Wikijunior:The Elements/Periodic Table
The Periodic Table of the Elements 
What is the Periodic Table of Elements? 
Although scientists know of many elements, the properties of many of them are similar. Thus if they are grouped on the basis of their properties, it becomes easy to study and compare their properties. The periodic table of elements is a way of organising all the known elements. In the early days, elements were classified into only two groups: metals and non-metals. But some elements showed the properties of both metals and non-metals. They were called as metalloids.
History of the Periodic Table 
Early History 
People have known about basic chemical elements such as gold, silver, and copper from antiquity, as these can all be discovered in nature in native form and are relatively simple to mine with primitive tools. Aristotle, a philosopher, theorised that everything is made up of a mixture of one or more of four elements. They were earth, water, air, and fire. This was more like the four states of matter (in the same order): solid, liquid, gas, and plasma, though he also theorised that they change into new substances to form what we see.
Hennig Brand was the first person to discover a new element. Brand was a bankrupt German merchant who was trying to discover the Philosopher's Stone — a mythical object that was supposed to turn inexpensive base metals into gold. He experimented with distilling human urine until in 1669 he finally obtained a glowing white substance which he named phosphorus. He kept his discovery secret, until 1680 when Robert Boyle rediscovered it and it became public.
By 1809, a total of 47 elements had been discovered. As the number of known elements grew, scientists began to recognize patterns in the way chemicals reacted and began to devise ways to classify the elements.
Antoine-Laurent de Lavoisier 
Antoine Lavoisier's Traité Élémentaire de Chimie (Elementary Treatise of Chemistry, 1789, translated into English by Robert Kerr) is considered to be the first modern chemical textbook. It contained a list of elements, or substances that could not be broken down further, which included oxygen, nitrogen, hydrogen, phosphorus, mercury, zinc, and sulfur. It also forms the basis for the modern list of elements. His list, however, also included light and caloric, which he believed to be material substances. While many leading chemists of the time refused to believe Lavoisier's new revelations, the Elementary Treatise was written well enough to convince the younger generation.
This model only classified elements into metals and non-metals and thus was not accepted.
Alexandre-Emile Béguyer de Chancourtois 
Alexandre-Emile Béguyer de Chancourtois, a French geologist, was the first person to notice the periodicity, the periodic or repetitive nature, of the elements — similar elements seem to occur at regular intervals when they are ordered by their atomic weights. He devised an early form of periodic table, which he called the telluric helix. With the elements arranged in a spiral on a cylinder by order of increasing atomic weight, de Chancourtois saw that elements with similar properties lined up vertically. His chart included some ions and compounds in addition to elements. His paper was published in 1862, but used geological rather than chemical terms and did not include a diagram; as a result, it received little attention until the work of Dmitri Mendeleev.
John Newlands 
John Newlands was an English chemist who in 1863 classified  the 56 elements that had been discovered at the time into 11 groups which were based on similar physical properties. He noted that many pairs of similar elements existed which differed by some multiple of eight in atomic weight.
The first periodic table 
Dmitri Mendeleev, also spelt Dmitry Mendeleyev, middle name (patronymic) Ivanovich, a Siberian-born Russian chemist, was the first scientist to make a periodic table much like the one we use today. Mendeleev arranged the elements in a table ordered by atomic mass. It is sometimes said that he played "chemical solitaire" on long train rides using cards with various facts of known elements. On March 6, 1869, a formal presentation was made to the Russian Chemical Society, entitled The Dependence Between the Properties of the Atomic Weights of the Elements. His table was published in an obscure Russian journal but quickly republished in a German journal, Zeitschrift für Chemie, in 1869. It stated
- The elements, if arranged according to their atomic weights, exhibit an apparent periodicity of properties.
- Elements which are similar as regards to their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs).
- The arrangement of the elements, or of groups of elements in the order of their atomic weights, corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, Ba, C, N, O, and Sn (probably an error for Li, Be, B, C, N, O and F, since the symbols for the elements weren't completely standardized yet at that time).
- The elements which are the most widely diffused have small atomic weights.
- The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
- We must expect the discovery of many yet unknown elements–for example, elements analogous to aluminium and silicon–whose atomic weight would be between 65 and 75.
- The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Thus the atomic weight of tellurium must lie between 123 and 126, and cannot be 128.
- Certain characteristic properties of elements can be foretold from their atomic weights.
- Mendeleev predicted the discovery of other elements and left space for these new elements, namely eka-silicon (germanium), eka-aluminium (gallium), and eka-boron (scandium). Thus, there was no disturbance in the periodic table.
- He predicted (often accurately as it turned out) properties of some of these then missing elements as well as properties of some of their compounds.
- He pointed out that some of the then current atomic weights were incorrect.
- He provided for variance from atomic weight order
- There was no place for the isotopes of the various elements.
- His table did not include any of the noble gases, which hadn't been discovered. But these were added by Sir William Ramsay as Group 0, without any disturbance to the basic concept of the periodic table.
Unknown to Mendeleev, Lothar Meyer was also working on a periodic table. In his work published in 1864, Meyer presented only 28 elements, classified not by atomic weight but by valence alone. Also, Meyer never came to the idea of predicting new elements and correcting atomic weights. Only a few months after Mendeleev published his periodic table of all known elements (and predicted several new elements to complete the table, plus some corrected atomic weights), Meyer published a virtually identical table. Some people consider Meyer and Mendeleev the co-creators of the periodic table, although most agree that Mendeleev's accurate prediction of the qualities of the undiscovered elements lands him the larger share of credit. In any case, at the time Mendeleev's predictions greatly impressed his contemporaries and were eventually found to be correct. An English chemist, William Odling, also drew up a table that is remarkably similar to that of Mendeleev in 1864.
Henry Moseley 
In 1914, Henry Moseley found a relationship between an element's X-ray wavelength and its atomic number and therefore resequenced the table by electronic charge rather than atomic weight. Before this discovery, atomic numbers were just sequential numbers based on an element's atomic weight. Moseley's discovery showed that atomic numbers had an experimentally measurable basis.
Moseley's research also showed that there were gaps in his table at atomic numbers 43 and 61 which are now known to be radioactive and not naturally occurring. Following in the footsteps of Dmitri Mendeleyev, Henry Moseley also predicted new elements.
Glenn T. Seaborg 
During his Manhattan Project research in 1944, Glenn T. Seaborg experienced unexpected difficulty isolating Americium (95) and Curium (96). He began wondering if these elements more properly belonged to a different series which would explain why the expected chemical properties of the new elements were different. In 1945, he went against the advice of colleagues and proposed a significant change to Mendelev's table: the actinide series.
Seaborg's actinide concept of heavy element electronic structure, predicting that the actinides form a transition series analogous to the rare earth series of lanthanide elements, is now well accepted in the scientific community and included in all standard configurations of the periodic table. The actinide series are the second row of the f-block (5f series) and comprise the elements from Actinium to Lawrencium. Seaborg's subsequent elaborations of the actinide concept theorized a series of superheavy elements in a transactinide series comprising elements 104 through 121 and a superactinide series inclusive of elements 122 through 153.