Each substance is not something solid, it consists of small particles that constitute the molecule.The molecules of atoms.From this we can conclude that determines the mass of a substance can be characterized molecules and atoms included elements.At the time, most of the works of Lomonosov shone the topic.However, many curious naturalists are always interested in the question: "In what units is expressed by the mass of the molecules, the atomic mass?»
But first, dip a bit in history
In the past per unit of mass of the atom always took a lot of hydrogen (H).And, based on this, we made all the necessary calculations.However, most of the compounds present in nature in the form of oxygen compounds, however atomic mass element calculated with respect to oxygen (O).What is quite uncomfortable, so they had to be taken into account in the calculation of the ratio of permanent A: N of 16: 1.In addition, studies have demonstrated the inaccuracy in the ratio, it is actually equal to 15.88: 1 and 16: 1,008.These changes have caused to convert the mass of atoms for many items.It was decided to leave about 16 weight value, and for H - 1,008.Further development of science has led to the disclosure of the nature of the oxygen.It was found that the oxygen molecule has several isotopes with masses of 18, 16, 17. For the physics is not acceptable to use the unit, which has an average value.Thus formed two scale atomic weights: in chemistry and physics.Only in 1961, scientists have concluded that it is necessary to create a single scale that is used today under the name of "carbon unit."As a result, the relative atomic mass of an element is the mass of an atom in terms of carbon.
methods of calculation
mass of molecules of any substance consists of a mass of atoms that form the molecule.We conclude that the mass of the molecule should be expressed in terms of carbon, as well as the mass of the atom, ie,relative atomic mass is determined by taking into account the relative molecular mass.As is known, using Avogadro's law can determine the number of atoms in the molecule.Knowing the number of atoms and molecules of the mass, we can calculate the atomic mass.There are several ways to define it.In 1858, Cannizzaro proposed method by which the relative atomic mass is determined by those elements which can form gaseous compounds.However, this does not have the ability to metals.Therefore, to determine their atomic mass method was chosen, using the dependence of atomic mass and heat capacity of the substance.But all the above methods only give approximate values of the atomic masses.
How was calculated exact mass of atoms of an element?
As shown by scientific studies of these approximate values can be determined accurate.It only need to compare this value with the equivalent.Equivalent elements equals the relative atomic weight of element to its valence compound.This relation was determined loyal relative atomic mass of each element.