When a vacancy, or hole, occurs in the K shell, one electron from an outer energy shell makes a transition to fill the hole. The result of this transition is the emission of an X-ray photon, called a K X ray, which has a definite wavelength, as indicated by the appearance of a line in the spectrum. How many lines will be in the spectrum of iron when K X rays are emitted?

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.