now using your periodic table as a reference type the symbols for the known elements into the appropriate groups and periods in a modified table below note that were using the international group numbers one through 18 followed by the American scheme for the a group elements in parentheses

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chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.