An element is: A. A compound formed when different atoms react and form bonds with each other.
B. A pure chemical compound consisting of multiple atomic numbers.
C. A pure chemical substance consisting of one type of atom.
D. Two or more compounds that are mixed together but not chemically bonded.
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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.