What are Isobars: Definition, Examples, Difference

When the atoms of different elements have the same mass number but different atomic number, then they are called isobars in chemistry. As the mass number is equal to the sum of the number of protons and neutrons present inside the nucleus of an atom, therefore, we can also define isobars as:

Atoms of different chemical elements that have different numbers of protons but the same total number of protons and neutrons (mass number) are called isobars. The word “isobar” has been derived from Greek word whose meaning is “equally heavy” (isos = equal and barys = heavy).

For example, carbon-14 (¹⁴₆​C) and nitrogen-14 (¹⁴₇​N) are isobars in chemistry. As you can observe in this example, these atoms have the same mass number 14, but different atomic numbers, which are 6 and 7. Therefore, they are called isobars. Another example of isobars is argon-40 (⁴⁰₁₈​Ar), potassium-40 (⁴⁰₁₉​K), and calcium-40 (⁴⁰₂₀​Ca).

Characteristics of Isobars in Chemistry

There are the following important characteristics of isobars that you should remember. They are:

• Isobars have the same mass number, which means that the sum of protons and neutrons which is equal to the mass number (A = n + p) is also the same.
• They must have different atomic numbers due to varying of number of protons.
• They have different number of protons, neutrons, and electrons.
• Isobars have different electronic configurations.
• They have different number of valence electrons.
• They have different physical as well as chemical properties because isobars have atoms of different elements.
• They are placed in different groups of periodic table because isobars have different atomic numbers.
• Stable isobars are more commonly observed among heavier nuclei compared with lighter ones.

Examples of Isobars

1. Let’s take an example of isobars with mass number 36: Sulphur-36 (³⁶S₁₆), Chlorine-36 (³⁶Cl₁₇), and Argon-36 (³⁶Ar₁₈). These elements have an identical mass number 36 but different atomic numbers varying from 16 to 18. You can see their structure in the below figure.

2. Tritium (³H₁) and Helium-3 (³He₂) are isobars because they have the same mass number of 3 but different atomic numbers 1 and 2. Tritium is an isotope of hydrogen, also known as hydrogen-3. It contains the number of proton 1 and the number of neutrons 2. On the other hand, Helium-3 is an isotope of helium with the number of protons 2 and number of neutron 1.

3. Calcium-40 (⁴⁰Ca₂₀) and Argon-40 (⁴⁰Ar₁₈) are isobars because they both have a mass number of 40 but different atomic numbers:

• Calcium-40 has an atomic number of 20, meaning that it has 20 protons.
• Argon-40 has an atomic number of 18, meaning that it has 18 protons.

Since they have the same mass number (i.e. total nucleons) but different numbers of protons, they are considered isobars.

Uses of Isobars

There are several important uses of isobars in various fields. They are as:

1. Radioactive Dating:

• Potassium-40, an isobar of calcium-40, is commonly used in the radiometric dating. It is used to determine the age of rocks and fossils.

2. Medical Field:

• Isobars are used in the medical field to treat blood clots and tumors.
• Isobars of iodine and phosphorus are used in treating thyroid disorders and blood cancer, respectively.

Difference between Isotopes and Isobars

Here, we have listed the most important difference between isotopes and isobars. They are as: