Periodic Table

There's antimony, arsenic, aluminum, selenium And hydrogen and oxygen and nitrogen and rhenium And nickel, neodymium, neptunium, germanium And iron, americium, ruthenium, uranium Europium, zirconium, lutetium, vanadium And lanthanum and osmium and astatine and radium And tantalum, technetium, titanium, tellurium And cadmium and calcium and chromium and curium... The Periodic Table is a chart which organises chemical elements into groups based on their various properties - ascending by atomic number and grouped by things such as reactivity and valency. While it was first organised by these macroscopic properties, it actually forms a map of the atomic orbital structure indicating how quantum mechanical concepts relate to chemical properties. The Table was invented by Russian chemist Dmitri Mendeleyev in the second half of the 19th century, though others were part of a push towards order in the chemical world.

Origins
Chemical behaviour is generally very messy, complicated and irregular. Chemists, in contrast, enjoy making neat and tidy lists of things. By the 17th century, chemists had begun to use the concept of an "element" in its modern sense (i.e. a substance which cannot be broken down into a simpler substance using chemical means) - which was nice and tidy - but, distressingly, they had also begun to discover dozens of new elements besides those which had been known for centuries - this was distressing untidy. A glimmer of hope was at hand, however: it was noticed that many of these newly discovered elements appeared to exhibit similar properties, seeming almost to belong to 'families'. Moreover, these similar properties often seemed to exhibit trends when the elements were placed in order of . Examples included "triads" such as chlorine, bromine and iodine, and copper, silver and gold.

The race was on amongst chemists to draw up some kind of nice tidy list which would arrange the ever-increasing numbers of these pesky new elements in some kind of elegant way which brought order to the increasing chemical mess. The breakthrough was finally made by Mendeleev. His Table managed to set out the elements in rows and columns, in order (more or less) of atomic weight, so that those elements with similar properties fell into nice tidy vertical groups. Mendeleev made two particular innovations over previous pedants:


 * in a couple of cases where listing the elements in order of atomic weight would have messed up his groups, he swapped elements around so that they fitted the pattern
 * in further cases where listing the elements in any order wouldn't fit the pattern at all, he declared there must be a "missing element" and left a gap for it in the table.

However, Mendeleev couldn't, at the time, provide any physical explanation of why there should be missing elements, or why atomic weights occasionally needed swapping around. The trends were evident, but the knowledge of atomic structure and the identity of the missing elements was still far off, meaning that there was no verifiable proof that the periodic table was as Mendeleev hypothesised. Still, he stuck to his hypothesis, and the testable predictions of what properties these missing elements ought to have if and when they were discovered. Luckily for him, his predictions were correct!

Later, the discovery of atomic structure (protons, neutrons and electrons) provided a sound rationalising for Mendeleev's quirky ad hoc tweaks, and vindicated his periodic table. It was discovered that the shared properties of similar elements was due to these their having the same number of electrons in their outer shells. Thus, silicon has similar reactivities to carbon, although their difference in physical size also affects these properties. These observed facts are combined in their arrangement in the modern Periodic Table of the Elements.

Elements in the table are sorted by the increasing number of protons in their nuclei, from 1 to 118. There is no necessary reason for the number of elements to end there, but heavier elements would have half-lives in minute fractions of a millisecond, so they'd be very difficult to detect. However, similarly to what Mendeleev did, it is possible to predict the properties of these hypothetical elements using both periodic trends and our current understanding of physics. The structure of the table is based on the electronic structure of the atom: in the "p" block for example, there are elements for which the outermost electron is in a "p" orbital.