Mass of an Atom

Atoms and molecules are material particles and they possess mass. The mass of one atom of an element is known as the atomic weight of the element. Atomic weight should not be confused with another concept called atomic number of an element. Atomic number A is the simple sum of the number of protons (Z) and neutrons (N) the element has. Thus A = Z + N and is an integral number as Z and N both are integer numbers.  The atomic mass or weight is an actual measure of the weight of the atoms in grams or kilograms.

The table below gives mass of some of the atoms.

We can see that when we deal with the masses, since they are very small, it is inconvenient to use such numbers in calculations. To make calculations less cumbersome,  weight of the atom of an element is compared to the weight of standard atom and their ratio is taken.

In 1962, it was decided by the international community, mass of the isotope of carbon, namely ¹²C, should be taken as 12. All the other atomic weights are compared to the weight of an atom of  ¹²C.

                                                                          Average weight of one atom of the element
Atomic weight of an atom of an element =     
                                                                            1/12 x  weight of one atom of ¹²C

                                                 Average weight of one atom of the element
Atomic weight of             =        x 12
an atom of an element                        weight of one atom of ¹²C

How to calculate the average weight of an element
Elements exist in various isotopes. The average weight of an element is calculated from the proportion in which the isotopes occur (relative abundance).

For example the atom chlorine has two isotopes : ³⁵Cl and ³⁷Cl. Their relative abundance is in the ratio of 3 : 1.

                                                                  3 x 35  + 1 x 37
Thus the average weight of Cl atom  =    = 35.5
                                                                          4

Please note that atomic weight is a ratio, hence does not have any units associated with it.

If an element does not have an isotope, then its average weight is same as that of its atomic number. Gold (Au) and Platinum (Pt) fall in this category.

The table below gives some of the atomic weights of elements accepted internationally.

Gram atomic weight or gram atomic mass : In chemical reactions, we deal with normal mass units such as grams (and kilograms). The quantity of element equivalent to its atomic weight taken in grams is called the gram atomic weight (GAW) or gram atomic mass (GAM). For example :  1 gram atomic mass of sodium will be equal to 22.99 gm. Similarly 1 gram atomic mass of carbon will be equal to 12.011 gm.  

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