Atom's structure

What is the mass number of an atom? the formula and definition

What is the mass number of an atom? the formula and definition

Understanding and learning the basics of chemistry can be an intimidating task. But once you understand the essential concepts, it becomes much easier to understand and memorize. One of those crucial concepts is the concept of mass number in an atom.

An atom's mass number represents the total number of protons and neutrons in its nucleus. It is crucial for understanding more complex chemistry processes and equations, such as nuclear reactions and fission. In this article, we will explore what is the mass number of an atom, its definition, along with its formula.

In chemistry, the mass number indicates that: "the mass number of an atom is the sum of the number of protons and neutrons in the atomic nucleus." In this case, the number of electrons around the nucleus is not considered.

Neutrons and protons are the two subatomic particles that make up the nucleus of an atom.

Do not confuse an element's atomic number with its mass number. In this case, the atomic number is the number that indicates only the number of protons in the nucleus.

Composition of an atom

Atoms are composed of protons, neutrons, and electrons. The protons and neutrons are in the nucleus, which is surrounded by electrons.

The number of protons in an atom determines its element. In addition, the number of neutrons in an atom determines its isotope.

For example, all atoms with 6 protons are carbon atoms. Isotopes are atoms of the same element that have different numbers of neutrons. For example, an atom of carbon has two common isotopes: 12C ( 6 protons and 6 neutrons) and 13C ( 6 protons and 7 neutrons present in the nucleus).

An electron has the same electrical charge as a proton. Knowing that neutrons have no electrical charge, a neutral atom implies that it has the same number of protons as electrons. 

The mass number formula

The value of the mass number can be calculated with the following formula:

Mass number (A) = atomic number (Z) + number of neutrons (N).

Increasing the atomic number, the mass number increases.

The atomic number is the total number of protons in the nucleus. This number is unique for each chemical element in the modern periodic table. Besides, the chemical elements are ordered according to this number in the periodic table.

According to the formula, subtracting the atomic number from the mass number, we can know the number of neutrons of the isotope. Furthermore, the mass number minus the number of neutrons equals the atomic number.

An isotope is a variant of an element with a different number of neutrons in its nucleus. 

How is the mass number of an atom represented?

The mass number of an atom is represented as a superscript to the left of the element symbol, for example, 206Pb. The notation Pb-206 is also used.

This heat can be represented by the letter A (which comes from German Atomgewicht). The periodic table of chemical elements lists the atomic mass of the most stable isotope.

How to find the mass number of an atom

The mass number of an atom is the sum of the number of protons and neutrons in the nucleus. The easiest way to find the mass number of an atom is to look at the periodic table. The mass number is listed above the symbol for each element. For example, the mass number of carbon is 12, and the mass number of oxygen is 16.

What is the difference between mass number and atomic mass?

According to the definition of mass number, this is the sum of the protons and neutrons in an atom. However, not all atoms of the same chemical element have the same number of neutrons, as happens, for example, with uranium. Different varieties of atoms with varying numbers of neutrons are called isotopes.

Relative atomic mass

On the other hand, an element's atomic (relative) mass is a weighted average over the mass numbers of the element's isotopes. The weighting is based on the relative presence of the isotopes in nature.

The standard atomic weight is the ratio of the average atomic mass of the different isotopes of that element (weighted by abundance) to the unified atomic mass unit.

Unit of atomic mass

The units amu (unified atomic mass unit) are used to express the mass of atoms and molecules. The amu is a standard unit of mass defined as one-twelfth (1/12) of the mass of an atom of carbon-12, not bonded and neutral in its ground state. Therefore, the atomic mass of a carbon atom is 12 amu.

Importance of mass number

This property is essential for several reasons:

Determining the identity of the element

The mass number is a unique characteristic of each isotope of an element. Changing the mass number will change the identity of the isotope and therefore of the element itself. This is important for classifying and distinguishing the different elements on the periodic table.

Chemical properties and reactivity

Although the mass number does not directly influence the chemical properties of an atom (which are mainly related to its electronic configuration), it does have an indirect impact on the stability and reactivity of an isotope.

Isotopes with very high or very low mass numbers compared to average can be unstable and undergo radioactive decay, which can influence how they interact with other atoms in chemical reactions.

Relative atomic masses

The mass number is also used to calculate the relative atomic mass of an element based on its isotopes and their relative abundances in nature. This information is important in chemistry and physics for stoichiometric calculations and in various scientific applications.

Applications in nuclear physics

In nuclear physics, the mass number is a key factor in the stability and nuclear properties of atomic nuclei. It affects nuclear binding energy, which is critical to understanding fission, nuclear fusion, and other nuclear processes.

Radioactive dating

The radioactive decay of certain isotopes is directly related to their mass numbers. These processes are used in radioactive dating to determine the age of archaeological, geological, and environmental materials, providing valuable information about the history of Earth and other celestial objects.

Technology and industrial applications

Understanding the mass numbers and properties of isotopes is essential in technological applications such as nuclear power generation, nuclear medicine, radiation therapy, and radiographic inspection.

Examples of mass numbers

Below is a table with some relevant isotopes in the context of nuclear power, along with their atomic numbers and mass numbers:



Atomic Number (Z)

Mass Number (A)





























Tritium (H-3)




Deuterium (H-2)












These isotopes are important in nuclear power due to their fission, fusion, and radioactivity properties, which are essential for electrical power generation and other nuclear applications.

Isotopes of uranium

Concerning nuclear energy, uranium is one of the essential chemical elements. Uranium is the nuclear fuel used in nuclear power plants.

Uranium is a naturally occurring atom, but it must be treated in nuclear fuel treatment plants to make it even more unstable.

They can have different numbers of neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Isotopes of uranium include uranium-233, uranium-235, and uranium-238.

  • Uranium-233 is created through the transmutation of thorium-232. It has a half-life of about 157,000 years and is used in nuclear weapons and reactors.

  • Uranium-235 has a half-life of about 700 million years and is used in nuclear reactors. The properties of this isotope make it prone to fission because they are very unstable. When a neutron collides with the nucleus of this isotope, it generates a nuclear fission reaction releasing a vast amount of energy.

  • Uranium-238 has a half-life of about 4.5 billion years and makes up more than 99% of naturally occurring uranium.


In conclusion, understanding the mass number of an atom is vital to mastering physical and chemical concepts. The formula for calculating it is straightforward as long as you understand the composition of each atom and how neutrons and protons contribute to its total weight.

This information can be used in various ways, from identifying different isotopes to estimating potential chemical reactions. With this knowledge in hand, you will have all the tools necessary to gain a better understanding of any element or molecule on the periodic table!

Publication Date: February 27, 2020
Last Revision: August 16, 2023