Coma

The unit of mass of the atoms unified (symbol U or uma ) is a Measuring unit standard, used to measure the Masse Atome S and Molécule S.

In physics

This unit does not belong to the international Système (IF), and its value is obtained in experiments. It is defined like 1/12 of the mass of an atom of the Nucléide 12C (Carbone), nondependant, at rest, and in its fundamental state. In other words an atom of < small > < sup > 12 a.c. a mass of 12 U and if one takes NR (Nombre of Avogadro) atoms of 12C, one will have a mass of 12 G; consequently a U or a uma is worth 1,66054 × 10-27 kg roughly.
Another useful correspondence (in chemistry): 1 U. NR = 1 G (NR: Number of Avogadro)

Value

In the units IF, CODATA of 2006 recommends the following value: m_u \ simeq 1,660 \ 538 \ 782 \ times 10^ {- 27} \ \ mbox {kg}

With a standard uncertainty of: \ plusmn \ 0,000 \ 000 \ 083 \ times 10^ {- 27} \ \ mbox {kg}

That is to say a relative uncertainty of: 8,3 \ times 10^ {- 8} \ \ mbox {kg}

In biochemistry

In the field of the Biochemistry, one also calls the unit of mass of the atoms the dalton (symbol Da ), in the honor of John Dalton for his work on the atomic Théorie.

A dalton is defined like equal to 1/12e mass of a carbon 12 atom, mass which proved then estimated starting from a mixture of several Isotope S. a dalton is with a rather good precision the mass of an atom of Hydrogène, the exact value being 1,00794 uma.

The kilodalton ( kDa ) is used much more than the dalton in biology and biochemistry, because of size of the studied molecules, being able to contain several thousands, even million atoms. As coarse reference mark, the majority of the cellular molecules typically have a size ranging between 20 and 100 kDa. An Amino-acid of Protéine accounts for approximately 110 Da, a bases DNA (with the Désoxyribose and phosphates it) approximately 330 Da.

Reference

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