Electrons in the first energy level of an atom
(a) have no energy
(b) have the lowest possible energy
(c) have the highest possible energy
(d) are in an excited state
(e) are in an unstable state

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.