Drill of UN SMA 2011 Kimia Paket 1 B

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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