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Carbon-12 Scale of Measurement of Mass

Measurement of the quantities of a substance forms an important part of our daily life. For example, in laboratories we measure volume and mass to find out the quantity of matter present in it. The knowledge of quantity helps us to avoid excess use of a substance. In chemistry, quantity of matter can be measured in two basic ways:

(i) mass quantity        (ii) amount of substance quantity


All type of substances are made up of atoms, molecules or ions which bound together to

make the total matter of the substance. As atoms are very tiny particles their absolute masses are difficult to measure. The problem was resolved by measuring the relative amounts, that lS, comparing amounts. For this purpose, “mass of one atom of hydrogen was assumed as unity and accepted as standard”.


Hydrogen is the lightest atom and is found to have a mass of 1.67 x 10-24 g i.e., 0.000 000 000 000 000 000 000 00167g. Similarly mass of the heaviest atom Lawrencium (Lr) is 4.29 x 10-22 g i.e., 0.000 000 000 000 000 000 000 429g. On comparing masses we get a simpler and more convenient measurement of relative mass:

It indicates that one atom of lawrencium is 257 times heavier than one atom of hydrogen.

In 1858, oxygen atom was adopted as a standard on account of the following reasons:


(i) It is much easier to obtain compounds of elements with oxygen than with hydrogen as oxygen is more reactive than hydrogen.

(ii) The atomic masses of most of the elements become approximately whole numbers but with hydrogen as standard the atomic masses of most of the elements are fractional.


The mass of one atom of oxygen was taken as 16.0. Thus, atomic mass of an element

Later on even the oxygen atom as reference was rejected in favour of C-12. It is because carbon has a more stable isotope C-12 whose mass is exactly taken as 12u. In 1961, the International Union of Chemists selected the most stable isotope of carbon (C-12 isotope) as standard for comparison of the atomic masses of various elements. The mass of C-12 isotope was taken as 12 atomic mass units. The scale in which the relative atomic masses of different atoms are expressed is called atomic mass unit scale or amu scale. (In the recent years, the new symbol ‘u’ (which means unified mass) is recommended to be used in place of amu.

The atomic mass unit or unified mass may be defined as one-twelfth of the actual mass of an atom of carbon ( carbon-12 isotope).

The atomic mass of an element tells us as to how many times an atom of the element is heavier than one-twelfth of an atom of carbon (C-12). This can be represented as

For example, atomic mass of magnesium is 24 u. It means that an atom of magnesium is 24 times heavier than one-twelfth of mass of a carbon atom (C-12).

The atomic masses of some elements on the basis of C-12 scale are given below:



                 Hydrogen                                                1.008 u

Oxygen                                                  16.00 u

Chlorine                                               35.453 u

Magnesium                                          24.305 u

Copper                                                  63.546 u

Iron                                                    55.847 u

Sodium                                                22.989 u

Zinc                                                        65.38 u

Silver                                                   107.868 u