Difference Between Mass Number and Atomic Mass

Mass Number vs Atomic Mass

Atoms are mainly composed of protons, neutrons and electrons. Some of these sub particles have a mass; therefore, they contribute to the total mass of the atom. However, some sub atomic particles like electrons do not have a significant mass. For each isotope of an element, there is a specific atomic mass and a mass number.

What is Atomic Mass?

Atomic mass is simply the mass of an atom. In other words, it is the collection of masses of all the neutrons, protons, and electrons in a single atom, specifically, when the atom is not moving (rest mass). Rest mass is taken because, according to the fundamentals of physics, it has been shown that when atoms are moving at very high velocity the masses increase. However, the mass of electrons is considerably very small compared to the masses of protons and neutrons. So we can say that the electrons’ contribution to an atomic mass is less. Most of the atoms in the periodic table have two or more isotopes. Isotopes differ from each other by having a different number of neutrons, even though they have the same proton and electron amount. Since their neutron amount is different, each isotope has a different atomic mass.

Moreover, the masses of atoms are extremely small, so we cannot express them in normal mass units like grams or kilograms. For our purposes, we are using another unit called atomic mass unit (amu) to measure the atomic mass. 1 atomic mass unit is one twelfth of the mass of a C-12 isotope. When a mass of an atom is divided by the mass of one twelfth of the mass of a C-12 isotope, its relative mass is obtained. However, in the general use when we say the relative atomic mass of an element, we mean their atomic weight (because it is calculated considering all the isotopes). Atomic mass and atomic weights are interchangeably used by most people. However, they bear different meanings, and it causes a significant error in bulk material calculations if these two are taken as one.

What is Mass Number?

Mass number is the total number of neutrons and protons in a nucleus of an atom. The collection of neutrons and protons is also known as nucleons. Therefore, mass number can also be defined as the number of nucleons in a nucleus of an atom. Normally, this is denoted in the left upper-corner of the element (as superscript) as an integer value. Different isotopes have different mass numbers, because their numbers of neutrons vary. Therefore, the mass number of an element gives the mass of the element in integers. The difference between the mass number and the atomic number of an element gives the number of neutrons it has.


What is the difference between Mass Number and Atomic Mass?

• Atomic mass is the mass of an atom. Mass number means the total number of neutrons and protons (nucleons) in a nucleus of an atom.

• Mass number is an integer value whereas the atomic mass often is a decimal value.