Courses for Kids
Free study material
Offline Centres
Store Icon

Atomic Mass Formula

Last updated date: 12th Apr 2024
Total views: 32.7k
Views today: 0.32k
hightlight icon
highlight icon
highlight icon
share icon
copy icon


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. 


Through, the Mass of the electrons are negligible and hence not always considered. Thus the Atomic Mass of an atom is generally said as the Mass of all the protons and neutrons present combined together. But due to the Mass of electrons, there is a little difference between the actual Mass of the atom as written on the periodic table than the Atomic Mass found by the ways of calculating Atomic Mass taught to the students.


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.


(Image will be Uploaded Soon)


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.


  1. Addition of Protons and Neutrons

In order to calculate the Mass of a single atom of an element, one can consider using the number of the protons and neutrons present in their. By adding the Masses of each proton and neutron present in the nucleus of a given atom, One can extract the Atomic Mass of that atom. In the vague terminology, Atomic Mass can also be defined as the number of the protons and neutrons.


For example - consider if the student has to find the Atomic Mass of the isotope of the atom of oxygen with 10 neutrons present in their nucleus. The student will be able to find via the periodic table that the atoms of oxygen have 8 protons and 8 electrons in them. And thus the sum of these two will result into 18, which will be the Atomic Mass of that certain atom of oxygen Isotopes in amu terms (Atomic Mass units). The average Atomic Mass of this isotope is 17.999 or 18 (approx.).


  1. The average Atomic Mass 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 of Elements


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 of Elements


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.

FAQs on Atomic Mass Formula

1. What's the significance of 1 amu?

Atomic Mass unit: A Mass precisely equal to one-twelfth of the atom's carbon - 12 mass. The carbon 12 (also expressed as C - 12) has a total of 6 protons and also 6 neutrons present in its nucleus.

The AMU (Atomic Mass Unit) is used to represent the relative masses of the element and thus also differentiate between various isotopes of the elements. In vague terms, 1 AMU can be said to be the average mass of the protons rest mass and the neutrons rest mass which is present at the nucleus. The rest mass of the neutrons and protons is 1.673 x 10 -24 gram (g). Then, The mass of an atom of an element in AMU can be defined as the total sum of the number of neutrons and protons present in the atoms.

2. What is an example of an atomic mass? 

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 sulphur-32 atom has a mass of 31.972 amu. The mass of an element in terms of AMU can also be defined as the total number of both neutrons and protons present in that element. For example - the atomic mass of the element sodium Na - 23 is equals to 22.989769 u or 23 (Approx.), The element has a total of 11 protons and 12 neutrons present in its nucleus, hence giving the total of 23 which also represent the atomic mass of the element.

3. How can you measure the atomic weight?

The sum of the number of protons and neutrons in the nucleus is called the mass number for any given isotope of a specific element. This is because each proton roughly weighs 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. Let's understand the concept better with the help of an example. For example: take the atom of the element calcium (Ca) which harbours 20 protons and also 20 neutrons in it. The sum of which will be 40 and 40 approximately equals to the atomic mass of the calcium atom ( 40.078).

4. What is the difference between atomic mass and atomic number of a given isotope of an element?

The points in which the atomic mass and atomic number differentiate are given below - 

  • Atomic mass represents the number of both protons and neutrons present in the nucleus combined together, while the atomic number just provides the number of protons present in the nucleus of the given element.

  • Atomic mass is associated with the average weight of each atom of an element, but atomic number just tells about the number of protons or electrons present in the atom.

  • The letter “A” is used to represent the atomic mass of an atom, while the atomic number is denoted by the letter “Z” ( or “Zahl”).

  • Different atoms of the element can have different numbers of the atomic mass (due to the isotopes), but they will always have the same value for the atomic number.

5. Write about some points to remember in the concepts of Atomic mass.

  • Atomic mass represents the average weight of the atoms of a given element and depends on the number of protons and neutrons present in the atom but the number of electrons present in those atoms does not have any notable impact on the mass of an element.

  • Different atoms of the same elements can have different numbers of the neutrons present in them, thus having different atomic masses. Such versions of an element with a different number of neutrons having different masses are called isotopes.

  • While in any mass calculation involving elements and compounds, One should prefer using the average atomic mass found on the periodic table.