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The most remarkable concept that Dalton’s atomic theory proposed was that of the atomic mass. According to him, each element had a characteristic atomic mass. The theory could explain the law of constant proportions so well that  scientists were prompted to measure the atomic mass of an atom. Since determining the mass of an individual atom  was a relatively difficult task, relative atomic masses were determined using the laws of chemical combinations and the compounds formed.

Let us take the example of a compound, carbon monoxide (CO) formed by carbon and oxygen. It was observed  experimentally that 3 g of carbon combines with 4 g of oxygen to form CO. In other words, carbon combines with 4/3  times its mass of oxygen. Suppose we define the atomic mass unit (earlier abbreviated as ‘amu’, but according to the latest IUPAC recommendations, it is now written as ‘u’ – unified mass) as equal to the mass of one carbon atom, then  we would assign carbon an atomic mass of 1.0 u andoxygen an atomic mass of 1.33 u. However, it is more convenient  to have these numbers as whole numbers or as near to a whole numbers as possible. While searching for various atomic mass units, scientists initially took 1/ 16 of the mass of an atom of naturally occurring oxygen as the unit. This  was considered relevant due to two reasons:

Imagine a fruit seller selling fruits without any standard weight with him. He takes a watermelon and says, “this has a  mass equal to 12 units” (12 watermelon units or 12 fruit mass units). He makes twelve equal pieces of the watermelon and finds the mass of each fruit he is selling, relative to the mass of one piece of the watermelon. Now  he sells his fruits by relative fruit mass unit (fmu), as in Fig. 3.4.

(a) Watermelon, (b) 12 pieces, (c) 1/12 of

watermelon, (d) how the fruit seller can weigh the fruits using pieces of watermelon

Similarly, the relative atomic mass of the atom of an element is defined as the averagemass of the atom, as compared to 1/12th the mass of one carbon-12 atom.

Atomic masses of a few elements : 


Atoms of most elements are not able to exist independently. Atoms form molecules and ions. These molecules or ions  aggregate in large numbers to form the matter that we can see, feel or touch. | Atoms and Molecules(Atomic Mass)

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