The sum of the number of protons and neutrons in the atomic nucleus is called the mass number. It is represented by the letter A and is located on the upper left side of the element symbol.

**periodic table**and is often confused with the atomic number or atomic mass. So learn about it and understand the difference between the two quantities.

Outline

## What is mass number?

Mass number refers to the approximate mass of an atom. It is represented by the capital letter **A.** According to theorist Ernest Rutherford, the mass of a chemical element is concentrated in the atomic nucleus. This is because electrons are about 1800 times lighter than protons and neutrons; they have a negligible mass. As the nucleus is composed of protons (positive charges) and neutrons (zero charges), the A value corresponds to the sum of these two units.

The correct way to represent the value of the mass of a chemical element X, is on the upper left side of the symbol for that element, that is, ^{A} X. There are cases in which the atomic number (Z) is also represented, however, it is located on the lower left side of the element, see below the example of how to correctly represent carbon-12, in other words, carbon that has 6 protons and 6 neutrons.

### How to calculate the mass number

To calculate mass number, it is necessary to know the number of protons and neutrons it has in the nucleus, since A represents the sum of these two terms. In this way, it is possible to find the value of A by the equation:

A = p + n

Where:

**A**: is the mass number;**p**: indicates the number of protons (positive charges) in the atomic nucleus;**n**: represents the number of neutrons in the nucleus.

By making mathematical modifications to it, such as rearranging the order of terms, it is possible to calculate any of the three corresponding quantities in the equation. Below are some examples of how to find the mass number of an element.

#### Example 1

**What is the mass number of an atom that has an atomic number of 60 and has 88 neutrons inside its nucleus?**

The values given are those of Z = 60 and of n = 88. As Z = p, the calculation can be done by the already known equation, where:

A = p + n

A = 60 + 88

A = 148

#### Example 2

**Knowing that potassium ( _{19} K) has 20 neutrons that stabilize the nucleus, what is its mass number?**

The number 19 next to the element symbol represents its atomic number (Z). Since Z is equal to the number of protons (Z = p), the calculation of A is given by:

A = p + n

A = 19 + 20

A = 39

As already mentioned, there may be questions that provide the value of A. In this way, the objective of the exercise becomes to calculate one of the other two quantities. Starting from the equation **A = p + n** it is possible to calculate any of them.

### Mass number and Atomic number

These two terms are very confusing, but they indicate different things. The **mass number** is equivalent to the sum of the number of protons and neutrons in the atomic nucleus. When different elements have the same mass number, they are known as *isobars*. On the other hand, the **atomic number** is the number of protons in the atoms and corresponds to the identity of the chemical elements, since two different elements cannot have the same Z.

In summary, the mass number of a chemical element corresponds to the sum of its protons and neutrons present in the atomic nucleus. Don’t stop studying here, see also about atomic number.