Discovery of Atomic mass and Molecular mass


Do you know that the modern definition of both molecular mass and atomic mass was meaningless about 150 years ago?

On the other hand, the roots of the periodic table seem to be rooted deep from the 18th century. The question that arises here is how the chemists of the 18th and 19th centuries like Dimitri, Mendeleev, and Ivanovich created the periodic table and managed to assemble all the known elements of their period so perfectly in order?

How did the scientist manage to make such an amazing tool that ultimately expanded and housed about 118 elements, whereas till then, only sixty-two elements were discovered? 

Discovery of atomic mass and atomic weight

The very first thing worth mentioning here is that Mendeleev is not the only person that can enjoy the overall credit since all the chemists deserve to be appreciated for putting their efforts into key discoveries regarding elements.

As we mentioned before, no one was aware of the concept of atomic mass 150 years ago because no one knew anything about isotopes.

Many scientists think of atoms as hard and solid entities that cannot be broke down into smaller units.

After Mendeleev’s

After about thirty years of Mendeleev’s concept of the periodic table, scientists found that the atom contains subatomic particles and these smaller pieces are the building blocks of atoms. 

Till 1932, no one was aware of the neutron thus, knowing about isotopes was impossible till then. The very first person who gave the idea of atomic weights was John Dalton. Dalton assigned the 1 weight of hydrogen and then found the supposed atomic weights of other elements by comparing this weight of hydrogen to them.

All this work was done in 1800 and when the proton was discovered by Ernest Rutherford in the 1900s, scientists started to believe that an atom’s total weight is only due to its protons as electrons are extremely light in weight and don’t contribute to the net weight. 

However, further research predicted that the weight of an atom is not only due to its protons but another entity that possesses weight equal to that of protons but has no charge at all. Now, we know that it’s the neutron. 

Thus, when the neutron was discovered by James Chadwick in May 1923, it acted as the missing piece of the jigsaw and scientists realized that the total atomic weight of an atom is the sum of its protons and neutrons. Along with the discovery of neutrons, scientists also identified the isotopes. That’s why, today, the formula we use to calculate the atomic mass of an element includes both the protons and neutrons but not the electrons. 

Remember that there’s a slight difference between atomic mass and atomic weight. Most of the time, this difference is not worth mentioning but in some cases, it can ruin all your chemical calculations. 

How to Calculate

The formula that we generally used for calculating the atomic mass of an atom is as follow:

Atomic Mass = No. of protons + No. of Neutrons


A = Z + n

here, Z represents the number of positive protons of the atom.

n is the number of neutrons available in an atom.

A is the atomic mass of that atom that we calculate by adding positive protons and neutrons to it.

But in the modern age of science, people get advanced according to technology and before consuming their time on such calculations, they saved their time by using an atomic weight calculator.

Discovery of molecular weight

Molecular weight is the total atomic weight of all the atoms present in a compound. The term of molecular weight is not so common in chemistry as the more accurate term is molecular mass.

If we talk about the molecule then it’s nothing but the chemical combination of two or more atoms together. It is because of the bond formation.

You can calculate molecular weight by using chemical methods, formulas, or instrumental techniques like mass spectrometry. 

Amedeo Avogadro proposed the terminology of molecules first time ever. That’s why he is called the father of molecules. He had stated in his research paper that the gas smallest particles are not simple atoms but they consist of numerous atoms that are combined by a special attraction to make a molecule. 

In 1819, the first method of finding molecular weight had given by Pierre Louis Dulong and Alexis-Therese Petit.

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However, that method did not defined enough due to a lack of knowledge and scientific evidence. The proper method of determining molecular weight had proposed by the discovery of atomic mass.

The process of determining molecular mass became handier after the invention of the mass spectrometer. Just like atomic mass and weight, the terms molecular mass, molecular weights, and molar mass used interchangeably. But all these terms possess sharp distinction. 

How to Calculate

There is no rocket science for calculating molecular weight. As we know, the molecule contains two or more elements or atoms. So for calculating molecular weight, we have to keep in mind the same thing.

Thus, if we have a molecule i.e. ABC then we have to calculate the atomic mass of each element in it. We can calculate the molecular weight of a molecule by adding the atomic masses of each atom.

Molecular Weight of ABC = Atomic Mass of A + Atomic Mass of B + Atomic Mass of C

But rather than evaluating each atomic mass of atoms or elements in the molecule, we will simply use a molecular weight calculator. It is quite simple to put your molecule in the input bar. Now just get your molecule’s weight in just a second.


So here we see How atoms and molecules discovered from the start of the journey. No doubt these are some different concepts but also these is related to each other. Infants atomic mass is a submodule of a molecular mass.

There is a great difficulty to identify atomic mass and weights at the start of their discovery. But modern technologies made it quite easy now. Thus we are able to find an atomic mass or molecular mass in seconds.


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