Döbereiner recorded this pattern as his “Law of Triads”, however, at the time this did not seem relevant to everyday life, as too few elements were known and atomic masses were uncertain, it was therefore deemed likely that this was just a curiosity.
It was later on in the century that a British chemist called John Newlands delved further into the periodicity of the elements. After many more elements were discovered, and atomic masses could be measured more accurately, he suggested that the elements were similar to an octave of music by stating that, when the elements were arranged in order of increasing atomic mass, “the eight element starting from a given one is a kind of repetition of the first.”
Newlands presented his “Law of Octaves” to the Chemistry Society in 1866, but they did not accept the theory as it seemed to be very self-limiting. It only seemed to apply to the first 16 elements, and it did not leave space for any new elements to be discovered.
Although the Chemistry Society did not accept it, we now recognise this as the very important pattern of periodicity, which means that the properties of elements have a regularly recurring, or “periodic” relationship with their relative atomic masses.
This may now be seen as a huge starting block for the construction of the periodic table, but the greatest credit for actually producing the table is always given to Dmiitri Mendeleev from Russia. He arranged the elements in the same way as Newlands did, and noticed regularly occurring patterns of properties. However, what made his method stand out from Newlands is that he was able to leave gaps so that elements which had not yet been discovered could be placed in groups with other similar elements.
Furthermore, he was able to predict the properties of the missing elements, such as the element between silicon and tin in Group IV. This element was then discovered 15 years later with the properties described by Mendeleev, proving his table was correct, and increasing its value, as a model or theory is always most valuable when it is used to explain and predict.
Mendeleev’s discoveries led to much more research and development and it has been greatly admired ever since. To prove its diversity, it was even able to cope with the discovery of a totally new group now known as the noble gases, even though Mendeleev himself had not predicted them.
The further research led a British scientist, Henry Moseley, in 1913, to prove that the real sequence in the periodic table is in order of atomic number (number of protons in a nucleus), not atomic mass which had been the accepted theory until this time. This sequence is very similar to that of the atomic mass but is slightly different and shows more accurate patterns.
Moseley’s work led to a final periodic table, the one which is today accepted all over the world. However, it did not answer questions about periodic variations in chemical and physical properties. The work done to explain this was not done until much later, and was discovered to be down to the numbers and distributions of the electrons orbiting the nucleus.
It was discovered that the periodic table then needed to be split into “blocks” to satisfy these variations. The blocks are dependant upon the electrons in the outer shell of the different elements.
The defining characteristics of the periodic table that we know today are its vertical Groups of elements labelled I to VII and 0, and the horizontal Periods labelled 1 to 7. The “blocks” are labelled as follows; Groups I and II – s block, Groups III to VII – p block, transition metals – d block, and the lanthanide and actinide elements – f block.
