Chemical Element and Atomic Weights

Topics: Chemical element, Isotope, Chemistry Pages: 6 (1943 words) Published: September 14, 2011
The history of the periodic table reflects over a century of growth in the understanding of chemical properties, and culminates with the publication of the first actual periodic table by Dmitri Mendeleev in 1869.[1] While Mendeleev built upon earlier discoveries by such scientists as Antoine-Laurent de Lavoisier and Stanislao Cannizzaro, the Russian scientist is generally given sole credit for development of the actualperiodic table itself. The table itself 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. -------------------------------------------------

Elemental ideas from ancient times
People have known about some chemical elements like 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.[2] However, the notion that there were a limited number of elements from which everything was composed originated with the Greek philosopher Aristotle. About 330 B.C Aristotle proposed that everything is made up of a mixture of one or more of four "roots" (originally put forth by the Sicilian philosopher Empedocles), but later renamed elements by Plato. The four elements were earth, water, air and fire. While the concept of an element was thus introduced, Aristotle's and Plato's ideas did nothing to advance the understanding of the nature of matter.

Age of Enlightenment
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[3] 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. This and related discoveries raised the question of what it means for a substance to be an "element". In 1661 Boyle defined an element as a substance that cannot be broken down into a simpler substance by a chemical reaction. This simple definition actually served for nearly 300 years (until the development of the notion of subatomic particles), and even today is taught in introductory chemistry classes. In 1817, Johann Wolfgang Döbereiner began to formulate one of the earliest attempts to classify the elements. He found that some elements formed groups of three with related properties. He termed these groups "triads". Some triads classified by Döbereiner are: 1. chlorine, bromine, and iodine

2. calcium, strontium, and barium
3. sulfur, selenium, and tellurium
4. lithium, sodium, and potassium
In all of the triads, the atomic weight of the second element was almost exactly the average of the atomic weights of the first and third element. Dimitri Mendeleev, a 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 byatomic weight, corresponding to relative molar mass as defined today. It is sometimes said that he played "chemical solitaire" on long train rides using cards with various facts of known elements.[10] 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: 1. The elements, if arranged according to their atomic weights, exhibit an apparent periodicity of properties. 2. Elements which are similar...
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