Periodic Table - chemistry.

Periodic Table - chemistry. I INTRODUCTION Periodic Table, table of the chemical elements arranged to illustrate patterns of recurring chemical and physical properties. Elements, such as oxygen, iron, and gold, are the most basic chemical substances and cannot be broken down by chemical reactions. All other substances are formed from combinations of elements. The periodic table provides a means of arranging all the known elements and even those yet to be discovered. Although all elements differ from one another, some show similarities that allow scientists to categorize them. Around 1870, Russian chemist Dmitry Mendeleyev used these similarities to construct the original periodic table. Since that time, newly discovered elements have been added and the table's layout has changed; however, the modern periodic table conveys essentially the same information as the one that Mendeleyev created. In the modern table, elements with similar properties fall into columns called groups or families. Group 1 of the periodic table, for example, contains a number of soft metals, all of which react vigorously with water to form hydrogen gas. II ARRANGEMENT OF THE TABLE The elements within the modern periodic table are arranged from left to right, top to bottom, in order of increasing atomic number. An element's atomic number is the number of protons in its nucleus. More than 90 elements have been found to occur naturally on Earth. The periodic table also includes artificially created elements. These additions have the highest atomic numbers on the table. They must be prepared through experiments with nuclear reactions. The most recent element to be synthesized has 116 protons in the nucleus of each of its atoms. The elements most recently produced have not yet been officially named. Whereas the ordering of the elements is completely determined by their atomic numbers, the arrangement into vertical columns, called groups, is determined by a number of factors. These factors include chemical properties, physical properties, and the number of electrons thought to exist in the outer shells of the element's atoms. (The electrons that surround the nucleus of an atom are arranged in concentric shells.) The placement of elements into groups within the periodic table is not completely clear-cut. Some scientists disagree about minor differences in the placement of elements such as hydrogen and helium. Helium, which does not react with other elements, is usually placed in group 18, which houses the noble gases. This group also includes neon, argon, and krypton, all of which are also very unreactive. Scientists who group the elements based primarily on the number of outer-shell electrons place helium with elements such as magnesium, calcium, and barium among the alkaline earth metals of group 2. Elements in group 2 have two electrons in their outermost shell. The periodic table has been published in various shapes and sizes, but the most commonly used modern form begins with a column of group 1 metals on the left-hand side, followed by a column of group 2 alkaline earth metals. These columns are followed by a block of 40 elements divided into ten columns of four elements each. The groups in this block, collectively called the transition metals, are numbered 3 through 12. Groups 13 through 18 make up the right-hand side of the table. A diagonal dividing line separates the nonmetals in the upper-right portion of this block, such as oxygen, carbon, and nitrogen, from the metals such as tin and lead in the lower left portion. There is an additional block of 28 elements, divided into two rows of 14 elements each, that is usually placed beneath the main table. These are the rare earth elements, whose properties are all remarkably similar. They are so similar to one another that chemists have difficulty separating them when they occur together as mixtures. This additional block really belongs between the first block, consisting of groups 1 and 2, and the transition metal block. For convenience it is placed at the bottom of the table rather than in its proper place. Otherwise the periodic table would be very wide and would not lend itself to being represented on wall charts. Scientists refer to the horizontal rows in the periodic table as periods. Periods vary in length. Moving through the table from top to bottom, the successive periods contain 2, 8, 8, 18, 18, 32, and 32 elements. These numbers correspond to the maximum number of electrons that can be accommodated in the largest electron shell in an atom of any element belonging to that period. III USES OF THE PERIODIC TABLE The periodic table is an important tool for scientists and students studying the chemical elements. If a person knows the main properties of each of the groups in the periodic table and how chemical properties vary within a group, he or she can predict the properties of any particular element with a reasonable degree of confidence. For example, if a student needs to know the properties of francium, she can predict that it, like other elements of group 1, will be a soft metal and will react even more vigorously with water than the elements above it. If another student wants to predict what compounds tellurium will form in combination with hydrogen, he can guess that the two elements will form H2Te because the other elements in tellurium's group form similar hydrogen compounds: H2O, H2S, and H2Se. IV HISTORY A number of chemists contributed to the development of the periodic table during the 19th century; however, Mendeleyev is regarded as the primary discoverer. His version of the table was the first to accommodate all the known elements and also successfully predicted the existence of several elements that had not yet been discovered. These undiscovered elements included gallium, scandium, and germanium. At first, the ordering of the elements was based on atomic weight--the weight of a single atom of the element. This method of ordering broke down in a few cases, such as tellurium and iodine. The atomic weight of tellurium is actually higher than that of iodine, suggesting that tellurium belongs in group 17 while iodine belongs in group 16. Judging by their chemical properties, however, tellurium belongs in group 16 and iodine in 17. This anomaly was resolved in 1914, when it was discovered that atomic number provides a better basis than atomic weight for ordering the elements. Tellurium has an atomic number of 52 and iodine's is 53. Elements with similar properties are placed in the same group of the periodic table, but for many years it was a mystery why these elements behaved similarly. At the beginning of the 20th century, when theories of physics changed rapidly, an approximate explanation for the repeating patterns within the periodic table was found. Scientists discovered that the elements within a single group of the periodic table possess the same number of outer-shell electrons, particles that had not even been discovered when scientists produced the first periodic tables. Contributed By: Eric Scerri Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.