Atomic Mass Formula

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Matter is made up of very small parts called atoms. It is described as something that takes up space and has mass. A physical property of matter is mass. The atomic mass is referred to as the mass of an atom or a molecule. In this article, we will study the atomic mass formula, the formula for molar mass, and the average atomic mass formula that will help to calculate the subatomic particles and also the mass of an atom.

Formula of Atomic Number

Atomic number represents the no. of protons in the nucleus of an atom. The formula for atomic number is-

Atomic number= no. of protons present in the nucleus of an atom.

Atomic Mass

The sum of the masses of protons, neutrons, and electrons in an atom or group of an atom is the atomic mass. In contrast with protons or neutrons, the electrons have much less mass, so the mass of electrons is not included in the calculation.

Atomic mass formula= Mass of protons + Mass of neutrons + mass of electrons

The relative atomic mass of an element is the total mass of the element's naturally occurring isotopes relative to the mass of a 12C atom that means a relative atomic mass of exactly 12 is given to one atom. 

To Calculate the Atomic Mass 

There are three different methods to calculate the atomic mass-

1. By having reference to the periodic table 

In the periodic table, an atomic number is typically indicated under the representation of an element.

For example-

  • Chlorine atomic number is 17 while its atomic mass is 35.5

  • Calcium’s atomic number is 20 while its atomic mass is 40.

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In general, however, the atomic mass of an atom will be very similar to its mass number, although the decimal places will have some variation.

2. Addition of protons and neutrons

3. The average atomic of various elements are determined by multiplying the atomic mass of each isotope by its fractional abundance and adding the value obtained. For example, chlorine contains two types of atoms having relative masses of 35u and 37u. The relative abundance of these isotopes in nature is in the ratio 3:1. Thus the atomic mass of chlorine is the average of these different relative masses.

Thus, the formula of the average atomic mass of chlorine = {(35 x 3) + (37 x 1)} / 4 = 35.5u


Atomic mass









Relative Atomic Mass

The relationship between the mass of an element and the number of atoms it contains is the element's relative atomic mass. To measure the masses of distinct atoms, the relative atomic mass scale is used. 

A relative atomic mass of 1 was initially allocated to the hydrogen atom, the lightest atom, and the relative atomic mass of other atoms as compared to this.


Relative Atomic Mass 







Solved Examples

Example 1: Find the element mass number whose atomic number is 15 and the number of neutrons present is 15.

Solution: This can be calculated using the formula for molar mass

Atomic number= number of protons present

Mass number= No. of protons + No. of neutrons



In chemistry, the definition of atomic weight is important, as most chemical reactions take place in accordance with simple numerical relationships between atoms. Since it is not possible to explicitly count the atoms in a substance, chemists quantify reactants and products by measuring and using atomic weight measurements to draw their conclusions.

FAQ (Frequently Asked Questions)

Question: What's the Significance of 1 amu?

Answer: Atomic mass unit: A mass precisely equal to one-twelfth of the atom's carbon-12 mass.

Question: What is an Example of an Atomic Mass?

Answer: The unit of atomic mass is defined as a mass equal to one-twelfth of the mass of carbon-12 atoms. In relation to the carbon-12 norm, the mass of an isotope of any element is expressed. One atom of helium-4, for example, has a mass of 4.0026 amu. A sulfur-32 atom has a mass of 31.972 amu.

Question: How Can You Measure the Atomic Weight?

Answer: The sum of the number of protons and neutrons in the nucleus is called the mass number for any given isotope. This is because each proton weights one unit of atomic mass and each neutron (amu). You can measure the mass of an atom by adding together the number of protons and neutrons and multiplying them by 1 amu.