Isotopes and Isobars: Overview, Questions, Preparation

The chemical elements are generally denoted as AZ X, where X represents the element, ’A’ its mass number (number of protons and neutrons, together known as nucleons), and ‘Z’ the atomic number (number of protons). Eg.: ¹₁H, ¹⁴₆C.

Isobars (iso means equal, and baros means weight) are atoms of different chemical elements (with different atomic numbers ) that have the same mass numbers. . They are also referred to as nuclides with the same mass number but different atomic numbers.

Consider the following set of elements -  ⁴⁰₁₆S, ⁴⁰₁₇Cl, ⁴⁰₁₈Ar, ⁴⁰₁₉K, ⁴⁰₂₀Ca. 

We can see that the mass number of every element in the set is 40, even though their atomic numbers are different. Although the total number of nucleons in each element is the same, isobars DO NOT have the same number of protons or neutrons. 

Variants of the same chemical element with different mass numbers are known as isotopes. (iso means equal and topos means place - since they are at the same position in the periodic table). Isotopes are also referred to as nuclides with the same atomic number but different mass numbers.

Now let us consider a different set - ¹²₆C, ¹³₆C, ¹⁴₆C.

Here, the atomic numbers are the same (6), but the mass numbers are different. This indicates that they have the same number of protons, while the number of neutrons varies. 

Note that they are all the same element, as the number of protons determines the number of electrons in an atom, which in turn controls the chemical characteristics of an element. The nucleus of isotopes will have varying nuclear properties as the number of neutrons influences nuclear behavior.

By virtue of its small mass, isotopes of hydrogen show exceptions to having indistinguishable chemical properties. The heavier isotopes tend to react somewhat more slowly than lighter isotopes, owing to the considerable change in mass as deuterium has twice the mass of protium, and tritium has three times the mass of protium.

Isotopes can be naturally occurring or artificially synthesized.

• Radioactive Isotopes: Isotopes that are radioactive and tend to decay or disintegrate into unstable forms are known as radioactive isotopes or radioisotopes. The time taken by radioactive elements to decay into half of their initial concentration is known as the half-life of the element.
• Stable Isotopes: Isotopes that do not tend to decay over time are known as stable isotopes.

It is observed that stable isotopes are predominant, hence abundant in nature (with a few exceptions where only one stable form is abundant even though the element has more than one stable isotope). This explains the predominance of protium (99.985%) and rarity of deuterium (0.015%) and tritium (extremely low) on earth.

Eg: 14C is a radioactive isotope, while ¹²C and ¹³C are stable isotopes of carbon.

They are discussed in Unit 2 - ‘Structure of Atom’. The chapter discusses the concept with examples. Although not a vast topic, this is a crucial concept necessary for a better grasp of advanced topics.

1. Identify the isobars and isotopes among the following:

 ³⁰₁₄A, ²₁B, ¹²6C, ³⁰₁₅D, ¹³₇E, ¹₁F.

(element notations for illustrative purpose only.)

Solution:  Isotopes - ¹₁F, ²₁B.

Isobars - ³⁰₁₅D, ³⁰₁₄A.

Q: What are isotones?

Q: What are the isotopes of Hydrogen?

Q: What is the number of known stable isotopes? Which element has the highest number of isotopes?

Q: What are primordial isotopes?

Q: What are primordial isotopes?

Q: Name any five applications of isobars and isotopes.