Atom's structure

Isotopes: Same Atomic Number and Different Mass Number

Isotopes: same atomic number and different mass number

Atoms whose atomic nuclei have the same atomic number but a different mass number are called isotopes. Not all atoms of the same element are identical, and each of these varieties corresponds to another isotope showing different physical properties.

This article will discuss what exactly isotopes are and how they can be used in various applications. We will also look at the potential risks associated with using isotopes and how to reduce these risks.

What Is an Isotope?

Isotopes are different forms of an element whit the same number of protons but a different number of neutrons with practically the same chemical properties. This term indicates that all types of chemical element atoms are located in the same place on the periodic table.

According to the isotope definition, each one that belongs to the same element has the same atomic number (Z) but a different atomic mass (A). The atomic number fits with the number of protons in the atom's atomic nucleus. The mass number is the sum of neutrons and protons in the core.

It means that the different isotopes of the same element differ only by the number of neutrons.

Isotopes: same atomic number and different mass number

Although they can have any number of neutrons, there are some preferred combinations of protons and neutrons in the different atoms.

Those that are light tend to equal the number of neutrons and protons, while heavier ones tend to have more neutrons than protons.

What Is Mass Number?

The mass number of an element is the sum of the number of protons and neutrons in its nucleus. This number is written as a superscript after the element symbol. For example, carbon-12 has 6 protons and six neutrons, so its mass number is 12.

An electrically stable atom has the same number of electrons and protons since both sub-particles have the same electrical charge with an opposite sign, and neutrons have no electrical charge.

What Is the Atomic Number?

The atomic number is the total number of protons in the nucleus of an atom. It is the number used to determine the element's place on the periodic table. Therefore, the atomic number of a chemical element cannot be changed.

Natural Occurring Isotopes

Items that we can find in nature can be in a wide variety of different configurations. The mass that appears in the periodic table of the elements is the average of all found naturally.

Isotopes of hydrogen can come in three different configurations: protium, deuterium, and tritium. These three are naturally occurring and are used as fuel for nuclear fusion. Concerning atomic weapons they are the essential elements that make up the hydrogen bomb.

Most natural elements are made up of several natural isotopes that physical procedures can only separate.

What Are Unstable Isotopes?

The unstable atoms are radioactive: their nuclei change or disintegrate, emit radiation, and become other elements. In addition, their nucleus breaks apart, undergoing radioactive decay

The unstable radioactive isotopes can be found with excess or lack of neutrons. These atoms may exist for some time, but they are unstable.

Isotopes: same atomic number and different mass numberIt is precisely this instability that is sought in nuclear fuel. This is because being unstable makes it much easier to generate nuclear fission reactions.

Usually, what makes an atom unstable is the large nucleus. It will be inconsistent if a core becomes large enough from the number of neutrons. To solve that issue, it tries to eject its neutrons or protons to achieve stability.

Isotope-ratio mass spectrometry (IRMS) measures the relative abundance of stable isotopes in a sample.

A radioactive isotope's half-life is the time it takes one-half of the isotope to decay. A radioactive isotope's half-life is constant, regardless of conditions, and is independent of its initial amount.

Some nuclei that emit radiation have very long half-lives. For instance, Uranium-238 has a half-life of 4.5 billion years.

Some Uses and Applications

They have nearly identical chemical properties, but they can have very different physical properties. For example, many isotopes are stable, meaning they do not undergo radioactive decay. Besides, some isotopes are radioactive, which will undergo radioactive decay to form a new element.

Isotopes can be used for a variety of purposes. Some of them are used in medicine, such as iodine-131, which is used to treat thyroid cancer. Others are used in research, such as carbon-14, which is used to date archaeological artifacts.
Finally, some isotopes are used in industry, such as uranium-235, which is used in nuclear power plants.

The Dangers of Isotopes

While all isotopes of an element are chemically identical, some are unstable and decay over time. Unstable ones emit radiation as they decay, which can harm living things. Radioactive isotopes are often used in medicine and industry, but they can be dangerous if not correctly handled.

In addition, they can also be stable, meaning they do not decay over time. Most natural elements are stable, but some artificial elements are unstable. While these elements are not harmful in small amounts, they can be dangerous if exposure is too high or prolonged.

Examples and Uses of Unstable Isotopes

These unstable atoms have many possible applications in our lives.

  • The isotopes of cobalt are used in nuclear medicine to stop the spread of cancer.

  • Radioactive isotopes can be used as tracers in patients to monitor various internal processes. 

  • In industry, these elements can measure the thickness of metal.

  • The unstable uranium atoms are used as fuel in nuclear power plants.

  • Archeologists use isotopes of carbon for dating. For example, carbon 14  is widespread in archeology to establish the dating of different elements.

  • Uranium, plutonium, and hydrogen can be used to build nuclear weapons such as the atomic bomb.


Isotopes are atoms that have the same atomic number but different mass numbers. These atoms can be used in various ways, such as to study radioactive decay and to help understand the makeup of materials.

Isotopes are an essential tool for scientists, who use them to better understand the properties of matter on an atomic level. They also provide insight into how elements interact with one another and give us a better understanding of our universe.


Published: January 2, 2013
Last review: November 25, 2020