An isotope of  an element has a standard atomic number but different molecular masses. An isotope of an element possesses almost similar chemical properties as other elements or other isotopes of that particular element. Isotopes have the same number of protons in their outer shell. They differ by the number of neutrons present in their nuclei.

The most common example of isotopes, which is studied extensively, is that of hydrogen. Hydrogen has a simple nucleus. The element of hydrogen has three isotopes, namely hydrogen, deuterium and tritium. All the isotopes of hydrogen have a single proton, but differ in the number of their neutrons. Hydrogen does not have any neutrons, deuterium has one neutron and tritium has two neutrons. The isotopes of hydrogen have mass numbers of one, two and three, respectively. 

Meaning of Isotope

An element is made up of one type of atom and these atoms are distinguished by their atomic number, which is the number of protons they contain in the nucleus. Atoms with different neutrons but an equal number of protons are known as isotopes. Isotopes are atoms of a particular element with the same atomic number but different mass numbers. 

Each isotope is identified by its atomic mass: the number of protons plus neutrons in each atom. For example, all carbon atoms have six protons but can vary in terms of neutrons; they can have six (atomic number), seven or eight (mass number). There are three naturally occurring carbon isotopes: carbon-12, carbon-13 and carbon-14. Carbon-12 has six neutrons and six protons, while carbon-13 has six protons and seven neutrons. Carbon-14 has six protons and eight neutrons. The only difference between carbon-12, carbon-13 and carbon-14 is in the number of neutrons.

In simple words, isotopes are “atoms with the same number of protons but different numbers of neutrons.”

Types of Isotopes

There are two types of isotopes: stable and radioactive isotopes.

1. Stable Isotope: Isotopes that do not decay into other elements are isotopes. Only certain elements can be used as stable isotopes. Stable isotopes found in nature are used to calculate the age of things like dinosaur bones. For example, carbon-12 and carbon-14 are stable isotopes of carbon with six protons and six neutrons or eight protons and six neutrons, respectively, in the nuclei. These do not decay radioactively since they have no excess neutrons or missing protons.
2. Radioactive Isotope: Isotopes that are unstable because they have one or more excess neutrons or missing protons are radioactive isotopes. These types of atoms are likely to undergo radioactive decay, a process by which a heavy nucleus loses energy by emitting ionizing particles (alpha particle, beta particle and gamma-ray) with resulting transmutation into a lighter atom with the same number of protons but fewer neutrons called daughter nuclide. The transmutation changes its atomic mass number as well as chemical properties. 

Radioactive isotopes can be used to measure volcanic activity or see how quickly corrosion is happening in a bridge that’s under construction. Scientists also create radioactive isotopes in laboratories for medical uses or in bombs. One such element is carbon-14 (C – 14), with six protons and eight neutrons for each atom. This radioactive isotope emits beta particles to become carbon-12 (C – 12) with six protons and six neutrons.

Examples of Isotopes  

Examples of commonly used and important isotopes and their physical and chemical properties:

•  Carbon 13 (C): (¹³C) has an atomic mass of 13 because it contains 3 protons and 10 neutrons. Its nuclei are made up of 6 protons and 7 neutrons, so the sum of protons and neutrons is equal to 13. It is also called carbon radioisotope because it can be used to trace radioactivity.
•  Oxygen 16 (16O): It is an isotope of oxygen with 8 neutrons in its nucleus. It is commonly found on earth.
•  Oxygen 18 (18O): It has an atomic mass of 17, so it contains 8 protons and 9 neutrons in its nucleus. It is not usually found naturally, but it can be made by bombarding nitrogen with a high-speed helium nuclei.
• Hydrogen 2 (²H): It is also called deuterium. Its chemical symbol is D because it contains 1 proton and 1 neutron in the nucleus. Hydrogen isotopes are used for scientific research.
• Argon 40 (Ar): This is a stable isotope of argon with an atomic mass of 40, which means it has 18 protons and 22 neutrons in the nucleus. It is found in rocks, soil, and the whole atmosphere.
• Lead 208 (Pb): This isotope is a naturally occurring one that has 82 protons and 126 neutrons in its nucleus.
• Uranium 235 (U): This is used to produce nuclear energy through fission. It contains 92 protons and 143 neutrons in the nucleus.
• Radium 226 (Ra): It is a radioactive isotope of radium with an atomic mass of 226, which means it contains 88 protons and 138 neutrons in the nucleus. It emits alpha particles, which are helium nuclei (two protons and two neutrons). These particles are highly charged. Therefore, they can cause damage to tissue by ionization. This is how it is used to treat cancer. The radiation produced by this isotope is directed at the tumour through shielding. 

We can see that some elements and even isotopes have different properties, but most importantly, they all come from nature.

Conclusion

The term isotope refers to a nucleus with the same proton but different neutrons. The commonest example of isotopes most widely studied is that of hydrogen. It has a simple nucleus. The element of hydrogen has three isotopes: hydrogen, deuterium and tritium. All the isotopes of hydrogen have a single proton, but differ in the number of their neutrons. Hydrogen does not have any neutrons, deuterium has one neutron and tritium has two neutrons. The isotopes of hydrogen have mass numbers of one, two and three, respectively.