- periodic table.
As stated in the definition, molecular weight - the sum of all the atomic masses of matter.To find the atomic masses of various chemical elements, need to look at the periodic table of chemical elements.
Now you need to throw a closer look at any of the items in the table.Under the name of any of the elements in the table has a numeric value.That it is the atomic weight of that element.
now is to make out a few examples of the calculation of molecular weight, based on the fact that the atomic weights are now known.For example, one can calculate the molecular weight substances such as water (H2O).The water molecule contains one oxygen atom (O) and two hydrogen atoms (H).Then, having found on the periodic table atomic weight of hydrogen and oxyg
en, it is possible to start counting and molecular weight : 2 * 1.0008 (after two hydrogen molecules) + 15.999 = 18.0006 amu (atomic mass units).
another example.Another substance, molecular weight which can be counted, let it be an ordinary table salt (NaCl).As seen from the molecular formula of a molecule of salt comprises a sodium atom Na and one chlorine atom Cl.In such a case, its molecular weight is considered to be as follows: 22.99 + 35.453 = 58.443 amu
It should be noted that the atomic weights of isotopes of various substances differ from the atomic masses in the periodic table.This is due to the fact that the number of neutrons in the nucleus of the atom and within the isotope of the same substance is different, so the atomic weight also differ markedly.Therefore, isotopes of different elements are usually denoted by the letter of the element, adding at the same time in the upper left corner of its mass number.An example is the isotope deuterium ("heavy hydrogen"), which is equal to the atomic mass unit is not like a normal atom and two.