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Atomic Mass – Definition and Elements

Sep 5, 2022

Some amount of mass is associated with every particle of matter, no matter if it’s small or large in size. Everything around us is composed of atoms, and the mass of these atoms is known as atomic mass. This is generally expressed in terms of atomic mass unit (amu) and can be measured almost accurately with the help of spectrometry.

The carbon atomic mass is also measured relative to the mass of a standardized atom by experimental means. Hydrogen is the first element in the periodic table and has an atomic number of 1, so its relative atomic mass is considered to be 1. 


The carbon atomic mass is described as the 1/12th of the mass of a single carbon-12 atom, and it equals to 1.66*10-24 grams approximately. Isotope abundance (relative to each other that exists on Earth) is taken into account to calculate the average atomic mass.

Suppose carbon-12 is the most abundantly found isotope of carbon on Earth. Its relative abundance is almost equal to 98.89%. 13.003 and 14.003 are the atomic masses of carbon 13 and carbon 14, respectively. From these isotopes, we calculate the average atomic mass of carbon as a whole which comes out to be equal to 12.011 amu. 


Atomic Mass

As discussed earlier, the atomic mass of sodium and other metals is defined as being equal to 1/12th of carbon-12 atom mass when it is in its ground state. The total average atomic mass of an atom is equal to the sum of the individual masses of its neutrons and protons. 


When the atomic weight of an atom is divided by the unified atomic weight in Daltons, it is a dimensionless number. It is known as the relative isotopic mass. The atomic masses of different elements are different. The variation is so much so that the atomic mass of the lightest element hydrogen atom, is 1.008, while that of the heaviest atom is 250 amu. 

The average mass of a molecule can be calculated by combining the average atomic mass of each individual atom it is made up of. 


Atomic Mass of Elements

Below is a table showing the average atomic mass of different elements:

ElementAtomic Mass
Chlorine35.453 u
Iron55.845 u
Copper63.546 u
Sulphur32.065 u
Nitrogen14.0067 u
Potassium39.0983 u
Phosphorus30.973762 u
Calcium40.078 u
Sodium22.989769 u

Average Mass

Atoms with the same atomic numbers but different mass numbers are called isotopes. The different mass numbers are because they contain different numbers of neutrons. The actual masses of the individual atoms are never whole numbers, although the mass numbers are.


This is how lithium has an atomic number of 6.942 Da. The isotopes of different atoms in the periodic table are arranged in order according to their atomic masses and abundance in nature. The isotopes that are more abundant in nature are given more weight. An element’s average mass is defined as: 

m(E)=∑ m(In)×p(In),



Where ∑ indicates an n-times summation of all the isotopes In of element E, and p(I) indicates the isotope I’s relative abundance.


Relative Mass

Traditionally when atomic masses were used in Chemistry, usage of any units was avoided. Even nowadays, a chemist will say that carbon-12 has a mass exactly equal to 12. However, as mass is not a dimensional entity, it is understood that it requires a unit.

The omission of the units used to be rationalized because of the concept of relative mass, which is a ratio of two masses instead of a single mass. For relative mass, the ratio of the mass is used rather than using a unit. This is constant mu is used analogously to the unit that is being avoided. Therefore, we can define the relative atomic mass of sodium and other metals as:

Ar = m/mu

Now, this is a dimensionless quantity. As the representation of relative mass is against the standards of modern metrology, its use is generally discouraged.

Integer Mass

Because protons and neutrons have almost the same mass, and the mass of an electron is very low compared to both of them, the mass of most molecules, if measured in daltons, is close to an integer value. Therefore, very commonly, only the integer mass of molecules is indicated. However, it is only meaningful if units of dalton are used. 

Accurate Mass

The mass of a molecule can be determined with an accuracy higher than the integer mass by many mass spectrometers. Therefore, this measurement is called the molecule’s accurate mass. The atomic masses and integer masses of isotopes are not identical due to a mass defect that results from the binding energy in the nucleus. 

What is Molecular Mass?

A compound’s molecular mass can be calculated by the sum of the atomic masses of the individual atoms that it is made up of. Molecular mass is sometimes also known as molecular weight.

For instance, there is one oxygen atom and two hydrogen atoms in a molecule of water. Its mass equals the average mass of an atom of hydrogen multiplied by two and added by the oxygen atomic mass. An element’s molecular mass is determined by the atomic masses of the atoms constituting it. 

Following are the methods by which the molecular mass of different molecules can be measured:

Mass spectrometry: The masses of small molecules can easily be measured by this method and reported as monoisotopic mass.

Hydrodynamic method: Mark-Houwink relations are used to determine the molecular weights in this method. Calibration is needed in this method; therefore, it is also defined as a method to determine the relative molecular weight. 

Static Light Scattering: The measurement of the amount of light scattered by the Zimm method is used to determine the molecular weight. 

Key Points

  • Atoms are the basic building blocks of all matter.
  • Substances that are made up of a single atom are called elements.
  • Every atom has a nucleus at the center, which is made up of neutrons and protons.
  • Electrons revolve around the nucleus in orbits.
  • The relative mass of protons is one, and those are positively charged.
  • The relative mass of neutrons is also one, but those are neutrally charged.
  • The mass of an electron is so small that it is almost negligible. It is negatively charged. 


Hope that the article helps to clear your doubts about the concept of carbon atomic mass, the atomic mass of calcium, the atomic mass of sodium and the atomic mass of other elements. As it is an important concept in both Physics and Chemistry, having a fundamental understanding of its concepts is important.

Frequently Asked Questions

1. Calculate the molar mass of sulphur trioxide.

The definition of molar mass suggests that it is equal to the molecular mass in grams. So we should first calculate its molecular mass.


A compound’s molecular mass can be defined as the sum of the atomic masses of its constituent atoms. Following is the molecular mass of sulphur trioxide with four atoms. 

1S = 1 x 32.06 amu = 32.06 amu

3O = 3 x 16.00 amu = 48.00amu

Therefore, 80.07 amu is the molecular mass of sulphur trioxide. 

2. Define the atomic mass of calcium in simple terms.

In simple terms, the atomic mass of calcium can be defined as the mass of a single atom of a chemical element. It is a combination of the masses of three subatomic particles, namely neutrons, protons, and electrons.

3. Mention the importance of oxygen atomic mass. 

Oxygen atomic mass is a vital concept in Chemistry as it establishes a connection between mass and moles that is measured in the laboratory. Almost everything you study in Chemistry depends on the properties of an atom, including its mass. 

4. Define atomic mass number.

This number can be defined as the combination of a total number of protons and a total number of neutrons in an atom. It is denoted by the symbol A.

5. How can molecular mass be determined?

Molecular mass is an entity that is equal to the number of nuclear masses of a molecule. It provides the weight of a molecule relative to that of the most abundant isotope of carbon-carbon 12, which is considered to have a weight of 12.

6. Is gold a heavy element?

Even though gold is soft and malleable, it is considered to be a heavy metal as each of its constituent atoms is heavy. 

7. Mention the fundamental unit of mass.

The concept of mass is used to calculate the weight of an object. Suppose you can determine the mass of your body when you step on a scale. Kilograms and grams are the most important mass units in the metric system. 


Atomic Mass


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