At the beginning of the 20th century scientists conducted a series of experiments doing their researches on particles carrying charge and moving in a magnetic field. They studied both beams of electrons, and beams of positive ions (which are atoms with loss of some of their electrons).

The beam of ions goes through some space where the magnetic and electric fields are crossed. In the electric field there is an upward force influencing the ions and described by the formula:

          (1)

          (2)

The instrument is constructed in the way, which won't let other ions than the ones with the not curved trajectory move out of the space of fields. These are those on which the magnetic and electric forces equilibrate:

          (3)

          (4)

Then the beam of ions, all having the same velocity, falls in the area of the magnetic field, which is perpendicular to the vector of their velocity.

          (5)

On the other hand:

          (6)

Connecting (5) and (6):

          (7)

Knowing the charge of the ions, the velocity (which was calculated before), and the radius of their movement we can define their mass:

          (8)

But it turned out that ions of the particular element, all having the same charge and velocity can move on different radii. It means that their masses are different. How to explain that?

It turned out that the answer lies in the neutrons. And this is how we explain it nowadays:

Atoms of respective elements vary in the number of protons in a nucleus. (So they also vary in the number of electrons on orbits.) For example a hydrogen atom has always one proton, and oxygen - always 8. The second constituent of a nucleus are neutrons. Nuclei of atoms of a particular element contain various numbers of neutrons. For example most hydrogen nuclei contain no neutrons. Some fraction of a per cent are nuclei consisting of one proton and one neutron, and the even smaller fraction of a per cent are nuclei consisting of one proton and two neutrons. Each type of them, however, contains only one proton - all of them are hydrogen nuclei. Atoms of a particular element but containing different number of neutrons are called isotopes. Customary a hydrogen isotope including one neutron is reffered to as deuterium, and including two neutrons - tritium. Other elements have even more isotopes. For example carbon nuclei contain 5 to as many as 10 neutrons, however always containing 6 protons. It was found that each type of isotopes occurs in nature always in the same percentage. For example there are about 75 per cent helium nuclei containing 18 neutrons, and about 25 per cent containing 20 neutrons. So in many tables the atomic mass of a particular element is given not as an integer because it is an arithmetic mean of masses of the isotopes. For example the chlorine atomic mass is often given as equal to 35,45.