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What is the mass number? Definition and examples

The mass number, also called the mass number, is the sum of the protons and neutrons contained in an atomic nucleus . It is usually represented by the letter “A” and in the periodic table it is indicated by a number that is generally above the name of the element.

What is the mass number

The mass number or mass number indicates the total number of particles that are in the atomic nucleus. Therefore, it can be defined as the sum of the protons and neutrons of an atom. It is indicated as a subscript to the left of the element. For example, the mass number of sodium is represented like this: 23 Na.

Electrons are not counted in the mass number because they have a much smaller mass than protons and neutrons, and therefore have almost no influence on it.

Differences between mass number and atomic number

Although these two concepts can sometimes be confused, there are some differences between the atomic number and the mass number:

• The atomic number is the total number of protons in an atom , while the mass number also includes neutrons.
• To identify both numbers, the letters that come from German are used: “Z” for the atomic number, from the German Zahl , and means “number”. Instead, for the mass number the letter “A” is used, which comes from the German term Atomgewicht and means “atomic weight”. However, it is important to note that the mass or mass number is not the same as the atomic mass. Atomic mass is measured in amu , that is, atomic mass units.
• The mass number is usually greater than the atomic number, usually twice.
• The mass number varies in each isotope. Therefore, the mass number of the most stable isotope is used as a reference.

How to Calculate Mass Number

To easily calculate the mass number of an element you can use the following equation:

A (mass number) = Z (atomic number) + N (number of neutrons)

In the same way, to know the number of neutrons in an atom, the following operation can be carried out: A – Z = N.

Both the mass number and the atomic number can be found on the periodic table of the elements.

Examples of mass numbers

In order to differentiate the mass number and the atomic number, in addition to recognizing the number of neutrons in an element, it is important to take into account some examples of mass numbers. For example, chlorine ( 37  17  Cl) has a mass number of 37 and an atomic number of 17. This means that its nucleus contains 17 protons and 20 neutrons.

Another common example is carbon ( 13 C). Its mass number is 13. To know, then, the number of neutrons in the carbon (C) atom, you just have to subtract the number of protons (atomic number) from the mass number. In this way, we can know that carbon-13 has 7 neutrons, since its atomic number is 6.

Other examples are:

• Oxygen ( 16 O)
• Uranium ( 238 U)
• Calcium ( 40 Ca)
• Iron ( 56 Fe)
• Aluminum ( 27 Al)
• Neon ( 20 Ne)
• Hydrogen ( 1 H). This is an exception, and the atomic number is indicated, because it has no neutrons.