What are Isotopes?
Isotopes can be defined as the variants of chemical elements that possess the same number of protons and electrons, but a different number of neutrons. In other words, isotopes are variants of elements that differ in their nucleon numbers due to a difference in the total number of neutrons in their respective nuclei. For example, carbon-14, carbon-13, and carbon-12 are all isotopes of carbon. Carbon-14 contains a total of 8 neutrons, carbon-13 contains a total of 7 neutrons, and carbon-12 contains a total of 6 neutrons.
Isotopes are primarily represented in two different ways:
- 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.
- By following the AZE notation (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-235 isotope can be represented as 23592U and the uranium-239 isotope can be represented as 23992U.
Determining the Number of Neutrons in an Isotope
The total number of neutrons in the nucleus of an isotope can be determined by subtracting the atomic number of the element from the mass number of the isotope. For example, the 12C isotope of carbon has a mass number of 12. The atomic number of carbon is 6. Therefore, the total number of neutrons in the carbon-12 isotope is equal to 6.
Stable Isotopes, Primordial Isotopes, and Radioactive Isotopes
- Some isotopes have unstable atomic nuclei that undergo radioactive decay. These isotopes are radioactive in nature and are, therefore, known as radioisotopes (or radionuclides). Examples of radioactive isotopes include carbon-14, tritium (hydrogen-3), chlorine-36, uranium-235, and uranium-238.
- Some isotopes are known to have extremely long half-lives (in the order of hundreds of millions of years). Such isotopes are commonly referred to as stable nuclides or stable isotopes. Common examples of stable nuclides include carbon-12, carbon-13, oxygen-16, oxygen-17, and oxygen-18.
- Primordial nuclides are the nuclides that have existed since the formation of the solar system. Of the 339 naturally occurring isotopes on Earth, a total of 286 isotopes are known to be primordial isotopes.
Comparison between Isotopes and Isobars
An isotope is a variation of an element that possesses the same atomic number but a different mass number. A group of isotopes of any element will always have the same number of protons and electrons. They will differ in the number of neutrons held by their respective nuclei. An example of a group of isotopes is hydrogen-1 (protium), hydrogen-2 (deuterium), and hydrogen-3 (tritium).
On the other hand, isobars are chemical species that have the same number of nucleons but different atomic numbers. Groups of isobars will differ in the atomic number, the number of protons, the number of electrons, and the number of neutrons. However, they will always have the same number of nucleons. Therefore, the sum of the number of protons and the number of neutrons will always be the same in a group of isobars. An example of a group of isobars is chlorine-40, argon-40, sulfur-40, calcium-40, and potassium-40.
To simplify, isotopes have the same atomic numbers but different mass numbers whereas isobars have the same mass numbers but different atomic numbers.
Frequently Asked Questions on Isotopes
What is the meaning of the term ‘isotope’?
The isotopes of a chemical element are a group of atoms of that have the same atomic numbers but different mass numbers. This implies that all isotopes of an element have the same number of protons in their atomic nuclei and the same number of electrons in the electron cloud surrounding the nucleus. However, they differ in the total number of neutrons present in their respective atomic nuclei.
What are the three isotopes of hydrogen?
The three isotopes of hydrogen are listed below.
- Protium, or hydrogen-1. This isotope of hydrogen contains 1 proton, 1 electron, and no neutrons.
- Deuterium, or hydrogen-2. This isotope of hydrogen contains 1 proton, 1 electron, and 1 neutron.
- Tritium, or hydrogen-3. This isotope of hydrogen contains 1 proton, 1 electron, and 2 neutrons. It can also be noted that this isotope of hydrogen is radioactive.
What are the applications of isotopes?
- An important application of isotopes is in the determination of the isotopic signature of element samples via isotope analysis. This is generally done via the process of isotope ratio mass spectrometry.
- The mechanism of a chemical reaction can be determined with the help of isotopic substitution. The change in the reaction rate can be measured based on the kinetic isotope effect.
- Isotopes can also be used to determine the concentration of many elements/substances via isotope dilution.
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