The atomic mass (or relative atomic mass) of a Isotope of a chemical element is the relative mass of an atom of this isotope; the comparison is made with the twelfth of the mass of the Carbone 12. The atomic mass is a number without dimension which is rather close to the number of Nucléon S in the core of the atom. The mass of an atom is generally expressed in uma, unit of mass of the atoms which is equivalent to the mass of the light isotope of Hydrogen. The molar mass of an atom is in fact the mass of a mole of a given chemical element and is worth as much G than the value of the atomic mass: it is the mass of a mole or 6.022 X10^23 atoms of the same element.

The atomic mass is also defined for a chemical element, it is then the weighted average of the atomic masses of the Isotopes in proportion to the frequency of these isotopes such as one finds them in nature.

Various references of mass

• 1805 : John Dalton fixes the atomic mass of the Hydrogène at 1

• 1865: Jean Stas fixes the atomic mass of the Oxygène 16 to 16
• 1961: The International union of chemistry pure and applied section for the current definition of the atomic mass

See too

• periodic Table

• molecular Mass
• Unit of atomic mass

Be-X-old: Атамнаямаса Simple: Atomic farmhouse

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