The atomic mass unit (AMU) is a universally applied measurement based on 1/12 of the total mass of a single carbon-12 atom. AMUs are used in every science except biology and biochemistry, and are convenient because one unit equals one atom of hydrogen. However, AMUs do not account for the energy that binds the nucleus of an atom together, making it a rough approximation. AMUs are used in relation to moles, and were first established by John Dalton in the early 1800s and later defined by the International Union of Pure and Applied Chemistry in 1961.
The atomic mass unit is the measurement system suitable for identifying each single unit of mass in atoms and molecules. Also known as the dalton, the atomic mass unit is a universally applied measurement based on 1/12 of the total mass of a single carbon-12 atom. This means that a carbon-12 atom has an atomic mass of 12 daltons. The designation for a standard atomic mass unit is uo or Da. Atomic mass units are used as a system of measurement in every science, except those involving biology and biochemistry, which use the designation dalton.
A convenient aspect of atomic mass units is that, while based on the mass of carbon, a single unit is also equal to one atom of hydrogen. This is because the combined mass of a single proton and neutron, the composition of a hydrogen atom, equals the measurement. Electrons, being only 1/1836 of the mass of a proton, are essentially negligible compared to the total mass of an atom.
One of the most problematic aspects of using the atomic unit of mass to define atoms is that it does not account for the energy that binds the nucleus of an atom together. Unfortunately, this is not a fixed mass due to differences between different atom types. As more protons, neutrons, and electrons are added to an atom to make a new element, the mass of this binding energy changes. This means that the measurement can be said to be a rough approximation rather than an exact constant.
One of the primary uses of the atomic mass unit involves its relationship to moles. A mole is the complete physical quantity of a single unit of a substance. For example, a single water molecule, made up of two hydrogen atoms and a single oxygen atom, is one mole of water. This means that it has the atomic mass of all three atoms.
The establishment of the atomic mass unit was first started by a chemist named John Dalton in the early 1800s. He used a single hydrogen atom as a platform for measurement. However, this was changed by Francis Aston with his invention of the mass spectrometer in the late 1800s. Aston defined an atomic mass unit as 1/16th the mass of a single oxygen-16 atom. It was not until 1961 that the International Union of Pure and Applied Chemistry defined the modern applications of measurement and related it to carbon-12.
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