Difference Between Atomic Mass and Atomic Weight

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What is Atomic Mass?

When it comes to Chemistry, then there are many things that students have to learn. One important Chemistry topic that all students should learn is the topic of atomic mass and atomic weight.


Atomic weight vs atomic mass is also one of those topics on which students always seem to have thousands of questions. This is why today we are going to look into all important aspects related to the topic of atomic weight vs atomic mass.


In this article, students will be able to learn about atomic mass and relative atomic mass. We will also talk about the difference between atomic mass and atomic mass numbers, along with the difference between atomic weight and molecular weight.


So, let’s begin by first looking at what atomic mass means. According to experts, atomic mass can be defined as the total number of protons and neutrons present in an atom. It is important to remember that, ideally, the mass of electrons is also counted. However, since the size of the electrons is very small, their mass is often neglected.


The unit for atomic mass is Dalton. This unit is represented by ‘Da.’ Also, students should also know that Dalton is the standard unit that is used for indicating the mass of an atom. The atomic mass of different elements varies as the number of protons and neutrons is different for different elements.


Now, let’s look at the meaning of relative isotopic mass. We have established that atomic mass is an absolute mass. But relative isotopic mass, on the other hand, is a dimensionless number that has no units.


This loss of units is the result of using a scaling ratio concerning the carbon-12 standard. Further, it is interesting to note that the word ‘relative’ in the term relative isotopic mass refers to this scaling about the carbon-12 standard.


Based on this information, it can be said that the relative isotopic mass is the mass of any given isotope when the final value is scaled by the mass of carbon-12. It is important to remember that the mass of carbon-12 should always be determined experimentally in this case.


On a similar note, it can also be stated that the relative isotopic mass of a nuclide or isotope is the mass of the isotope relative to 1/12th of the mass of a carbon-12 atom.


Learn to View the Atomic Mass and Other Details in the Periodic Table

Students must now have a basic idea of the meaning of atomic mass and relative atomic mass. The next important step is to learn about atomic weight and the difference between atomic mass and atomic mass number. However, before that, students need to know how to spot some details in the periodic table.


What we mean by this is that all students must know how to spot the atomic number, atomic symbol, and atomic mass of an element on the periodic table. To help students succeed in this task, we have attached a labelled image below. Students can refer to this image to learn how to spot the atomic number, atomic symbol, and atomic mass of an element on the periodic table.


(image will be uploaded soon)


What is Atomic Weight?

It is a commonly known fact that a single element can have several isotopes. This means that if one wants to calculate the atomic weight, then it is important for that person to consider the masses of different isotopes. The percentage of different isotopes might also vary.


It is also important to remember the values of isotope relative abundance and isotope mass while calculating the atomic weight. Also, atomic weight is known as the relative atomic mass. It can be defined as the ratio of the mean mass of the atoms of a chemical element to a certain standard.


Further, the normal unit of atomic weight has been 1/12th of the atomic mass of the carbon-12 isotope since the year 1961. An isotope can also be referred to as one of two or more species of atoms with different atomic mass numbers of the same chemical element.


Students should also be aware of the fact that atomic weight is also measured in the units of atomic mass. This is usually abbreviated to AMU. It is also known as Daltons. Before we proceed to discuss the difference between atomic weight and molecular weight, some students might also be interested to learn about the history of this concept.


According to various sources, John Dalton was the first scientist who considered that the value of one should be allotted to the hydrogen atom. This was considered as the base of his scale of atomic weights.


After that, for several decades, nuclidic masses were explained on a scale based on 1/18th the mass of neutral atom oxygen. The chemical atomic weight unit at that time was 1.000272 times the physical atomic mass unit.


This period of confusion was eventually brought to an end in 1961. This was done after the acceptance of the 1/12th mass of carbon as the unit for both atomic weights and nuclidic masses.


Till now, we have discussed the meaning of atomic mass and atomic weight. We have also looked at various aspects related to both of these topics. Now, it’s time to look at the difference between both of these concepts.


To make it easier for students, we have created a table that highlights the differences that exist between atomic mass and atomic weight. That table is mentioned below.


The Difference Between Atomic Mass and Atomic Weight

Atomic Mass

Atomic Weight

It is defined as the sum of protons and neutrons that exist in an atom.

It is defined as the average ratio of all atoms present in an element.

In the calculation of atomic mass, isotopes are not included.

Isotopes are included while calculating the atomic weight.

The final value that is obtained is a whole number.

The final value obtained may or may not be a whole number.

Mass spectroscopy is used to carry out the analysis.

An analysis can be done through mass spectroscopy.


Fun Facts About Atomic Mass and Atomic Weight

Did you know that there were two groups of scientists who discovered the concept of relative atomic masses? John Dalton and Thomas Thomson discovered this concept in 1803 and 1805, respectively, while Jons Jakob Berzelius discovered this concept between 1808 and 1826.


Further, earlier, the relative atomic mass was defined with that of the lightest element. However, Prout’s hypothesis stated that the atomic mass of all elements would have to be proven to be exact multiples of the lightest element. Many other theories and experiments were carried out before finally arriving after using the 1/12th carbon-12 standard.

FAQ (Frequently Asked Questions)

1. Explain the Topics of Atomic Number and Atomic Weight in Brief.

Answer: Atomic number is the number of protons that are present in a chemical element. It is also defined as the number of electrons present in an element. Atomic weight, on the other hand, is defined as the total number of protons plus the number of neutrons present in a chemical element.

2. Where is the Atomic Mass of an Element Present?

Answer: According to various experimental evidence, the large majority of an atom’s mass is present in its nucleus. This is because the nucleus contains both protons and neutrons. The mass number of a chemical element is also represented by the letter ‘A,’ and it is the total number of protons and neutrons present in an atom.

3. What is Atomic Weight, Also Known as? And Why is the Concept of Atomic Mass Important?

Answer: Atomic weight is also known as the relative atomic mass. And in chemistry, the concept of atomic mass is vital because it is the relation between mass and moles. The value for mass can be calculated in the laboratory, and moles can be calculated by referring to the atomic numbers. Further, a lot of research that is carried out in chemistry is done based on atomic ratios.

4. What is the Difference Between Mass and Weight?

Answer: According to sources, the mass of an object can be defined as the measure of the inertial property of the object. It can also be defined as the sum of matter that is found within the object. The weight of an object, on the other hand, is the measure of the force that is exerted by gravity on the object. It can also be viewed as the force that is required to sustain that force.