Chemistry

Atoms and Molecules

Atomic Mass Of An Element

Actual masses of the atoms of the elements are very,very small. For example,one atom of hydrogen (H) has a mass of 1.673 x 10~24 gram or 0.000000000000000000000001673 gram. It is not convenient to use such small and complicated figures in our calculations, therefore, it was necessary to define atomic masses in such a way that we get simple figures for them. In order to understand the present definition of atomic mass, we should first know the meaning of carbon-12 atom (read as carbon twelve atom).Carbon-12 atom is that atom of carbon which has 6 protons and 6 neutrons in its nucleus so that its mass number is 12.
Carbon-12 atom has been assigned an atomic mass of exactly 12 atomic mass units (Atomic mass unit was earlier written in short as 'amu' but these days atomic mass unit is denoted by the letter 'u'). This means that a carbon-12 atom has been assigned an atomic mass of exactly 12 u. Now, since a carbon-12 atom has been assigned an atomic mass of 12 atomic mass units,therefore, the atomic mass unit should be equal to

\frac{1}{12}

(one-twelfth) the mass of a carbon-12 atom. That is :
Atomic mass unit = $\frac{1}{12}$ the mass of a carbon-12 atom
or 1u =

\frac{1}{12}

the mass of a carbon-12 atom
Thus,one atomic mass unit (1 u) is defined as exactly one-twelfth the mass of an atom of carbon-12.It is clear from this discussion that for the purpose of determining atomic masses of various elements, the mass of a carbon-12 atom is taken to be the standard (having a value of 12 units on the atomic mass scale).The atomic masses of all other elements are determined by comparing the mass of their atom with the mass of a carbon-12 atom.Keeping these points in mind, we can now define the atomic mass of an element as follows.
The atomic mass of an element is the relative mass of its atom as compared with the mass of a carbon-12 atom taken as 12 units.The atomic mass of an element indicates the number of times one atom of the element is heavier than

\frac{1}{12}

of a carbon-12 atom. For example, the atomic mass of magnesium is 24 u which indicates that one atom of magnesium is 24 times heavier than

\frac{1}{12}

of a carbon-12 atom. It is clear from the above discussion that the present standard for the atomic masses is carbon-12 atom which has been assigned an atomic mass of exactly 12 u.The atomic masses of some of the important elements are given below.

Notes

Atoms and Molecules - Notes

1. Drawbacks of Dalton’s Atomic Theory