The key difference between atomic mass unit and atomic mass is that the atomic mass unit is the unit that we use to measure the mass of an atom whereas the atomic mass is the mass of a particular single atom.
Expressing the weights of atoms or molecules was a problem for scientists in the early stages. Since atoms are extremely small, we cannot measure their masses using regular units like kilograms or grams or even in micrograms. Therefore, scientists came up with a new concept to measure these.
What is Atomic Mass Unit?
The masses of atoms are extremely small. Thus, we cannot express them in normal mass units like grams or kilograms. Hence, we need to use another unit called atomic mass unit (amu) to measure the atomic mass. One atomic mass unit is one-twelfth of the mass of a C-12 isotope, which is 1.66 X 10−27 kg. When we divide the mass of an atom by one-twelfth of the mass of a C-12 isotope, we can get its relative mass. And, this value is a small number, which is easy to use in calculations and for other purposes. However, in general use, when we say the relative atomic mass of an element, we mean their atomic weight (because we calculate it considering all the isotopes).
Before using carbon-12 as the standard for measuring the atomic mass unit, people used other elements. For instance, H-1 was used first. Later, they changed this in order to reduce the errors. After that, they used elements with higher masses. Next standard was oxygen-16. Later, with the discovery of the presence of oxygen isotopes and other problems related to it, the atomic mass unit was measured relative to carbon-12 isotope.
What is Atomic Mass?
Atoms mainly contain protons, neutrons and electrons. 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). We should take the rest mass because, according to the fundamentals of physics, when atoms are moving at very high velocity the masses increase.
However, the mass of electrons is considerably small compared to the masses of protons and neutrons. Therefore, we can say that the contribution of an electron to the 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. The average of the entire isotope mass is the atomic weight. Therefore, the mass of a specific isotope is the atomic mass in an atom, which has several isotopes.
What is the Difference Between Atomic Mass Unit and Atomic Mass?
Atomic mass is the mass of a specific atom (without taking the average mass of isotopes). Atomic mass unit is the 1/12th of the mass of carbon -12 isotope. Hence, the key difference between atomic mass unit and atomic mass is that atomic mass unit is the unit that we use to measure the mass of an atom whereas atomic mass is the mass of a particular single atom. Moreover, we can use atomic mass unit to indicate the relative masses of other atoms in relative to the C-12 mass.
The below infographic on the difference between atomic mass unit and atomic mass summarizes all these differences.
Summary – Atomic Mass Unit vs Atomic Mass
Atomic mass unit is the unit of measurement of atomic mass of a single atom. The key difference between atomic mass unit and atomic mass is that the atomic mass unit is the unit that we use to measure the mass of an atom whereas the atomic mass is the mass of a particular single atom.
1. Helmenstine, Anne Marie, Ph.D. “Atomic Mass and Atomic Mass Number (Quick Review).” ThoughtCo, Jun. 13, 2018. Available here