Structure of Atom


We all have come across the word “radioactivity” somewhere or the other. Do you know an isotope form the basis of radioactivity? Many times terms like nuclear energy, nuclear reactors, and nuclear weapons have popped up in news and textbooks. We have read stories and watched movies about superheroes born out of some sort of radiation exposure. What is radioactivity and how it is associated with isotopes?

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Radioactivity is one of the properties of atoms. Every radioactive atom will have an unstable nucleus. Thus it has a tendency to release subatomic particles to obtain stability thereby releasing radiation or energy in the process. Elements or atoms are present both in radioactive and non-radioactive variety.

The number of neutrons in both the varieties will be different. These different varieties of the same element are termed isotope. Read through this article to find out more about isotopes.


Elements with the same atomic number but a different mass number are defined as “Isotopes”. The number of protons and neutrons combined together is called atomic mass or mass number of an element, whereas the total number of protons gives the atomic number of an element.

In a certain element, the number of protons will always remain constant. However, the number of neutrons can change. The number of neutrons varies but the number of protons always remains same in an isotope of a single element. Thus, the definition “Elements with the same atomic number but a different mass number are termed as Isotopes.”

For Example

Hydrogen has three most stable isotopic forms- protium, deuterium, as well as tritium. All the three isotopic forms of hydrogen have the same number of protons but vary in the number of neutrons. The number of neutrons in protium is zero, the number of neutrons in deuterium is one and the number of neutrons in tritium is two.

Isotopes of Hydrogen

Isotopes of Hydrogen (Source: Wikipedia)

Carbon has three isotopic forms- Carbon-12, Carbon-13, as well as Carbon-14. The numbers 12, 13, and 14 represents the atomic masses of different isotopic forms of carbon. As discussed, atomic number is the unique property by which we can determine the element. Therefore, the atomic number 6 of carbon in all the forms is constant. Carbon-12 is the stable isotope of the carbon element whereas carbon-14 is the radioactive isotope.

Examples of Isotopes

  • Isotopic forms of Oxygen – Oxygen -16, Oxygen -17, Oxygen -18
  • Isotopic forms of Uranium- U-235, U-238
  • Chlorine- 35, Chlorine – 37 are the isotopic forms of chlorine
  • Isotopic forms of Fluorine – Fluorine 17, Fluorine 18, Fluorine 19
  •  Hydrogen – 1, Hydrogen – 2, Hydrogen – 3 are the isotopic forms of Hydrogen
  • Isotopic forms of Carbon-Carbon– 12, Carbon – 13, Carbon- 14

There are approximately 275 different isotopes of 81 stable elements. There are more than 800 natural and synthetic radioactive isotopes present. A single element present in the periodic table can have multiple isotopic forms.

Chemical properties

Generally, the chemical properties of isotopes of any element are almost identical. The exception to this case is the isotopes of hydrogen because the numbers of neutrons have a major effect on the size of the nucleus of a hydrogen atom.

Physical properties

The physical properties of isotopes in a particular element vary from each other. This is because the physical properties of any isotope depend on the mass. The mass of each isotope of a single element varies from one another. Processes such as fractional distillation and diffusion are used to separate isotopes from one another. We make use of the fact that isotopes have different physical properties.

Uses of Isotopes

  • Carbon dating makes use of Carbon-14, an isotope of Carbon. This isotope of carbon is present in the atmosphere as radioactive carbon. The amount of carbon-14 obtained in fossils help palaeontologists to calculate the age of the fossils.
  • Uranium Isotopes are popular for its use in nuclear reactors. U-235 is used as a fuel in nuclear reactors.
  • Radioactive Isotopes are used for medicinal purposes. They are used for detection of tumours, blood clots, etc. Arsenic-74, An isotope of arsenic, is used for determining the presence of a tumour. Similarly, sodium-24 is used for the detection of blood clots.
  • Cobalt (cobalt-60) isotope of carbon is applied in cancer treatments.
  • Iodine (Iodine-131) isotope of carbon helps in the treatment of goitre.

Learn more about Isobars here.

Solved Question for You

Question: Isotopes of a single element vary in the number of

  1. Neutrons
  2. Protons
  3. Electrons
  4. All of the above

Solution: The answer is 1 (Neutrons). The number of protons will always constant in a single element. However, the number of neutrons can change. The number of neutrons varies in isotopes of an element but the number of protons always remains same.

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