. What is Atomic Mass

Best Answer

Analysis of water shows that it contains 88.89% oxygen and 11.11% of hydrogen by mass. Thus the ratio of masses of hydrogen and oxygen in water is 11.11: 88.89 or 1:8. Moreover the ratio of number of hydrogen and oxygen atoms in water molecule can be shown to be 2 : 1.

Therefore oxygen is 16 times heavier than hydrogen. Therefore relative atomic mass of oxygen is 16 units if we take mass of hydrogen atom as 1 unit. In 1961 International Union of Pure and Applied Chemists (IUPAC) selected the most stable isotope of carbon, C–12 as the standard for comparison of atomic masses of elements. The mass of C–12 atom is taken as 12 atomic mass unit.

The scale in which the relative atomic masses of different elements are expressed is called atomic mass unit or amu.

Atomic mass unit (AMU)= 1/12 the massof a C-12 atom = 1.660539 x 10-24 gm


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