboratory.
Natural radioactivity refers to the spontaneous disintegration of unstable atomic nuclei found in nature without any external influence. Certain heavy elements such as uranium, thorium, and radium emit alpha (α), beta (β), and gamma (γ) radiations naturally. These emissions cause the nucleus to change into a new element until a stable isotope is formed.
Examples of Natural Radioactivity:
1. Uranium decay to thorium:
$$^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^{4}_{2}\text{He}$$
2. Thorium decay to radium:
$$^{232}_{90}\text{Th} \rightarrow ^{228}_{88}\text{Ra} + ^{4}_{2}\text{He}$$
3. Radium decay to radon:
$$^{226}_{88}\text{Ra} \rightarrow ^{222}_{86}\text{Rn} + ^{4}_{2}\text{He}$$
These decays occur naturally over time and form part of long decay series that end in a stable isotope such as lead-206.
Artificial radioactivity, also called induced transmutation or artificial transmutation, occurs when stable nuclei are made radioactive by bombarding them with high-speed particles such as neutrons, protons, or alpha particles. This process was first discovered by Irène and Frédéric Joliot-Curie in 1934 when they bombarded aluminum with alpha particles, producing radioactive phosphorus.
Examples of Artificial Radioactivity:
1. Aluminum bombarded by alpha particles produces radioactive phosphorus:
$$^{27}_{13}\text{Al} + ^{4}_{2}\text{He} \rightarrow ^{30}_{15}\text{P} + ^{1}_{0}\text{n}$$
2. Nitrogen bombarded by alpha particles produces oxygen:
$$^{14}_{7}\text{N} + ^{4}_{2}\text{He} \rightarrow ^{17}_{8}\text{O} + ^{1}_{1}\text{H}$$
3. Cobalt made radioactive by neutron bombardment:
$$^{59}_{27}\text{Co} + ^{1}_{0}\text{n} \rightarrow ^{60}_{27}\text{Co}$$
Artificial radioactivity allows scientists to create isotopes used in medicine, agriculture, and industry — such as cobalt-60 for cancer treatment and iodine-131 for thyroid therapy.
The table below highlights the major differences between natural and artificial radioactivity based on their origin, process, control, and applications.
| Natural Radioactivity | Artificial Radioactivity |
|---|---|
| Occurs spontaneously in nature without any external cause. | Induced by bombarding stable nuclei with subatomic particles such as neutrons or alpha particles. |
| Found in naturally radioactive elements such as uranium, radium, and thorium. | Occurs in stable elements that become radioactive when bombarded, such as aluminum and cobalt. |
| Cannot be controlled or initiated by human effort. | Can be initiated, controlled, and reproduced in laboratories or reactors. |
| Responsible for background radiation on Earth. | Used for producing useful isotopes for medicine, agriculture, and industry. |
| Occurs in nature through long decay series ending in stable isotopes (often lead). | Produces radioactive isotopes with specific half-lives that decay to stability. |
| First discovered by Henri Becquerel in 1896. | First demonstrated by Irène and Frédéric Joliot-Curie in 1934. |
| Examples: uranium-238, thorium-232, radium-226. | Examples: phosphorus-30, cobalt-60, iodine-131. |
| Occurs continuously and cannot be accelerated or stopped by external means. | Can be produced at will using particle accelerators or nuclear reactors. |
In summary, both types of radioactivity involve the emission of nuclear radiation and transformation of one element into another. However, natural radioactivity occurs without human intervention, while artificial radioactivity is deliberately induced for scientific, medical, and industrial purposes.