The terms Mass Number and Atomic Mass are frequently confused, yet they represent distinct concepts. While both relate to the mass of an atom, one is a simple count of particles in a single atom, and the other is a calculated average representing the element as it appears in nature. Understanding the difference between these two values is foundational to interpreting the properties of chemical elements.
Understanding the Mass Number
The Mass Number is a straightforward count of the particles found in an atom’s nucleus. This number is determined by adding the total number of protons and the total number of neutrons in a specific atom. Since it is a tally of whole particles, the Mass Number is always a whole number.
The Mass Number is a characteristic of a single, specific atom or a particular version of an element called an isotope. For example, Carbon-12 has six protons and six neutrons (Mass Number 12). Carbon-13 has six protons and seven neutrons (Mass Number 13).
The Mass Number gives a quick, close approximation of the mass of a single atom. We exclude the mass of electrons from this count because their mass is extremely small compared to that of protons and neutrons.
Understanding the Atomic Mass
Atomic Mass, also frequently called Atomic Weight, is the value listed beneath the element symbol on the periodic table. Unlike the Mass Number, the Atomic Mass is rarely a whole number; it is usually a decimal. This decimal value represents the weighted average of the masses of all the naturally occurring versions (isotopes) of that element.
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons, resulting in different masses. The Atomic Mass accounts for the fact that a typical sample of any element contains a mixture of these isotopes.
The average is “weighted” because it considers the relative abundance of each isotope found in nature. An isotope that makes up 90% of an element’s atoms will have a much greater influence on the final Atomic Mass value. The standard unit for Atomic Mass is the unified atomic mass unit, symbolized as ‘u’ or ‘amu’.
Why Mass Number and Atomic Mass Are Not the Same
The primary distinction is that the Mass Number applies to one specific atom, serving as a specific-atom identifier. In contrast, the Atomic Mass represents the entire element as a whole, providing the practical, real-world mass used for most chemical calculations.
Consider the element chlorine, which clearly illustrates this difference. Chlorine has two main isotopes found in nature: Chlorine-35 (Mass Number 35) and Chlorine-37 (Mass Number 37). The Mass Number for any single chlorine atom will be either 35 or 37.
In any naturally occurring sample of chlorine, the lighter Chlorine-35 isotope is far more common, making up about 75% of the atoms. The heavier Chlorine-37 isotope accounts for the remaining 25%.
When a weighted average calculation is performed using these natural abundances, the resulting Atomic Mass for the element chlorine is approximately 35.45 u. This weighted average of 35.45 u is the value found on the periodic table, and it accurately reflects the mass of a typical chlorine atom from a natural sample.