Atomic and Molecular Mass: Definition and Measurement

Before understanding the Atomic and Molecular Masses, let us learn about matter, elements and atoms. Anything which has mass and occupies space is called matter, and matters are made up of atoms. A pure substance made up of only one kind of atom is called an element. The atomic mass of an element is the mass of each atom. The sum of the atomic masses of atoms in a molecule or compound is the molecular mass of that mass.

One or more atoms combine to give molecules. In this article, Atomic and molecular masses, you will acquire knowledge on the basic concepts of atomic mass, molecular mass determination of atomic and molecular masses by spectroscopy method, atomic masses of some common elements in detail. Read on to find more.

What is an Atom?

An atom is the smallest particle of an element that takes part in a chemical reaction and maintains its chemical identity throughout all physical and chemical changes. All matter is made up of atoms, whether it be an element, a compound, or a combination. This suggests that atoms are the fundamental constituents of all matter. Atom is made up of even smaller particles called electrons, protons, and neutrons.

What is Atomic Mass?

Masses of individual atoms are very-very small. Instead of measuring atomic masses in kilograms, it is far more practical to have a system that allows us to easily compare relative atomic masses. The scientist decided on using Carbon\(-12\) as the reference atom by which all other masses would be compared.

Hence, the atomic mass of an element may be defined as follows, the atomic mass of an element is the number of times an atom of that element is heavier than \({\rm{1/1}}{{\rm{2}}^{{\rm{th}}}}\) of an atom of carbon taken. From the definition, one atomic mass unit is \({\rm{1/1}}{{\rm{2}}^{{\rm{th}}}}\) of the mass of one \({\rm{C – 12}}\) isotope.

According to the latest recommendations of IUPAC, a unit of atomic mass instead of atomic mass unit (amu) will be called ‘unified mass’ and written as \({\rm{‘u’}}.\) The atomic mass of an element (A) indicates the number of times one atom of the element under consideration is heavier than \({\rm{1/1}}{{\rm{2}}^{{\rm{th}}}}\) of the \({\rm{C – 12}}\) atom. 

For example, the atomic mass of magnesium is \(24,\) which means a magnesium atom is \(24\) times heavier than \({\rm{1/1}}{{\rm{2}}^{{\rm{th}}}}\) of \({\rm{C – 12}}\) atom. In other words, the atomic mass of magnesium is \(24{\rm{u}}.\)
One atom of oxygen is \(16\) times heavier than \({\rm{1/1}}{{\rm{2}}^{{\rm{th}}}}\) of \({\rm{C – 12}}\) part of the atomic mass of \({\rm{C – 12}}\) atom, so the atomic mass of oxygen is \(16{\rm{u}}.\)

How is Atomic Mass Measured?

The atomic mass of an atom is determined by an experimental technique called mass spectroscopy. This technique separates different isotopes of an element to allow the determination of the per cent abundance of the isotopes of the element. Each isotope of the element appears as a peak in the mass spectrum. The intensity (height) of each peak depends on the abundance of that isotope in the sample, and the unique location of the peak on the \({\rm{x – }}\)axis indicates the mass to charge ratio of the isotope. Knowing the charge of the isotope, one can calculate the mass of the isotope. From the relative abundance of different isotopes of the element, the average relative atomic mass is calculated.

Average Atomic Mass

The atomic mass of an element can be defined as the average relative mass of its atoms as compared with an atom of carbon\(-12\) taken as \(12.\)

In terms of percentages of the different isotopes, the average atomic mass may be calculated as follows:

\({\rm{A = }}\frac{{{\rm{\Sigma }}{{\rm{p}}_{\rm{i}}}{{\rm{A}}_{\rm{i}}}}}{{{\rm{100}}}}\)

Where \({{\rm{p}}_{\rm{i}}}\) is the percentage abundance of the isotope with atomic mass \({{\rm{A}}_{\rm{i}}}\)

Atomic Masses of Some Common Elements

Atomic masses of some common elements are given below:

Molecules

A molecule is the smallest particle of an element or a compound that can exist in the state independently under ordinary conditions and exhibits all the properties of the substance (element or compound). 

Molecules are characterized by the number of atoms present in them. A molecule may contain the atoms of the same or the different kinds. A molecule may contain one, two, three, or more atoms of the same or different kinds.

Molecular mass (M)

Molecular mass is a number representing the mass of one molecule of the chemical substance as compared with the mass of an atom of \({\rm{C – 12}}{\rm{.}}\)

For example, by saying that the molecular mass of carbon dioxide \(\left( {{\rm{C}}{{\rm{O}}_{\rm{2}}}} \right)\) is \(44,\) and we mean that \(1\) molecule of carbon dioxide is \(44\) times heavier than one-twelfth \((1/12)\) of an atom of carbon\(-12.\)

The molecular mass of a compound is calculated by adding the atomic masses of all the atoms present in one molecule of the compound.

Solved Examples

Q.1. Molecular mass of water: The molecular formula of water is \({{\rm{H}}_{\rm{2}}}{\rm{O}}{\rm{.}}\) Hence,
Ans: The molecular mass of water\( = (2 \times {\rm{Atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{hydrogen)}} + (1 \times {\rm{Atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{oxygen)}}\)
\({\rm{Molecular}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{water}} = (2 \times 1) + (1 \times 16) = {\rm{18u}}\)

Q.2. Molecular mass of nitric acid \(\left( {{\rm{HN}}{{\rm{O}}_3}} \right):\) The molecular formula of nitric acid is \({{\rm{HN}}{{\rm{O}}_3}}.\) Hence,
Ans: The molecular mass of nitric acid\( = (1 \times {\rm{Atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{hydrogen)}} + (1 \times {\rm{Atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{nitrogen) + }}\left( {3 \times {\rm{Atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{oxygen}}} \right)\)
\({\rm{Molecular}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{nitric }}{\rm{acid}} = (1 \times 1) + (1 \times 14) + \left( {3 \times 16} \right)\)
\({\rm{Molecular}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{nitric }}{\rm{acid}} = 1 + 14 + 48 = 63\,{\rm{u}}{\rm{.}}\)

Determination of Molecular Mass

Mass spectrometry: It is the most accurate method for calculating the molecular mass of the compound and its elemental composition. In this method, molecules are bombarded with a beam of energetic electrons to ionize the molecules. Each type of ion gives a particular mass to charge ratio (m/e ratio). The m/e ratio will give the molecular mass of the ion.

Static Light Scattering (SLS): It is a method used to calculate the absolute molecular weight of macromolecules like a polymer or a protein in solution. In this method, a high-intensity monochromatic light is passed through a solution of macromolecules. The scattering intensity of light is measured in one or many angles using detectors. The intensity of light scattered by a molecule determines its molecular weight and size. 

Hydrodynamic method: It is used to find the molecular weight, size, shape, flexibility, electrical properties, and interactions of biological macromolecules. It is widely used to study the small proteins in dilute solution, DNA molecules of almost macroscopic size, molecules in membranes, and whole cells.

Summary

In this article, Atomic and Molecular Masses serve you to understand the meaning of atom, molecule, atomic mass, and molecular mass. The calculation of molecular mass, its unit, determination of molecular mass by mass spectroscopy, static light scattering, hydrodynamic method, atomic masses of some common elements are also clear with this article. This knowledge will be applicable for the calculation of the number of moles, number of particles in a molecule or compound.

FAQ

Q.1. What are molecular mass and atomic mass?
Ans: The atomic mass of an element is the number of times an atom of that element is heavier than \({\rm{1/1}}{{\rm{2}}^{{\rm{th}}}}\) of an atom of carbon taken as \(12.\)
Molecular mass is a number representing the mass of one molecule of the chemical substance as compared with \({\rm{1/1}}{{\rm{2}}^{{\rm{th}}}}\) of the mass of an atom of \({\rm{C – 12}}.\) It is also defined as the sum of the atomic masses of all the atoms present in one molecule of the compound.

Q.2. How do you calculate atomic mass and molecular mass?
Ans: Atomic mass is calculated by adding the number of protons and neutrons present in an atom.
The molecular mass of a compound is calculated by adding the atomic masses of all the atoms present in one molecule of the compound.

Q.3. What are the Atomic and Molecular Masses of iron and potassium?
Ans: The atomic mass of iron \(\left( {{\rm{Fe}}} \right)\) is \({\rm{55}}{\rm{.8}}\,{\rm{u,}}\) and molecular mass is \(55.8\;{\rm{g}}/{\rm{mol}}.\)
The atomic mass of potassium is \({\rm{39}}{\rm{.1}}\,{\rm{u,}}\) and molecular mass is \(39.1\;{\rm{g}}/{\rm{mol}}.\)

Q.4. What are the Atomic Mass and Atomic Mass numbers?
Ans: Atomic mass refers to the naturally occurring mixture of isotopes, whereas mass number refers to an individual isotope. Atomic mass is an average and never an exact integer; the mass number is a sum of the number of protons and the number of neutrons and is always an integer.

Q.5. What is the basic unit for mass?
Ans: The basic unit for mass is the kilogram \(\left( {{\rm{Kg}}} \right){\rm{.}}\)

Q.6. Why is the atomic mass important?
Ans: Atomic mass is important to calculate the molecular mass of an element or compound. From the atomic mass, the number of protons and neutrons can be calculated since atomic mass number\(=\)Number of protons\(+\)Number of neutrons.

Q.7. What is the simple definition of atomic mass?
Ans: Atomic mass is defined as the average relative mass of its atoms as compared with 1/12th of an atom of carbon\(-12\) taken as \(12.\)

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