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Isobars - JEE Important Topic

Last updated date: 23rd Jul 2024
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Discovery of Isobars

Different kinds of atomic models about the structure of the atom such as Thomson's model, Rutherford's model and others were offered to explain the knowledge of nuclear mass and number. For better understanding, these elements are split into many groups, such as isotopes, isobars, and isotones. At the same time, it aided in explaining many aspects of nature that had the same atomic mass or atomic number.

Alfred Walter Stewart coined the term 'isobar' in 1918, combining the Greek terms 'isos', which means equal, and ‘bar’, which refers to a type of weight. Protons and neutrons are found in isobars, which have an equal number of nucleons. Isobars contain different chemical elements with the same atomic mass and have various chemical properties. However, varying amounts of protons and neutrons will be present. In isobars, the sum of protons and neutrons will remain constant. Isobars with various atomic numbers always have different atomic structures.

What are Isobars?

The total number of nucleons in the isobars or the sum of protons and neutrons will always be the same, but the total number of protons and neutrons will vary. The number of neutrons determines the difference in the number of nucleons. Due to atomic number mismatches, isobars always have a distinct atomic structure. Different chemical elements with the same atomic mass are always present. As a result, isobars have other chemical characteristics. The number of electrons present and their configuration within an atom determines an element's chemical attributes.

Isotopes have the same amount of electrons, but their mass numbers differ due to differences in physical properties. They do, however, have chemical features in common. As a result of their various atomic numbers, isobars have different chemical characteristics. 

Examples of Isobars

Isobars have comparable physical qualities, so their atomic masses are the same. For example, atomic numbers 18 and 20 are found in argon and calcium, respectively. However, they have the same atomic mass of 40. There are also times when a group of elements with the same nuclear mass exists. Similarly, iron and nickel have atomic numbers of 26 and 28, respectively. On the other hand, the mass number is found to be 58.

Elements and their Mass Numbers

Here are some more examples regarding isobars, which are divided based on atomic number and mass number:


Atomic Number

Mass Number

${ }_{32} Ce^{76},~{ }_{34} Se^{76} $



${ }_{11} Na^{24},~{ }_{12} Mg^{24} $



${ }_{27} Co^{64},~{ }_{28} Mg^{64} $  



${ }_{18} Ar^{40},~{ }_{19} K^{40},~{ }_{20} Ca^{40}$ 



${ }_{26} Fe^{58},~{ }_{27} Ni^{58} $ 



The Mattauch Isobar Rule

According to the Mattauch isobar rule, if two neighbouring elements on the periodic table have isotopes with the same mass number, at least one of these isobars must be radionuclides. The branch decay of the middle isotope may occur if the first and last isotopes remain constant while three isobars with sequential elements are present. There are no observably stable isobars for mass numbers 5, 8, 147, 151, or 209 and above. Beta-decay stable nuclides for mass numbers can theoretically suffer alpha decay with no stable nuclides with the same mass number.

Uses of Isobars

Several uses of isobars can be seen in everyday life, which are used for the benefit of human life, some of which are described below:

  • Goitre was treated with the iodine isobar.

  • Cancer is treated using cobalt isobars.

  • Cancer, blood clots, and other medical disorders are treated with isobars.

  • In nuclear reactors, uranium isobars were used.

  • Iodine is used to treat thyroid disorders.

  • Uranium isobars can be used in atomic reactors.

  • Blood cancer is treated with phosphorus isobars.

Application of Isobar in Nuclear Physics

Isobars are often studied and used, mainly in nuclear physics and radioactivity. Data on naturally occurring beta decay research is also gathered during this time. These isobars are used as a secondary criterion for classification. Isobars differ from isotopes in various ways, the most notable of which is their chemical characteristics. They also have different atomic numbers. Iodine isobars are used to cure goitre, while cobalt isobars are primarily employed in cancer treatment.


Isobars have unique physical features that cannot be compared. Isobars are defined as elements with the same mass number but different atomic numbers. Due to changes in the electron count, isobars exhibit a wide range of chemical characteristics, which could lead to the usage of uranium isobars in nuclear reactors. 

The mass number, the sum of the protons and neutrons is the same for all isobars. Its application should be made not just for humanity's benefit, but also for the benefit of the entire cosmos. Various uses related to this are provided in the above article.

Competitive Exams after 12th Science

FAQs on Isobars - JEE Important Topic

1. Differentiate between isotopes and isobars.

Isotopes and isobars are types of nuclei. Isobars are nuclei with the same mass number i.e. the total sum of protons and neutrons will be the same for isobars. Isotopes are atoms having the same proton number but a different number of neutrons. Isotopes are components that have a similar atomic number but the mass number will be different.

20Ca40 and 19K40 have the same mass number i.e 40 and are examples of isobars. On the other hand, 1H2 and 1H3  have the same atomic number i.e. 1 but different mass numbers and are examples of isotopes.

2. What are the types of questions that can be asked from isobars in the JEE exam and is it necessary to revise the previous year’s question papers?

Isobars is an important concept for JEE exams as multiple-choice questions like finding and recognising isotopes have been asked in previous year's JEE exams. With proper revision of the concept, you can easily secure 4- 8 marks in JEE Main.  

Revising and going through previous year's question papers will give you a better idea about the types of questions asked. It will also boost your confidence and will keep any nervousness and anxiety at bay.