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What are Isotopes?

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  • Last Updated : 04 Jul, 2022
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An atom is the smallest unit of an element consisting of protons  (positive charge), electrons (negative charge), and neutrons(neutral). The protons are positively charged particles and electrons are negatively charged particles that determine the valency of the element.  An atom can be divided into two regions the first one is the nucleus which contains protons and neutrons, and another one is the outer shell that carries electrons. Electrons of an atom keep revolving around the nucleus on its own orbit just as we can see in the solar system where planets keep revolving around the Sun. Protons and neutrons share similar mass but electrons are comparatively negligible mass than proton or neutron. The atomic mass is totally determined by the nucleus as it carries protons and neutrons.

What is an isotope?

Isotopes are different types of elements that have the same number of protons but a different number of neutrons. Carbon, potassium, and uranium, for example, all have numerous naturally occurring isotopes. Isotopes are characterized first by their element, then by the total of their protons and neutrons. 

The stability of isotopes varies. Carbon-12 (12C) is the most prevalent isotope of carbon, accounting for 98.89% of all carbon on Earth. Carbon-14 (14C) is an unstable element that exists only in tiny amounts. Unstable isotopes are known to release alpha particles (He2+) and electrons. Through a process known as radioactive decay, neutrons, protons, and positrons can be released and electrons collected to achieve a more stable atomic configuration (lower amount of potential energy). The newly formed atoms may be in a high-energy state and release gamma rays, reducing energy but not converting the atom into a different isotope. These atoms are known as radioactive isotopes.

Let’s now discuss the isotopes of different elements like Hydrogen, Carbon, Oxygen, and Neon as below:

Isotopes of Hydrogen 

Hydrogen has atomic number one and is the first element in the periodic table. Isotopes are elements that have the same atomic number but a different mass number. There are three hydrogen isotopes: protium 1H1, deuterium 1H2 or D, and tritium 1H3 or T. Because of the varying amount of neutrons in them, the isotopes differ.

Hydrogen has three naturally occurring isotopes, which are represented by the letters 1H, 2H, and 3H. Other very unstable nuclei (4H to 7H) have been produced in the lab but have not been detected in nature. Each isotope has distinct characteristics. These isotopes have been widely utilized to date.

The three isotopes of hydrogen are briefly discussed below:

  • Protium (1H1): Protium gets its formal name from its structure as it consists of only one proton. Protium is denoted 1H. It is the most commonly found isotope as its abundance counts 99.98% in nature. It is a stable isotope with an atomic mass is 1.007825 a.m.u.  They are combustible and can be combined with other atoms or compounds.

Atomic Structure of Protium

  • Deuterium (2H1): Deuterium is one of the stable isotopes of hydrogen which is also known as 2H. The nucleus of deuterium consists of one proton and one neutron in its nucleus. Its mass is twice of protium. Deuterium is not radioactive so it doesn’t affect humans negatively. It is also known as heavy water. Generally, heavy water is used as a neutron moderator. Deuterium is also applicable for NMR  spectroscopy and is used as the prototype fusion reactor.

Atomic Structure of Deuterium

  • Tritium (3H1): Tritium is the most stable natural isotope of hydrogen. It is also known as 3H. It was first prepared by bombarding deuterium in 1935. Its consists of one positively charged proton and two negatively charged neutrons. Tritium decays into very light particles which automatically change into helium. It is formed in the upper atmosphere by the reaction of cosmic rays and interaction with gases present in the atmosphere. Tritium is used in making luminous paints and self-powered lighting devices. It is also used as a radiolabel in radiolabeling experiments as well as hydrogen bomb secondaries.

Atomic Structure of Tritium

Isotopes of Carbon

Carbon has 15 known isotopes among which Carbon-12, Carbon-13, and Carbon-14 are the major naturally occurring isotopes. They are stable and occur naturally.

  • Carbon-12: Carbon 12 was initially selected to replace oxygen as it consists of 6 protons, 6 neutrons, and 6 electrons. Carbon-12 is the amplest isotope of carbon, amounting to 98.93% of the element carbon. Symbolically, it is represented by 12C . Carbon 12 is used to define mole as a substance or element. Its isotopic mass counts 12u. is It is used as a standard from which the atomic mass of all nuclides is measured.
  • Carbon-13: Carbon-13 is a stable isotope that is specially used for medical investigation and physiological research. Its natural abundance is 1.109%. It contains six protons and seven neutrons. The isotopic mass of carbon-13 is 13.035u. Symbolically, it is represented by 13C. They are used in medical diagnostic tests.
  • Carbon-14: Carbon-14, also known as radiocarbon, is a radioactive isotope of carbon discovered by Martin Kamen and Sam Ruben in 1940.  It contains six protons and eight neutrons. They are used for radiocarbon dating and radiolabeling. Carbon-14 is also used in the medical field for a breath test to detect ulcer-causing bacteria. Symbolically, It is represented by 14C. Its isotopic mass counts 14.10032u.

Isotopes of Oxygen

Oxygen consists of 16 isotopes in which there are three main isotopes  oxygen-16, oxygen-17, and oxygen-18. Oxygen-16 is the most stable isotope among them. So, the isotopes of oxygen are briefly described below: 

  • Oxygen-16: Oxygen-16 is the most stable isotope of oxygen has a natural abundance of 99.76%. It consists of eight protons and eight neutrons. It has a mass of 15.994914619 u. It contributes  99% of natural oxygen. Symbolically, Oxygen-16 is represented by  16O. Radioactive N-13 is produced by using oxygen- 16 which is used for PET imaging and myocardial perfusion.
  • Oxygen-17: The stable and natural isotope of oxygen that has low abundance is known as oxygen-17. It consists of 8 protons and 9 neutrons. Symbolically, it is represented by 17O. Its isotopic mass counts  16.9991315 u. t is used in the field of medical research and helps to improve the practice of medicines in the field of neurology and cardiology. 
  • Oxygen- 18: Oxygen-18 is a stable environmental isotope. It is also generally termed heavy oxygen. Symbolically it is represented by 18O. It consists of 8 protons and 10 neutrons in its nucleus. The natural abundance of Oxygen -18 is 0. 2% and its isotopic mass count is 17.9991610 u. It is generally used in pharmaceutical industries. It is also used to produce heavy water by the combination of oxygen-18 and tritium.

Isotopes of Neon

Neon is the second-lightest noble gas which has 20 isotopes. It has three major isotopes which are neon-20, neon-21, and neon-22. The other 17 isotopes of neon are no long-lived radioactive isotopes.

  • Neon-20: Neon-20 is the major stable isotope of neon. It consists of ten protons and ten neutrons. Symbolically, it is written as 20 Ne. Its atomic mass is 19.99244018 a.m.u. The natural abundance of Neon-20 ranges 90.48%. It is used to produce F-18. Neon-20 is also used to study quantum physics in Masters.
  • Neon-21: Neon-21 is one of the stable naturally occurring isotopes of Neon. Its symbolic representation is given by    21Ne. The natural abundance of Neon-21 is 0.27%.   It has 3/2 spin and is therefore quadrupedal. Its atomic mass is 20.99 a.m.u. Neon-21 is a naturally occurring isotope.
  • Neon-22: Neon-22 is a stable isotope with a natural abundance of 9.25%. It is composed of ten protons and 12 neutrons. Symbolically, it is written as 22Ne. The atomic mass of neon-22 is 21.9913851 a.m.u. They are used in vacuum tubes, television tubes, wavemeter tubes, etc.

Sample Problems 

Problem 1: Which isotope of hydrogen is least radioactive?


Tritium is the most stable isotope of hydrogen. Hence, it is the least radioactive among all the isotopes of hydrogen which were produced by scientists and researchers as they are volatile and cannot exist freely.

Problem 2: What do you mean by heavy hydrogen?


Deuterium oxide is known as heavy hydrogen as deuterium has enriched molecules of water instead of normal hydrogen. It doe does not have any radioactive properties.

Problem 3: Which is the most common isotope of hydrogen?


Protium is the commonly found isotope of hydrogen.  It is so-called because it consists of only one proton. Its atomic no. is equal to its atomic mass and accounts for nearly 99 percent of universal hydrogen.

Problem 4: What causes an isotope?


Atoms of the same element with different numbers of neutrons are called isotopes. Elements contain one or more radioactive isotopes. Their nuclei are unstable, breaking down or declining and emitting radiation.

Problem 5: Why do isotopes exist?


Isotopes exist because different elements have different types of atoms which are referred to as isotopes. They have the same number of protons, but different neutron numbers.

Problem 6: How are isotopes applicable?


Isotopes of an element all have a similar kind of behaviour, but the unstable isotopes experience a spontaneous decline due to which they emit radiation and reach a stable state. The radioisotope activity is useful for:

  • Food preservation
  • Archaeological abject identification
  • Medical diagnosis and treatment, etc.

Problem 7: Are all isotopes are radioactive?


Those elements with atomic numbers of more than 83 are radioisotopes, which means that they have unstable nuclei and are radioactive.

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