Chemistry Drill Solutions of UN Exam 2011 SMA Paket 5

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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