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Define the Term Isotope?

Isotope definition or isotope meaning can be given as the chemical element variants that possess the same count of electrons and protons but with a different neutron count. In other terms, isotopes are the variants of elements that vary in their nucleon numbers because of the difference in their respective nuclei's total neutron number. For example, carbon-12, carbon-13, and carbon-14 are all the isotopes of carbon. Carbon-12 comprises a total of 6 neutrons, carbon-13 contains a total of 7 neutrons, and Carbon-14 comprises a total of 8 neutrons.


Representation of Isotopes

Isotopes can be majorly represented in 2 different ways, which are given as follows:

By writing the name of the element followed by a hyphen and the mass number of the isotope. For example, uranium-235 and uranium-239 are two different isotopes of the element uranium.


Following the AZE notation (which is also known as the standard notation). This involves writing the symbol of an element and prefixing the atomic number in subscript and the mass number in superscript. For example, the uranium-239 isotope can be represented as 23992U, whereas, uranium-235 isotope can be represented as 23592U.


Determining the Neutron Number in an Isotope

The total neutron number present in an isotope nucleus is determined by subtracting the element's atomic number from the mass number of the isotope. As an example, the atomic number of carbon is 6, and the 12C isotope of carbon contains a mass number of 12. Thus, the total neutron number present in the carbon-12 isotope is equal to 6.


Stable Isotopes, Radioactive Isotopes, and Primordial Isotopes

  • A few isotopes contain unstable atomic nuclei that experience radioactive decay. These specific isotopes are radioactive in nature and are, thus, called radioisotopes (otherwise radionuclides). A few examples of the radioactive isotopes can be given as chlorine-36, uranium-235, uranium-238, and carbon-14, tritium (hydrogen-3).

  • Primordial nuclides are the ones that existed because of the solar system formation. Out of 339 naturally occurring isotopes on Earth, a total of 286 are known to be the primordial isotopes.

  • A few isotopes are known to have extremely long half-lives (in the order, hundreds of millions of years). Such types of isotopes are commonly known as either stable isotopes or stable nuclides. Some common examples of stable nuclides are given as oxygen-16, oxygen-17, oxygen-18, carbon-12, and carbon-13.

Comparison Between the Isobars and Isotopes

An isotope is described as a variation of an element that possesses a similar atomic number but with a different mass number. A group of isotopes with any element will always contain a similar number of electrons and protons. Moreover, they will vary in the number of neutrons held by their respective nuclei. One example of a group of isotopes is given as hydrogen-1 (so-called, protium), hydrogen-2 (so-called, deuterium), and hydrogen-3 (so-called, tritium).


On the other side, isobars are described as the chemical species that contain a similar number of nucleons, with different atomic numbers. The isobar groups will differ in the number of protons, in the atomic number, the number of neutrons, and the number of electrons. However, they will always contain a similar number of nucleons. Thus, the sum of the number of neutrons and the number of protons will always be the same in an isobar group. An example of an isobar group is given by argon-40, chlorine-40, sulfur-40, potassium-40, and calcium-40.


To simplify more, isotopes hold similar atomic numbers but different mass numbers. On the other side, the isobars contain similar mass numbers but with different atomic numbers.


Applications of Isotopes

  • One of the important applications of isotopes is given in the determination of the isotopic signature of the element samples through isotope analysis. In general, this is done via the process of isotope ratio mass spectrometry.

  • The chemical reaction mechanism can be determined using isotopic substitution. The change in the rate of reaction can be measured on the basis of the kinetic isotope effect.

  • Isotopes are also used to determine the concentration of several substances/elements via isotope dilution.

An Isotope of an Element

The isotope of an element can be described as one of the many variants of the specific chemical element that carries a similar number of electrons and protons as the atomic number of the element but contains a variable number of neutrons when compared to the other variants (isotopes) of the element. Alternately, isotopes can be defined as variants of elements that differ in their nucleon numbers because of the difference in the total number of neutrons in their respective nuclei.


Isotopes of Hydrogen

The three isotopes of hydrogen can be listed as follows.

  • Hydrogen-1 or Protium  

This isotope of hydrogen holds 1 electron, 1 proton, and zero neutrons.

  • Hydrogen-2 or Deuterium 

This isotope of hydrogen holds 1 electron, 1 proton, and 1 neutron.

  • Hydrogen-3 or Tritium 

This isotope of hydrogen holds 1 electron, 1 proton, and 2 neutrons. It should also be noted that this isotope of hydrogen is radioactive.

FAQ (Frequently Asked Questions)

Q1. Explain the Meaning of the Word 'Isotope'?

Ans. The isotopes of a chemical element are defined as atoms with similar atomic numbers but with different mass numbers. This indicates that all the isotopes of an element contain a similar number of protons in their atomic nuclei and a similar number of electrons in the nucleus's electron cloud. However, they differ in the total neutron count present in their respective atomic nuclei.

Q2. Mention the First Person to Find the Multiple Stable Isotopes of an Element?

Ans. The British physicist named J.J. Thomson was the first person to discover the evidence for multiple isotopes of the element neon (Ne) in 1912. Later, another physicist and English chemist, named F.W. Aston, discovered several other stable isotopes of elements using a mass spectrograph.

Q3. Give Examples of Isotopes?

Ans. Any atom having a similar amount of electrons and protons but with a different number of neutrons. This is most important in nuclear physics/chemistry when it comes to things such as enriching Uranium-235 (purifying it of the Uranium-238) or enriching Plutonium-239 (purifying it of the Plutonium-240).

Q4. Give the Use of Isotopes?

Ans. Use of isotopes in the diagnosis of the disease

  • Haemoglobin Contains Iron

The rate where the injected radioactive Fe(56) appears in the blood provides the measurement of red blood cells' production.

  • Test of Cancer Growth

Co(60) gamma rays emit high energy, which can be used for testing the deeply seated cancer growth.

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