The only thing that you need - the periodic table.It allocated to each element of a well-defined place - "cell" or "cell."In every cell there is information containing the following information: the symbol of the element, consisting of one or two letters of the Latin alphabet, a sequence (atomic) number corresponding to the number of protons in the nucleus of the atom and the value of its positive charge, the distribution of electrons in the electron levels and sublevels.And there is another very important quantity - the same relative atomic mass, indicating how many times an atom of that
element is heavier than the standard carbon units.
Consider a specific example.Take the alkali metal sodium, located in the periodic table at number 11. Its relative atomic mass, said there - about 22.99 amu(atomic mass units).This means that each sodium atom is heavier than carbon unit adopted as the standard of comparison, about 22, 99 times.Rounded can take this value of 23. Therefore, its mass is equal to 23 * 1.66 * 10 to the power of -24 = 3.818 x 10 to the power of -23 grams.Or 3,818 * 10 to the power of -26 kg.You calculated the absolute weight of the sodium atom.
But, of course, to use in the calculation of such values ​​is extremely inconvenient.Therefore, as a rule, are relative atomic mass.A relative atomic mass for the same sodium is about 22.99 amu
For every element in the periodic table contains its relative atomic mass.If the need arises, it can be easily calculated and the absolute atomic mass multiplying the relative atomic weight by unit of carbon (1.66 * 10 -24 degree grams).