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Minggu, 20 Februari 2011

Theory of Chemistry UN SMA 2011

Minggu, 20 Februari 2011

Chemistry (the etymology of the word has been much disputed is the science of matter and the changes it undergoes. The science of matter is also addressed by physics, but while physics takes a more general and fundamental approach, chemistry is more specialized, being concerned with the composition, behavior (or reaction), structure, and properties of matter, as well as the changes it undergoes during chemical reactions. Disciplines within chemistry are traditionally grouped by the type of matter being studied or the kind of study. These include inorganic chemistry, the study of inorganic matter; organic chemistry, the study of organic (carbon based) matter; biochemistry, the study of substances found in biological organisms; physical chemistry, the study of chemical processes using physical concepts such as thermodynamics and quantum mechanics; and analytical chemistry, the analysis of material samples to gain an understanding of their chemical composition and structure. Many more specialized disciplines have emerged in recent years, e.g. neurochemistry the chemical study of the nervous system (see subdisciplines).
The word chemistry comes from the earlier study of alchemy, which is a set of practices that encompasses elements of chemistry, metallurgy, philosophy, astrology, astronomy, mysticism and medicine. Alchemy in turn is derived from the Arabic word "كيمياء" meaning "value"; it is commonly thought of as the quest to turn lead or another common starting material into gold. Many believe that the Arabic word "alchemy" is derived from the word Chemi or Kimi, which is the ancient name of Egypt in Egyptian. The Arabs added the Arabic definite article "al" to the word, resulting in the word "الكيمياء" (al-kīmiyā). Thus, an alchemist was called a 'chemist' in popular speech, and later the suffix "-ry" was added to this to describe the art of the chemist as "chemistry".

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, nitrogen and oxygen.

Download Theory of Chemistry UN SMA 2011 according to grid UN 2010/2011

Save for the hydrogen and helium in the universe, which are thought to have been mostly produced in the Big Bang, most chemical elements are thought to have been produced by later processes.
These processes are divided into:
cosmic ray spallation (important for lithium, beryllium and boron, though some of these may have formed in the Big Bang), and
stellar nucleosynthesis which produces all elements heavier than boron (with carbon being the first of this series). The very heaviest elements (those beyond element 94, plutonium) decay with half lives too short to allow them to be observed naturally on Earth.
In total, 118 elements have been characterized as of March 2010, and new elements of higher atomic number are "discovered" from time to time, as new synthetic products of artificial nuclear reactions. Of the known elements, the first 92 occur naturally on Earth. Of these, oxygen is the most abundant element in the Earth's crust. About 80 elements have stable isotopes: namely all elements with atomic numbers 1 to 82, except elements 43 and 61 (technetium and promethium). About half of the 80 stable elements are expected to be radioactive with such long half lives that their decay happens only in theory, and is too slow to have yet been detected by experiment (see the list of nuclides). These elements (such as bismuth, only recently measured as unstable) have half lives at least 100 million to 1000 million times longer than the estimated age of the universe.
Elements with atomic numbers 83 or higher (bismuth and above) are unstable to the point that their instability has been detected, and they undergo radioactive decay. The elements from atomic number 83 to 94 are composed entirely of radioactive isotopes. Other radioactive elements continue to be produced in natural processes, such as production by cosmic rays, or as shorter-lived daughter nuclides or transmutation products from natural decay of longer-lived radioactives.
When two distinct elements are chemically combined, with the atoms held together by chemical bonds, the result is termed a compound. Chemical compounds may result in elements combined in exact whole number ratios of atoms (a familiar example is water). The term "compound" does not always imply an exact combination ratio, however, inasmuch as chemical bonding of many types of elements results in crystalline solids and metallic alloys, for which exact formulas do not exist. Most of the solid substance of the Earth is of this latter type: the atoms that are present in the substance of the Earth's crust, mantle, and core are combined into chemical compounds of many compositions, but these do not have precise empirical formulas.


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