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Radioactive Isotopes – Definition, Properties, Uses

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The main components of an element are its atoms, atoms of different elements are different from each other on the basis of atomic number. But it was thought that atoms of an element are the same throughout, But with advancements in chemistry, It was found out that atoms of the same elements can also differ, They do not differ on the basis of the protons or electrons, so their atomic number is same, But they may differ on the basis of their mass number.

Mass Number and Atomic Numbers

The number of protons in a chemical element’s nucleus is its atomic number. The atomic number of hydrogen, for example, is 1. The total number of protons and neutrons in an atom is referred to as the mass number. Carbon, for example, has a mass number of 12. (6 protons and 6 neutrons.)


Isotopes are variations of an element that have the same number of protons but differ in the number of neutrons in the atom. The isotopes have differing weights due to uneven numbers of neutrons. One or two stable isotopes exist for elements with odd atomic numbers, such as hydrogen and lithium. Even-numbered elements, such as sulphur and oxygen, are likely to have at least three stable isotopes. Carbon, helium, and beryllium are the only exceptions.

Isotopes of Hydrogen and Carbon

The three stable hydrogen isotopes are protium, deuterium, and tritium. They all have the same number of protons, but they have varying numbers of neutrons. Tritium has two neutrons, while deuterium only has one. Protium has zero neutrons while deuterium has one and tritium has two. Carbon has three isotopes: Carbon-12 (stable isotope), Carbon-13, and Carbon-14 (radioactive isotope), with atomic weights of 12, 13, and 14 respectively.

Types of Isotopes

  1. Isotopes are either stable or radioactive.
  2. Isotopes that are radioactive are called radioisotopes or radionuclides.
  3. Stable isotopes or stable nuclides are isotopes that do not decay radioactively. On the planet Earth, there are around 339 naturally occurring nuclides or isotopes.
  4. There are 286 primordial nuclides which are believed to have existed since the formation of the Solar System.

What Are Radioactive Isotopes?

An atom of an element is different from the atoms of another element on the basis of its atomic number, But Some atoms of the same elements exhibit different properties, such as radioactivity and change in average molar mass. When investigated this change was due to the change in the mass number of the atoms. The change in mass number is sole because of the presence of different numbers of neutrons and as neutrons are electrically neutral they don’t change the atomic number.

Atoms of elements that have the same atomic number but a different mass number are called isotopes.

For example:

  • Carbon has three isotopes. Carbon-12, Carbon-13 and Carbon-14.

  • Also, Hydrogen has three isotopes: Protium (1H1), Deuterium (2H1) and Tritium (3H1).

Properties of Isotopes

  • D2O (D denotes deuterium) is called heavy water and is used as a neutron moderator in nuclear reactors.
  • Tritium is radioactive when the other two are not.
  • Carbon-14 is also radioactive and is used in carbon dating (carbon dating is an archaeological technique of determining the age of fossils).
  • Other than this oxygen, uranium and thorium also show isotropy of great significance.

Applications of Radioactive Isotopes

  1. Medical Field: Cobalt-60 is a radiation source used in cancer treatment to decrease the growth of the disease. Other radioactive isotopes are used as tracers in metabolic research and diagnostics. Carbon-14 is used in a breath test to detect the ulcer-causing bacteria Helicobacter pylori.
  2. Industry: In industry, radioactive isotopes are used to determine the thickness of metal or plastic sheets, with the strength of the radiations that penetrate the substance being studied revealing the precise thickness. They can also be used to generate modest amounts of electricity. One example is the use of Plutonium-238 in spacecraft.

What is Radioactivity?

Radioactivity is a nuclear phenomenon. In which there is a spontaneous emission of radiations from the nuclei of atoms during their decay. The atom generally decays and energy is released in the form of alpha, beta and gamma radiations. 

In 1896 it was Sir Henry Becquerel who discovered the phenomenon of radioactivity. In a dark room once he left a uranium salt was placed on a photographic plate wrapped in black paper. Some days later when he investigated, he was surprised to found that the photographic plate had been affected. Later on, the same observation was made with the other salts of uranium. 

From these observations, he concluded that uranium and the number of its salts by themselves emit some kind of radiation that can pass through the cover (i.e. black paper, glass or wood, etc.) of the outside the photographic plate and affect it. These radiations were called the Becquerel rays. Later renamed as alpha, beta and gamma rays.

Radioactive Emission

The radioactive substances emit three types of radiations as stated earlier, due to the imbalance and instability in their atoms due to the varying number of neutron

  1. Alpha Rays: These are positively charged beams of particles, or precisely helium nuclei (4He2). When the number of neutrons is more than that of the proton, the excessive neutron converts into a proton and in this process an alpha particle is released, Generally observed for elements heavier than lead. This results in decreasing in Mass number A and Atomic Number Z.
  2. Beta Rays: These are negatively charged streams of particles. When an unstable nucleus contains neutrons more than the protons, a neutron may change into a proton by emitting an electron. The electron given out from the nucleus at a high speed is called a beta particle. This result is increasing in atomic number and no change in the mass number.
  3. Gamma Rays: Gamma rays are fast-moving streams of uncharged particles. Sometimes after alpha or beta emission is found to be followed by the Gamma-emission. It occurs when the daughter or the parent nucleus is in a state of excitation (i.e. it has an excess of energy). This extra energy is released in the form of electromagnetic radiation known as gamma-radiation (or y-ray photon). The gamma-ray is massless and has no electric charge this implies, no neutrons or protons are lost, hence the nucleus does not decay into a different nucleus, i.e, there is no change in the mass number A and atomic number Z of the nucleus in gamma emission.

Radioisotopes and Conditions for Isotopes to show Radioactivity

In isotopes the number of neutrons is greater than the original atom, which adds excessive mass, Due to excessive mass, the nuclei get unstable and henceforth it results in an explosion, causing dissipation of energy in the form of radioactive waves.

The isotopes have an unstable combination of neutrons and protons or excess energy in their nucleus. are called radioactive isotopes

The instability of the nucleus can be natural or artificially created by altering the atoms. Some manmade isotopes are produced in nuclear reactors or in cyclotrons. Examples of natural radioisotopes are Uranium-235 (about 99.3% uranium is radioactive in nature, only 0.7% is Uranium 238). Examples of artificially created radioisotopes are fluorine-18 and molybdenum-99.

Uses of Radioactive Isotopes

The use of radioisotopes is vast, they are used in various fields, most extensively in medicine, food preservation, sterilization and research. Some used are listed below:

  1. Medicines: Radiopharmaceuticals i.e. diagnosis and therapy of a specific organ. (molybdenum-99 decays to technetium-99). Carbon-11 is used in Positron emission Tomography, Iodine-123 and Iodine-131 are used in imaging to monitor thyroid function and detect adrenal dysfunction of the human body, Lutetium-177 and Samarium-153 are used in the diagnosis and treatment of various tumors of the human body.
  2. Research: Carbon-14 used in carbon dating. Chlorine-36 is used to measure sources of chloride and the age of water (up to 2 million years). and Lead-210 is used to date layers of sand and soil. Also, Uranium-235 is used as a fuel in nuclear reactors.

Sample Questions

Question 1: Name the three types of radiation that are emitted in radioactive decay. How will they get deflected in a negative electric field?


The three types of radiations that are emitted due to radioactivity are Alpha rays, Beta Rays and Gamma Rays.

  1. Alpha rays are positively charged hence the negative electric field will attract them.
  2. Beta rays are negatively charged, hence they will be repelled by the electric field.
  3. Gamma rays have no charge, so they will pass un-deviated from their path.

Question 2: Which substance was the first to be detected radioactive? Also, state its isotopes.


Uranium was the first radioactive substance to be detected. It has three isotopes, U-238 (most abundant), U-235, U-234.

Question 3: What is the condition for gamma-ray emission? Is there a change in atomic number after a gamma emission?


Gamma rays are emitted when the excited nucleus releases energy in form of radiations and gets back to a stable state. There is no change in the atomic configuration because only excessive energy possessed by the nucleus is released.

Question 4: Which isotope of carbon is used in carbon dating? State all the isotopes of carbon.


Carbon-14 is used in carbon dating as it is the radioactive isotope of carbon. Carbon has three naturally occurring isotopes C-12, C-13, C-14.

Question 5: Do all isotopes show radioactivity? if not then state the specific condition.


Not all isotopes of every element are not radioactive. For example, out of three isotopes of hydrogen, only tritium is radioactive. It is not possible to predict whether an isotope will be radioactive or not, other than the fact that radioactivity is shown only if there is a significant difference between the number of protons and neutrons. 

Last Updated : 04 Mar, 2022
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