Isotopes: Types, Applications, Representation and Examples

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Anjali Mishra

Content Writer-SME | Updated On - Jun 14, 2024

Isotopes are the elements that have the same number of electrons and protons but a varying number of neutrons. In other words, they have same atomic number but different atomic mass.

  • The atomic number of an element is determined by number of electrons. 
  • Isotopes are categorized into three types: Stable, Primordial, and Radioactive isotopes. 
  • Hydrogen is group I element which has three isotopes: Hydrogen, Deuterium, and Tritium.
  • Isotopes plays an important role in nuclear reactors and treatment of cancers.

Read also: Basic Concepts in Chemistry


What are Isotopes?

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Isotopes are defined as those nuclear species which have the same atomic number but a different mass number. The difference in mass number is due to the presence of a different number of neutrons in the elements. 

  • The most common example of isotope is hydrogen which exists in three different forms: 1H1,1H2 ,1H3.
  • Stable isotopes are found naturally and are unreactive in nature. 
  • The isotopes which are highly reactive in nature are called unstable or radioactive isotopes. For example- Uranium.

Isotopes

Calculating the Neutrons in an Element

The nucleus of any element comprises a set of neutrons and protons. A total number of neutrons and protons together represents the mass number of an element. 

Therefore the number of neutrons is determined by the difference between the mass number and the atomic number. For example, the mass number of tritium is 3 and the atomic number is 1.

The formula for number of neutrons is given as: 

Number of neutrons = Mass No. – Atomic No.

Therefore, using the formula, number of neutrons in tritium = 3 - 1 = 2

Hence, tritium has 2 neutrons in excess as compared to protium.

Atom

Atom

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Representation of Isotopes

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For the representation of isotopes, atomic number and atomic mass of an element must be identified first. Moreover, the symbolic representation of isotopes are done by following ways:

  1. First, the name of the element is written, followed by the hyphen and the mass number of the respective isotope.
  2. In this method of representation, we follow the standard, AZE notation.
Representation of Isotopes

Representation of Isotopes

Here E represents the element, A represents the mass number, and Z represents the atomic number. The mass number A is written in superscript, and Z is written in the subscript, i.e., ZAE.

For example, isotopes of hydrogen protium, 11H, Deuterium: 12H, and tritium: 13H.


Types of Isotopes

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Isotopes are broadly categorized into three categories:

  • Stable isotopes
  • Primordial isotopes
  • Radioactive isotopes

1. Stable Isotope

The Isotopes which are having extremely long half-lives (half-life is defined as the period in which the compound undergoes decomposition and attains the concentration or weight equal to half, or 50% of its initial concentration or weight), are termed as stable isotopes. Some stable isotopes are

  • Carbon: Carbon-12, Carbon-13
  • Oxygen: Oxygen-16, Oxygen-17, Oxygen-18

2. Primordial Isotope

These are the isotopes possessing primordial nuclei. Primordial nuclides are the ones that have existed since the formation of our solar system. There are a total of 339 naturally occurring isotopes on earth, out of these 339, 286 isotopes are primordial in nature.

3. Radioactive isotopes

Certain isotopes have very short lives and decay rapidly characterized by the emission of radioactive waves. Such isotopes are termed radioactive isotopes. Some radioactive isotopes are:

  • Hydrogen: Tritium
  • Carbon: Carbon-14
  • Chlorine: Chlorine-36
  • Uranium: Uranium-235 and Uranium-238

Examples of a few isotopes

A few naturally occurring isotopes are mentioned in the table below:

Element Number of isotopes Isotope
Hydrogen three 11H, 12H, and 13H
Helium two 23He and 24He
Carbon three 612C, 613C, and 614C
Nitrogen two 714N and 715N
Oxygen three 816O, 817O, and 818O
uranium two 92235U and 92238U

Difference between Isotopes and Isobars

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Isotopes Isobars
Isotopes are defined as those chemical species which have the same atomic number but a different mass number. Isobars are defined as those chemical species which have varying atomic numbers but similar mass numbers.
The difference in mass number is due to the presence of a different number of neutrons in the elements The difference in atomic number is due to the presence of the same number of protons and neutrons in the elements
They are the same elements They are different elements
They have different numbers of neutrons but a similar number of protons They have the same number of protons and neutrons in total, but different numbers of protons individually.
Example:
  • Oxygen: Oxygen-16, Oxygen-17, Oxygen-18
  • Carbon: Carbon-12, Carbon-13
Example: A group of isobars is comprising of:
  • chlorine-40,
  • argon-40,
  • sulfur-40,
  • calcium-40, and
  • potassium-40.

Applications of Isotopes

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  1. Isotopes provide specific properties to specific elements having the same atomic number. Hence, isotopes are used for the chemical analysis of the element.
  2. Isotopes are often used to determine the rate of reaction or change in concentration of the reactants and products since they are easier to determine.
  3. They are also used in analytical procedures such as mass spectrometry, Nuclear magnetic resonance, etc.
  4. Isotopes of uranium are used in nuclear reactors, while isotopes of cobalt and iodine are used in cancer treatment.

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Things to Remember based on Isotopes

  1. Isotopes are defined as those chemical species which have the same atomic number but a different mass number.
  2. The difference in mass number is due to the presence of a different number of neutrons in the elements.
  3. We can find out the number of neutrons in an element by subtracting the atomic number from its mass number.
  4. There are three types of isotopes: Stable, Radioactive, and Primordial
  5. Isotopes and Isobars are not the same, Isobars are defined as those chemical species which have a different atomic number but have the same mass number.

Sample Questions based on Isotopes

Ques. What is an isotope? (1 mark)

Ans. Isotopes are defined as those chemical species which have the same atomic number but a different mass number. The difference in mass number is due to the presence of a different number of neutrons in the elements.

Ques. Which of the following pairs represent an isotope: (1 mark)
(A) 1 1H, 2 1H
(B) 12 6C, 12 6C,
(C) 12 6C, 13 8C,
(D) 235 92U 238 93U

Ans. Option A represents an isotope pair

Ques. The radioactive isotope amongst the flowing are: (1 mark)
(A) 2 1H
(B) 12 6C
(C) 1 1H
(D) 235 92U

Ans. Option D is radioactive.

Ques. Differentiate between isotopes and isobars. (3 marks)

Ans. 1. Isotopes are defined as those chemical species which have the same atomic number but a different mass number whereas Isobars are defined as those chemical species which have different atomic numbers but have the same mass number.

  1. Isotopes have different numbers of neutrons but a similar number of protons while isobars have the same number of protons and neutrons in total, but different numbers of protons individually.
  2. Example of Isotopes: Oxygen: Oxygen-16, Oxygen-17, Oxygen-18

Example of Isobars: chlorine-40, argon-40, sulphur-40

Ques. Mention all the isotopes of hydrogen. (3 marks)

Ans. Hydrogen has three naturally occurring isotopes. They are: hydrogen-1 (protium), Hydrogen-2 (deuterium), and hydrogen-3 (tritium).

  • Hydrogen-1 has 1 electron, 1 proton, and no neutrons,
  • Hydrogen-2 has 1 electron, 1 proton, and 1 neutron, and
  • Hydrogen 3 has 1 electron, 1 proton, and 2 neutrons.

Ques. What are the different types of isotopes? (3 marks)

Ans. Isotopes are broadly categorized into three categories:

  • Stable isotopes: The Isotopes which are having extremely long half-lives are termed stable isotopes.
  • Primordial isotopes: These are the isotopes possessing primordial nuclei.
  • Radioactive isotopes: They have very short lives and decay rapidly characterized by the emission of radioactive waves. 

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CBSE X Related Questions

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