Atomic Insights: Atomic Mass and Molecular Mass in Chemistry

by Yuvi K - December 16, 2023

Atomic Mass and Molecular Mass in Chemistry

Atomic Mass and Molecular Mass are two important measures of the mass of various elements in chemistry. Atomic Mass is the average mass of all the isotopes of an element, while Molecular Mass is the sum of the masses of all the atoms of a particular molecule. Both terms are related, but they measure different aspects of the mass of an element or a molecule.

Atomic Mass

Atomic Mass is the average mass of all the isotopes of an element. An isotope is an atom of an element with a different number of neutrons. For example, hydrogen has two common isotopes, hydrogen-1 and hydrogen-2, which have 1 and 2 neutrons, respectively. The Atomic Mass of hydrogen is thus, the average mass of both of these isotopes. It takes into account the different mass of each isotope.

The Atomic Mass of an element is typically expressed in atomic mass units (amu) or as a relative atomic mass (Ar). Atomic mass units (amu) are convenient for expressing the masses of atoms and atomic particles. One amu is equal to a mass of 1/12 of a carbon-12 atom. The relative atomic mass (Ar) of an element is equal to the ratio of the average mass of one of the atom’s isotopes to 1/12 of the mass of a carbon-12 atom.

Examples of Atomic Mass

Let’s take the Atomic Mass of hydrogen as an example. The most abundant isotope of hydrogen is hydrogen-1, which has a mass of 1.00783 amu, and the less common hydrogen-2, which has a mass of 2.0141 amu. The Atomic Mass of hydrogen is thus, the average mass of both of these isotopes. Thus, the Atomic Mass of hydrogen is 1.0079 + 2.0141 = 3.022 amu.

Molecular Mass

Molecular Mass, on the other hand, is the sum of the masses of all the atoms of a particular molecule. It is typically expressed in units of grams per mole (g/mol). A mole is a way of measuring the number of atoms or molecules of a particular substance. One mole is equal to the number of atoms in 12.0 grams of carbon-12 atoms.

Molecular Mass is often confused with Molecular Weight, but they are not the same. Molecular Mass is the sum of the mass of all the atoms of a particular molecule, while Molecular Weight is the sum of the atomic weight of all the atoms of a particular molecule. Atomic weight is the average mass of the atoms of a particular element, while Molecular Mass is the sum of the individual mass of all the atoms of a particular molecule.

Examples of Molecular Mass

Let’s take the example of water. Water is composed of two hydrogen atoms and one oxygen atom. The Atomic Mass of hydrogen is 1.0079 amu, and the Atomic Mass of oxygen is 15.9994 amu. Thus, the Molecular Mass of water is (1.0079 + 1.0079 + 15.9994) = 18.0153 g/mol.

Atomic Mass vs. Molecular Mass

Atomic Mass and Molecular Mass are two different measures of mass in chemistry. Atomic Mass is the average mass of all the isotopes of an element, while Molecular Mass is the sum of the masses of all the atoms of a particular molecule. Atomic Mass is typically expressed in atomic mass units (amu) or as a relative atomic mass (Ar), while Molecular Mass is typically expressed in grams per mole (g/mol).

Conclusion

Atomic Mass and Molecular Mass are important measures of the mass of various elements in chemistry. Atomic Mass is the average mass of all the isotopes of an element, while Molecular Mass is the sum of the masses of all the atoms of a particular molecule. They are related, but measure different aspects of the mass of an element or a molecule.