What is Atomic Mass Unit?

PrintPrintEmailEmailSaveSave
Image Credit: 
ehs.washington.edu
Main Image: 
Atomic Mass Unit.

The unified atomic mass unit (u) or Dalton ( Da) is a unit that is used for indicating mass on an atomic or molecular scale. One Da is approximately equal to the mass of one proton or one neutron. An atomic mass unit or amu is one twelfth of the mass of an unbound atom of carbon-12. This means that a carbon-12 atom has the atomic mass of 12 daltons. Mass is the property which reflects the quantity of matter within a sample. Carbon-12 is the more abundant of the two stable isotopes of the element carbon accounting for 98.89% of carbon. Atomic mass units are used as the system of measurement in every science, except for those involving biology and biochemistry, which use the dalton designation. Although not an SI unit, the unified atomic mass unit is accepted by the CGPM for use with SI. CGPM, from the French, stands for Conference Generale des Poids et Mesures. The CGPM is an assembly of delegates from all the nations that have signed the Meter Convention, at present 46 nations.

 

What is the History of Atomic Mass Unit?

The first atomic weight basis was suggested by John Dalton in 1803 a single hydrogen atom, H-1. In spite of the initial mass of H-1 being used as the natural unit for atomic weight, it was suggested by Wilhelm Ostwald that atomic weights would be best expressed in terms in units of 1/16 weight of oxygen. This evaluation was made prior to the discovery of the existence of elemental isotopes, which occurred in 1912. It wasn't until 1961 that the International Union of Pure and Applied chemistry defined the modern applications of the measurement and linked it to carbon-12.

 

What is the significance of Atomic mass Unit?

The mass of a single atom is so small that a unit of measurement has been created to describe it--the atomic mass unit (amu). A unique characteristic of atomic mass units is that, while based on carbon mass, a single unit is also equal to one hydrogen atom. The reason is because the combined mass of a single proton and neutron, the composition of a hydrogen atom, is equal to the measurement. Being only 1/1836 the mass of a proton, electrons are fundamentally negligible to the overall mass of an atom. A proton has about 2,000 times the mass of an electron.

 

What is the correlation of Atomic mass unit with SI units?

The definition of the mole (unit of measurement for the amount of substance), an SI (International System of Units base unit), was accepted by the CGPM in 1971 as:

  • The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. It is represented as "mol". 
  • When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

The definition of the mole also determines the value of the universal constant that relates the number of entities to amount of substance for any sample. This constant is called the Avogadro constant, denoted as NA or L, and is equal to 6.023 * 1023entities per mole. Granted the unified atomic mass unit is one twelfth the mass of one atom of carbon-12, meaning the mass of such an atom is 12u, it follows that there are NA atoms of carbon-12 in 12 grams of carbon-12. This can be expressed mathematically as

  • NA (12u) = 12 grams, or 
  • NA u = 1 gram

 

What is the primary use of Atomic Mass units?

The primary use of the atomic mass unit involves its relationship with moles. Mole is defined as the amount of substance that contains as Avogadro number of elementary entities (e.g., atoms, molecules, ions, electrons) in it . For instance, one mole of water molecules is 18 grams, as it consists of two moles of hydrogen (2 grams) atoms and one mole of oxygen atoms (16 grams). This means that one mole of water has 6.023 * 1023 atomic mass units of all three atoms (2 hydrogen atoms and one oxygen atom).

 

What is the disadvantage of Atomic Mass Unit?

The most challenging aspects in using the atomic unit of mass to define atoms is that it does not explain for the energy that binds together an atom’s nucleus (core). Shockingly, this is not a fixed mass due to the differences between each different type of atom. As more protons, neutrons and electrons are added to an atom to create 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. Binding energy is the energy needed to remove a particle from an atom. 

External References
Related Videos: 
See video