Atom is the smallest portion of any chemical element, which can not be divided and which has independent existence. Atoms are composed of electrons orbiting a nucleus with neutrons and protons.
The atoms of different elements have different amounts of protons. The atomic number (which is identified with the letter Z, by the German term zahl ) indicates the number of protons that is present in the nucleus of an atom. This number, therefore, is in charge of defining the electronic configuration of the atom and allows the ordering of the various chemical elements in the periodic table, which starts with hydrogen (Z = 1) and continues with helium, lithium, beryllium, boron, carbon and nitrogen.
We must also add that the number of protons that exists in the nucleus of a particular atom is equal to the number of electrons that surrounds it in what is called the cortex.
The list of chemical elements established based on the atomic number of them can be determined that is headed by these ten elements: with 1 hydrogen, with number 2 helium, 3 is for lithium, 4 is beryllium, 5 is occupied by boron, 6 is carbon, 7 is nitrogen, 8 is oxygen, 9 is fluorine and 10 is neon.
To this we can add also that in total said list is formed by a total of 115 chemical elements, the last one being the ununoctio. Presumably this is a colorless noble gas that has as a temporary symbol the following: Uuo.
It is important to keep in mind that an unaltered atom is electrically neutral: this means that its atomic number will always be the same as its number of electrons.
Although the periodic table of the elements is usually attributed to Dimitri Mendeleiev (responsible for ordering the elements according to the variations of the chemical properties), who concretized the organization according to the physical properties of the atoms was Julius Lothar Meyer.
However, we can not ignore the important role played by the English physicist and chemist Henry Moseley who established his own law of atomic numbers, called Moseley's Law. In 1913 it was when it became known that it established that there was a clear relationship of a systematic type between the wavelength of the X-rays that were emitted by the atoms and the atomic number of the same.
In this way, with this discovery, this British chemist came to oppose or counteract the proposals made four decades earlier by the aforementioned Mendeleev.
We have already said that the number of protons (expressed by the atomic number) is equal to the number of electrons. The mass number ( A ), on the other hand, indicates the number of particles that an atom has in its nucleus. Therefore, it expresses the sum of the protons and neutrons. It is possible to establish, in short, the following relationship between the atomic number and the mass number: A = Z + N, where N equals the number of neutrons.