All the atoms of a particular chemical element have the same number of protons in their nuclei, but the number of neutrons may be different. Such atoms have the same atomic number, but mass numbers are different because of a different number of neutrons. These atoms are called isotopes of the element. Thus, we can define isotopes as:
Definition: Atoms of an element which have the same atomic number but different mass numbers are called isotopes of that element. These isotopes are variants of the same element in which the number of protons remains constant, but the neutron count varies. For example, the hydrogen element has three isotopes:
- Protium (H)
- Deuterium (D)
- Tritium (T)
All the three isotopes have the same atomic number 1, but their mass numbers are 1, 2, and 3, respectively due to different number of neutrons in their nuclei.
Table of Contents
Meaning of Isotope
The term “isotope” is originated from the Greek words “isos” and “topos” whose meaning are “equal” and “place“, respectively. Scottish doctor and writer Margaret Todd coined it in 1913, and British chemist Frederick Soddy made it popular. This term refers to the fact that isotopes of an element occupy the same place or position on the periodic table, despite having different mass numbers.
Representation of Isotopes
To denote an isotope, we use the symbol of the element along with its atomic number (Z) and mass number (A). The general representation of an isotope is AZX.
Where:
- X is the symbol of the element.
- Z is the atomic number (i.e. number of protons).
- A is the mass number which is the sum of protons and neutrons present in the nucleus of an atom.
In the above notation of an isotope, the mass number (A) and atomic number (Z) are shown as a superscript and a subscript respectively on the left side of the symbol.
Thus, we can represent the three isotopes of hydrogen as: 11H, 21H, and 31H.
Examples of Isotopes
There are numerous naturally occurring and artificial isotopes. Below are some well-known isotopes, their characteristics, and uses.
Structure of Isotopes of Hydrogen
Hydrogen element has three isotopes namely, protium (hydrogen), deuterium (also called heavy hydrogen) and tritium. These isotopes only differ in their mass number because they have a different number of neutrons inside their nuclei. The structure of three isotopes of hydrogen is shown in the below figure.
Protium: This is the most abundant isotope of hydrogen, which contains just one proton and no neutrons.
Deuterium (also denoted as D): This isotope of hydrogen contains one proton and one neutron. It is a stable and found in small quantities in water known as “heavy water”. It has applications in nuclear fusion research and as a non-radioactive label in scientific experiments.
Tritium: This isotope is radioactive, with one proton and two neutrons. It has applications in nuclear fusion, as well as in self-luminous devices like watch dials.
The characteristics of these three isotopes of hydrogen in the table form are as follows:
| Particles | Protium | Deuterium | Tritium |
|---|---|---|---|
| No. of electrons | 1 | 1 | 1 |
| No. of protons | 1 | 1 | 1 |
| No. of neutrons | 0 | 1 | 2 |
| Atomic number | 1 | 1 | 1 |
| Mass number | 0 | 1 | 2 |
Structure of Isotopes of Carbon
Carbon, whose atomic number is 6, has three isotopes, namely carbon-12, carbon-13, and carbon-14.
Carbon-12: This is the most common isotope of carbon, accounting for about 98.9% of carbon atoms. It is represented by the symbol 126C. This is a stable isotope, which contains six protons and six neutrons.
Carbon-13: This stable isotope of carbon accounts for about 1.1% of carbon atoms, which have 6 protons and 7 neutrons. It is represented by the symbol 136C. This isotope is widely used in various scientific research fields, including metabolic studies involving isotope-labeled compounds.
Carbon-14: This is a radioactive isotope containing 6 protons and 8 neutrons. It is represented by the symbol 146C. Carbon-14 is used in radiocarbon dating to determine the age of archaeological artifacts and fossils. It has a half-life of approximately 5,730 years.
The structure of isotopes of carbon atoms is shown in the below figure.
The characteristics of three isotopes of carbon in the table form are as follows:
| Particles | Carbon-12 | Carbon-13 | Carbon-14 |
|---|---|---|---|
| No. of electrons | 6 | 6 | 6 |
| No. of protons | 6 | 6 | 6 |
| No. of neutrons | 6 | 7 | 8 |
| Atomic number | 6 | 6 | 6 |
| Mass number | 12 | 13 | 14 |
Isotopes of Chlorine
There are two stable isotopes of chlorine: chlorine-35 (³⁵Cl) and chlorine-37 (³⁷Cl).
Chlorine-35: This is the most abundant isotope of chlorine, making up about 75.78% of naturally occurring chlorine. It has 17 protons and 18 neutrons in its nucleus. It is represented by the symbol 3517Cl.
Chlorine-37: This stable isotope makes up about 24.22% of naturally occurring chlorine. It has 17 protons and 20 neutrons in its nucleus. It is represented by the symbol 3717Cl.
Both isotopes only differ in their number of neutrons, having 18 and 20, respectively. The number of protons in the nucleus of the chorine atom is the same. These two isotopes contribute to the average atomic mass of chlorine, which is approximately 35.45 atomic mass units (amu). Since both isotopes are stable therefore, they exist in a fixed ratio in nature. They do not undergo radioactive decay.
Isotopes of Nitrogen
Nitrogen has two stable isotopes: Nitrogen-14 and Nitrogen-15.
Nitrogen-14: This is the most abundant nitrogen isotope, making up about 99.63% of naturally occurring nitrogen. It is represented by the symbol 147N. Nitrogen-14 isotope contains 7 protons and 7 neutrons in its nucleus. Its mass number is 14.
Nitrogen-15: This isotope accounts for about 0.37% of nitrogen found in nature. It is often used in scientific research, such as in nitrogen cycle studies. Nitrogen-15 isotope is represented by the symbol 157N, which contains 7 protons and 8 neutrons inside its nucleus.
Both isotopes of nitrogen are stable and play a vital role in biological processes, including the nitrogen cycle.
Characteristics of Isotopes
Isotopes are atoms of the same element that have an identical number of protons but different numbers of neutrons. Some important characteristics of isotopes are as follows:
(1) Same Atomic Number:
As all the isotopes of a given element have the same atomic number, the number of electrons present in them is also the same. Due to the same number of electrons, their electronic configuration is also identical.
(2) Different Mass Numbers:
The number of neutrons varies between isotopes, resulting in different mass numbers. This gives a unique atomic mass for each isotope.
(3) Similar Chemical Properties:
Since the chemical properties of an element mainly depend on the number of protons in the nucleus, and the number of electrons in an atom, hence, different isotopes of an element show an identical chemical properties. For example, all the isotopes of carbon atoms on burning produce carbon dioxide (CO2). However, rates of chemical reactions are affected by the isotopic mass. Heavier isotopes react slower than lighter one.
(4) Different Physical Properties:
Since all the isotopes of an element differ in their atomic masses, therefore, their physical properties such as density, melting point, boiling point, rates of diffusion, etc. are different.
(5) Stability and Radioactivity:
Some isotopes are stable in nature, meaning that they do not change over time. Others are radioactive isotopes, which undergo decay, and emit radiation in the process. Radioactive isotopes are used in applications like radiometric dating and medical imaging.
(6) Natural Abundance:
Certain isotopes of most elements are more abundant in nature than others. This natural abundance contributes to the average atomic mass listed on the periodic table.
Z, A, and N of Isotopes of Common Elements
Most elements have two or more stable isotopes, while 20 elements with odd atomic numbers have only one stable isotope. Some of these are 94Be, 199F, 2311Na, 2711Al, 3115P, and 5325Mn.
In general, atoms with odd atomic numbers have no more than two isotopes, and their nuclei contain an even number of neutrons. Atoms with even atomic numbers have several isotopes containing an odd number of neutrons, but their most common isotopes are those that contain an even number of neutrons.
Here, we have listed isotopes of some common elements with atomic numbers, mass numbers, and neutrons in the table form which you should keep in mind.
Atomic Number, Mass Number, and Neutrons of Isotopes of Common Elements:
| Element | Isotopes | Atomic number | No. of electrons | Mass number | No. of protons | No. of neutrons |
|---|---|---|---|---|---|---|
| Hydrogen | 11H 21H 31H | 1 1 1 | 1(1) 1(1) 1(1) | 1 2 3 | 1 1 1 | 1 – 1 = 0 2 – 1 = 1 3 – 1 = 2 |
| Carbon | 126C 136C 146C | 6 6 6 | 6 (2, 4) 6 (2, 4) 6 (2, 4) | 12 13 14 | 6 6 6 | 12 – 6 = 6 13 – 6 = 7 14 – 6 = 8 |
| Oxygen | 168O 178O 188O | 8 8 8 | 8 (2, 6) 8 (2, 6) 8 (2, 6) | 16 17 18 | 8 8 8 | 16 – 8 = 8 17 – 8 = 9 18 – 8 = 10 |
| Neon | 2010Ne 2110Ne 2210Ne | 10 10 10 | 10 (2, 8) 10 (2, 8) 10 (2, 8) | 20 21 22 | 10 10 10 | 20 – 10 = 10 21 – 10 = 11 22 – 10 = 12 |
| Chlorine | 3517Cl 3717Cl | 17 17 | 17 (2, 8, 7) 17 (2, 8, 7) | 35 37 | 17 17 | 35 – 17 = 18 37 – 17 = 20 |
| Uranium | 23592U 23892U | 92 92 | 92 (2, 8, 18, 32, 21, 9, 2) 92 (2, 8, 18, 32, 21, 9, 2) | 235 238 | 92 92 | 235 – 92 = 143 238 – 92 = 146 |
| Potassium | 3919K 4119K | 19 19 | 19 (2, 8, 8, 1) 19 (2, 8, 8, 1) | 39 41 | 19 19 | 39 – 19 = 20 41 – 19 = 22 |
| Nitrogen | 147N 157N | 7 7 | 7 (2, 5) 7 (2, 5) | 14 15 | 7 7 | 14 – 7 = 7 15 – 7 = 8 |
Uses of Isotopes
Isotopes have a wide range of applications across various fields, such as medicine, industry, environmental science, and archaeology. Some important uses of different isotopes are as follows:
(1) Medical Uses of Isotopes
Diagnostic Imaging: Radioactive isotopes such as iodine-131 and technetium-99m are used in medical imaging techniques like PET (Positron Emission Tomography) scans and SPECT (Single-Photon Emission Computed Tomography) scans. These isotopes emit radiation that can be detected by imaging devices, which allows doctors to see inside the body and diagnose conditions like cancer or heart disease.
Cancer Treatment: Radioisotopes such as cobalt-60 and iridium-192 are used in radiation therapy to destroy cancer cells. The radiation damages the DNA of cancerous cells, and prevents them from growing and spreading.
Sterilization: Radioactive isotopes like gamma rays are used to sterilize medical equipment to free from bacteria and other pathogens.
(2) Industrial Uses of Isotopes
Radiography: Radioactive isotopes like iridium-192 are used in industrial radiography to inspect the integrity of welds, pipes, and structural components without causing damage.
Tracer Studies: Isotopes such as phosphorus-32 are used as tracers in various industrial processes, including oil exploration and environmental monitoring. By adding a small amount of radioactive material to a system, scientists can track its movement and behavior, helping them understand complex processes.
Nuclear Power: Uranium-235 plays a vital role in fuel in nuclear reactors. It undergoes fission to release large amounts of energy. This energy is used to generate electricity in nuclear power plants.\
(3) Environmental and Archaeological Uses
Radiocarbon Dating: Carbon-14 isotope is used to determine the age of historical and geological materials. By measuring the ratio of carbon-14 to carbon-12 of a material, scientists estimate how long it has been since the organism died.
Climate Studies: Isotopes of oxygen (like oxygen-18) are used to study past climate conditions.
Hydrology: Isotopes like tritium and deuterium are used to study the movement of water in the environment, including groundwater flow and ocean circulation. These studies help to improve our understanding of the water cycle and predict water availability.
(4) Agricultural Uses
Fertilizer Studies: Nitrogen-15 is used to study how plants absorb nitrogen from fertilizers. This helps farmers to optimize fertilizer use and improves crop yields.
