Let’s Understand the Important 3I’s i.e. Isotopes, Isobars, and Isotones

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What are Isotopes?

All atoms of a given element are not always identical, contrary to Dalton’s Atomic Theory. Most elements, in fact, have been discovered to be made up of two or more types of atoms combined in a specific proportion.

(1) The atomic number of such an element is the same since all of its atoms contain the same amount of protons.

(2) The nucleus of atoms that differ from one another has a distinct number of neutrons. As a result, their atomic masses differ.

Isotopes are atoms of an element with the same number of protons but a different number of neutrons.

Isotopes can also be defined as an element’s atoms with the same atomic number but varying atomic masses or mass numbers.

Symbolic Respresentation of Isotopes

The following notation has been standardized worldwide to denote certain isotopes of an element. The element’s symbol has the atomic mass at the top and the atomic number at the bottom. Alternatively, the element’s name is followed by the atomic mass, separated by a hyphen (-).

Identification of Isotopes

Positive rays emitted by a discharge tube are made up of atom nuclei. In an electric and magnetic field, the deflection of positive rays is proportional to e/m, which is the charge on the particle divided by its mass.

The nuclei formed by a mixture of isotopes will all have the same positive charge, so their deflection will be inversely proportional to their masses.

Thus, we can identify the isotopes present in a given element by using a suitable electric and magnetic field. Following ways can be used to identify an isotope-
(1) Aston’s Mass Spectrograph
(2) Dempster’s Mass Spectrograph

Separation of Isotopes

Because isotopes have identical chemical properties, chemical separation is out of the question. Their difference in physical properties that depend on the mass of the atom has been used to separate them.

The following methods are commonly used for this purpose:

  1. Gaseous Diffusion
  2. Thermal Diffusion
  3. Distillation
  4. Ultracentrifuge
  5. Electromagnetic Separation
  6. Fractional Electrolysis
  7. Laser Separation

Examples of Isotopes

Because all isotopes of an element have the same atomic number, they all have the same number of protons. They have different atomic masses, which can be explained by the different numbers of neutrons in the nucleus. As a result, different isotopes of an element have a different number of neutrons in their nucleus.

An isotope with atomic number Z and mass number A (atomic mass in amu) has the following atomic structure:

(1) The number of extranuclear electrons = Z
(2) The number of protons in the nucleus = Z
(3) The mass number A is equal to the total number of protons (Z) and neutrons (N) in the nucleus. That is,

A = Z + N

∴ N = A – Z

Isotopes of Hydrogen

Hydrogen has three isotopes: protium, deuterium, and tritium. Protium is by far the most abundant element in natural hydrogen, with deuterium accounting for about 0.015 percent and tritium accounting for only one out of every 10,000,000 hydrogen atoms.

Isotopes of Oxygen

There are three isotopes of oxygen. These have relative abundances of 99.759, 0.037, and 0.204, respectively.

Read: Precipitation Titration- Principle, Methods, Applications

Difference between Isobars and Isotones

Isobars are atoms that have the same mass number but different atomic numbers.

The term isobar, which means ‘equally heavy,’ comes from the Greek words isos (equal) and barys (heavy). Because isobars have the same mass number, the number of protons and neutrons in the nucleus of each is equal.

Because the number of protons is given by the atomic number (Z), the number of neutrons is (A – Z), where A is the mass number. The total number of extranuclear electrons equals (Z).

Whereas, Isotones are atoms that have different atomic numbers and masses but the same number of neutrons. Isotones are distinct elements with distinct atomic structures. They differ in their physical and chemical properties.


The atoms of an element that have the same number of protons and different number of neutrons are called

Ans. Isotopes.

Isotopes of an element have______atomic number but______mass numbers.

Ans. same, different

The atoms which have the same mass number but different atomic numbers are called

Ans. Isobars

Atoms that have different atomic numbers, different mass numbers but the same number of neutrons are called

Ans. Isotones

The electronic configuration of isotopes of an element is _____

Ans. Same

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