What is Mass Number?

The sum of number of protons and neutrons is termed as the mass number of an atom.

It is represented by using the letter ‘A.’

The symbol A is derived from a German word Atomgewicht means atomic weight. Protons and neutrons present in the nucleus of an atom together, are also called nucleons.

For instance, an atom of Oxygen has 8 protons and 8 neutrons in the nucleus of its atom. Thus, its mass number is 8+8= 16.

The number of protons is the same in all atoms of an element that means, it is unique for all elements, the number of neutrons can vary. Therefore, atoms of the same element can have different mass numbers.

These types of structure are named as isotopes.

The weight of an electron in an atom is nearly insignificant. Therefore, the atomic mass of an atom is approximately the same as its mass number.

Mass Number vs Atomic Number

Atomic Number	Mass Number
The atomic number is defined as the total number of protons present in the nucleus of an atom.	The total number of protons and neutrons present in the nucleus of an atom gives us the mass number of an atom.
It is commonly signified with the letter ‘Z.’	It is commonly represented using the letter ‘A.’
All the atoms of a specific element have the same number of protons, and henceforth they have the same atomic number.	Protons and neutrons present in the nucleus of an atom together, are called nucleons.
For example, all carbon atoms have an atomic number equal to 6, however all atoms of Oxygen have 8 protons in their nucleus thus atomic number of oxygens is 8.	For instance, an atom of carbon has 6 protons and 6 neutrons. Thus, the mass number of carbon atom is 12.

If we want to find the valency of a substance, then we see the electrons present in the outermost shell of the atom. But to further find the atomic number or the mass number, we need to know the total number of protons and neutrons present in the nucleus of an atom.

Uses of Mass Number

The mass number of an element is convenient to determine the isotopic mass which is generally calculated in the atomic mass units or amu.

An isotope of an element is referred to the elements having same atomic number but different mass number. Isotope thus differs in the number of neutrons.

Usually, atomic mass and mass numbers are two different terms and can vary only by an insignificant value. In most of the cases, they are different. Though, as the weight of an electron is insignificant so it can be said that the atomic mass of an atom is almost equal to mass number.

There are some substances which are named as Isobars. These are defined as the atoms of different elements having the same mass number, but different atomic numbers.

For example, Chlorine-37 and Argon-37 have the same mass number. But their atomic numbers are 17 and 18 respectively. Hence, they are called isobars.

The heavier atoms frequently practice alpha decay where they terminate 2 protons and 2 neutrons from their radioactive nucleus respectively, the mass number of elements is then altered consequently.

In conclusion, the mass number is altered by total 4 units.

Notation of Atom

To write the notation of any atom, we need to know the symbol of the element, the atomic number, and also the mass number.

The mass number of the atom goes in superscript of the symbol and the atomic number is written below as a subscript as shown below.

For example: potassium (K) has an atomic number 19 and mass number is 39 is written as;

₁₉K³⁹

Mass Number Calculation

If the number of protons and the mass number of an element is given, then we can also find the number of neutrons simply by subtracting its atomic number from its mass number as shown below;

No of neutron = mass number – atomic number

N = A – Z