Early in the 19th cent., a number of chemists had noticed certain relationships between the properties of elements and their atomic weight. In 1829 J. W. Döbereiner stated that there existed some three-element groups, or triads, in which the atomic weight of the middle element was the average of the other two and the properties of this element lay between those of the other two. For example, calcium, strontium, and barium form a triad; lithium, sodium, and potassium, another. The English chemist J. A. Newlands found (1863-65) that if the elements are listed according to atomic weight starting with the second, the 8th element following any given element has similar chemical properties, and so does the 16th. This became known as the law of octaves. About the same time, A. E. de Chancourtois arranged the elements according to increasing atomic weight in the form of a vertical helix with eight elements in a turn, so that elements having similar properties fell along vertical lines.The Periodic Table
D. I. Mendeleev was the first to state the periodic law close to its present form. He proposed in 1869 that the properties of elements are periodic functions of the atomic weight and grouped the elements accordingly in a periodic system. Working independently and not aware of Mendeleev's work, Lothar Meyer arrived at a similar system, publishing his results about a year after Mendeleev's. When Mendeleev devised his periodic table a number of positions could not be fitted by any of the then known elements. Mendeleev suggested that these empty spaces represented undiscovered elements and by means of his system accurately predicted their general properties and atomic weights.
The work (1913-14) of H. G. Moseley on the X-ray spectra of elements (see X ray) led to the present form of the periodic law. He found that the wavelength of the X-radiation of elements decreased with increasing atomic weight. However, the relationship was not a strict one. He assigned a new set of numbers, called atomic numbers, to the elements he had studied, so that there was a relation between the wavelength and the atomic number. The atomic number is the number of positive charges, or protons, contained in the atomic nucleus (see atom) or, equivalently, the number of negative charges, or electrons, outside the nucleus in a neutral atom. The periodic law can be explained on the basis of the electronic structure of the atom, which is believed to be the main factor underlying the chemical properties and many of the physical properties of the elements. In turn, the electronic structures of atoms have been successfully accounted for by the quantum theory.
In spite of its great success, the periodic system that had been introduced by Mendeleev had some discrepancies. Arranged strictly according to atomic weight, not all elements fell into their proper groups. Better arrangement could be made if the positions of certain neighboring couples were interchanged. For example, to suit the chemical order of the table, the inert gas argon (at. wt. 39.948) should come before the chemically active metal potassium (at. wt. 39.0983). Through Moseley's work, it was found that although the atomic number of an element is roughly half its atomic weight, the atomic weight does not always increase with increasing atomic number. The discrepancies occur just for those elements where Mendeleev's law failed. Based on atomic number, the periodic law now has no exceptions. Although all the missing elements in the periodic table have been found (with the aid of the periodic table itself), the table retains its usefulness to the chemist as a reliable check for disputed or uncertain data concerning some of the known elements.
The periodic table is a tabular method of displaying the chemical elements. Although earlier precursors exist, its invention is generally credited to the Russian chemist Dmitri Mendeleev in 1869. The table is a visual representation of the periodic law which states that certain properties of elements repeat periodically when arranged by atomic number. The table arranges elements into vertical columns (Groups) and horizontal rows (Periods) to display these commonalities.
Hennig Brand was the first person recorded to have discovered 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 1649 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 1869, a total of 63 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.
Newlands noted that many pairs of similar elements existed which differed by some multiple of eight in atomic weight. However, his law of octaves, likening this periodicity of eights to the musical scale, was ridiculed by his contemporaries. It was not until the following century, with Gilbert N. Lewis' valence bond theory (1916) and Irving Langmuir's octet theory of chemical bonding (1919) that the importance of the periodicity of eight would be accepted.
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 (Eng., "Chemistry Magazine"), in 1869. It stated:
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 cocreators 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.
Moseley's research also showed that there were gaps in his table at atomic numbers 43 and 61 which are now known to be Technitium and Promethium, respectively, both radioactive and not naturally occurring. Following in the footsteps of Dmitri Mendeleyev, Henry Moseley also predicted new elements.
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.