What are isobars?

The isobars  are those atomic species having the same mass but which come from different chemical elements. As a consequence of this, it can be said that they are made up of different numbers of protons and neutrons.

Both protons and neutrons are found in the nucleus of their atoms, but the net number of neutrons and protons present in each nucleus remains the same. In other words, an isobar species originates when a pair of atomic nuclei shows the same net number of neutrons and protons for each species.

However, the number of neutrons and protons that make up that net quantity is different. One way to notice it graphically is to observe the mass number (which is placed on the upper left side of the symbol of the chemical element that is represented), because in isobars this number is the same.

The etymology of the term isobarus comes from the Greek words isos  (which means “equal”) and baros  (which means ” weight “), which refers to the equality of weights between both nuclear species.

– Isobars have certain similarities with other species whose nuclei have coincidences, such as isotones, which have the same number of neutrons but with different mass numbers and atomic numbers, such as pairs 13 C and 14 N or 36 S and 37 Cl.

– The term “nuclide” is the name that has been coined to each of the nucleon sets (structures made up of neutrons and protons) that can be formed. So nuclides are possibly distinguished by their numbers of neutrons or protons, or even by the amount of energy possessed by the structure of their conglomeration.

– Likewise, a daughter nucleus arises after the β decay process and this, in turn, is an isobar of the parent nucleus, because the number of nucleons present in the nucleus remains unchanged, unlike what happens by means of α decay.

– It is important to remember that different isobars have different atomic numbers, confirming that they are different chemical elements.


To denote the different nuclides, a specific notation is used, which can be represented in two ways: one consists of placing the name of the chemical element followed by its mass number, which are joined by a hyphen. For example: nitrogen-14, whose nucleus is made up of seven neutrons and seven protons.

The other way to represent these species is to place the symbol of the chemical element, preceded by a numerical superscript that indicates the mass number of the atom in question, as well as a numerical subscript that designates its atomic number, as follows way:

In this expression X represents the chemical element of the atom in question, A is the mass number (result of the addition between the number of neutrons and protons) and Z represents the atomic number (equal to the number of protons in the nucleus of the atom) .

When these nuclides are represented, the atomic number of the atom (Z) is usually omitted because it does not provide relevant additional data, so it is frequently represented as A X.

One way to show this notation is by taking the previous example (nitrogen-14), which is also denoted as 14 N. This is the notation used for isobars.

Examples of isobars

The use of the expression “isobars” for species known as nuclides that have the same number of nucleons (the same mass number) was proposed at the end of the 1910s by the British chemist Alfred Walter Stewart.

In this order of ideas, an example of isobars can be observed in the case of the 14 C and 14 N species : the mass number is equal to 14, this implies that the number of protons and neutrons in both species is different.

Indeed, this carbon atom has an atomic number equal to 6, so there are 6 protons in its structure, and in turn it has 8 neutrons in its nucleus. So its mass number is 14 (6 + 8 = 14).

For its part, the nitrogen atom has an atomic number equal to 7, so it is made up of 7 protons, but it also has 7 neutrons in its nucleus. Its mass number is also 14 (7 + 7 = 14).

You can also find a series in which all the atoms have a mass number equal to 40; this is the case of the isobars: 40 Ca, 40 K, 40 Ar, 40 Cl, and 40 S.

Differences between isobars and isotopes

Nuclides describe the different classes of atomic nuclei that exist, according to the number of protons and neutrons they have.

Also, among these types of nuclides are isobars and isotopes, which will be differentiated below.

In the case of isobars, as mentioned before, they have the same number of nucleons —that is, the same mass number—, where the number of protons by which one species is greater than the other agrees with the number of neutrons. that are in deficit, so the total is the same. However, its atomic number is different.

In this sense, isobar species come from different chemical elements, so they are located in different spaces of the periodic table and have different characteristics and specific properties .

On the other hand, in the case of isotopes, the opposite happens, since they have the same atomic number but different amounts of mass; that is, they have the same number of protons but a different number of neutrons inside their atomic nuclei.

Furthermore, isotopes are atomic species belonging to the same elements, so they are located in the same space on the periodic table and have similar characteristics and properties.

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