The Periodic Table of the Elements
Elements are arranged systematically according to the Periodic Law
A key tool for chemists throughout the world is the systematic arrangement of all known elements in the Periodic Table
Until the mid-nineteenth century, chemists worked without any systematic understanding of the elements. When the Periodic Law was proposed, finally some order came to the study of chemistry.
Atomic Weights
In 1860 there was a conference of scientists in Karlsruhe, Germany at which the relative atomic weights of the sixty known elements were agreed far more accurately than before. This gave several scientists, including John Newlands in London, Julius Meyer in Germany, Alexandre Béguyer in France, and Dimitri Mendeleev in Russia, a new idea. They discovered that when the elements were arranged in order of increasing atomic weight, there was a repeating pattern of properties.
Mendeleev
It was Mendeleev who succeeded in producing the best arrangement of atoms in a table published in 1869. This was despite the fact that many elements were unknown and others were wrongly understood. Once he saw that there were patterns in some of the elements, he grouped those elements together. He was the first chemist to make this “periodicity” more important than the calculated values for atomic weight.
Periodicity
The other scientists had assumed that the known elements were complete. Mendeleev stuck to his patterns of periodicity and left gaps for unknown elements. He even predicted their properties, which have all turned out to be correct as more elements have been discovered.
Periodic Table
Today the periodic table of Mendeleev appears very crude in comparison to the one used by every student of chemistry. The development of models for the atomic structure have led to the realisation that the best way to arrange the elements is by atomic number or the number of protons each element has in the nucleus. The elements that caused most trouble for the early pioneers of the Periodic Law were the Transition Metals. These have been set apart from the other elements in the central section of the Periodic Table. Their strange properties have been explained by the discovery of different orbital types in the electron shells.
Groups
The current Periodic Table begins with Hydrogen and Helium, the smallest and simplest elements. Then come the two groups of reactive metals, the Alkali Metals (Group 1) and the Alkaline Earth Metals (Group 2). After the Transition Metal Section comes the p-block groups characterised by increasingly less metallic properties. In the second period (row) the familiar elements Carbon, Nitrogen, Oxygen demonstrate the changing properties until the halogens are reached. These are extremely reactive non-metals and to the opposite extreme to the Alkali metals, chemically speaking.
Order in Chemistry
The final group contains the Noble Gases. These are all almost chemically inert gases due to their full outer shells of electron. So the Law of Periodicity demonstrates the order of the chemical world and has proved invaluable to the modern science of Chemistry.
