The periodic table is one of the most commonly used references for a chemist, and provides order and understanding to what would be a chaotic and confusing array of elements.
The periodic table is a natural arrangement of all possible elements that exist. This arrangement complies with the periodic law which states the chemical elements, when arranged in order of increasing atomic number to form a chart, show repeating properties'.
An atomic number is the amount of protons in the nucleus of the atom. This is also the same amount of electrons, which is a major contributor to the reactivity and properties of that element. Another contributor to the properties of the element is how many neutrons are present, and thus forming isotopes. The mass of the nucleus is subsequently changed and with it the spacing between the electron shells and the nucleus. .
Since the birth of humankind, ways to order the chemical elements have been created, no matter how crude. At one stage in history, it was stated that there were four elements, earth fire air and water. That theory although remarkably perfect in its simplicity, didn't last for very long. Greek thinkers in about 4000BC said that there were "atoms' which were the smallest parts of matter, which made up everything. Further on down the track, many discoveries were made about atoms, their structure and many elements had been discovered and isolated. This was when a need to order those elements became apparent. Many schemes to order the elements were created, but none combined all their properties into a neatly arranged manner.
Johann DÖbereiner in 1829 found that their were chemical triads. Three elements where the properties of the middle massed could be determined from the heavier and lighter ones. E.g. lithium, sodium and potassium and sulfur, selenium and tellurium.
These triads were important as these triads needed to be incorporated into the order.