Atomic Mass Unit vs Atomic Mass
Expressing the weights of atoms or molecules had been a problem for scientists in early stages. Since atoms are extremely small, they couldn’t be measured using regular units like kilograms or grams or even in micrograms. Therefore, scientists came up with a new concept to measure these.
Atoms are mainly composed of 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). 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 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. The average of the entire isotope mass is known as the atomic weight. Therefore, the mass of a specific isotope is the atomic mass in an atom, which has several isotopes.
Atomic Mass Unit
The masses of atoms are extremely small, so we cannot express them in normal mass units like grams or kilograms. For this purpose, 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, which is 1.66 X 10−27 kg. When a mass of an atom is divided by one twelfth of the mass of a C-12 isotope, its relative mass is obtained. This value is a small number, which is easy to use in calculations and for other purposes. 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).
Before using carbon-12 as the standard for measuring atomic mass unit, other elements were used. H-1 was used first. Later this was changed in order to reduce the errors, and elements with higher masses were used. Next standard was oxygen-16. Later, with the discovery of presence of oxygen isotopes and other problems related to it, the atomic mass unit was measured in relative to carbon-12 isotope.
What is the difference between Atomic Mass and Atomic Mass Unit?
• 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.
• Atomic mass unit is used to indicate the relative masses of other atoms in relative to the C-12 mass.