The alchemists of the Middle Ages and the Renaissance worked in vain to transmute lead into gold. Modern nuclear physicists are the heir of their legacy, substituting Bunsen burners and beakers for reactors and accelerators in an attempt to transform the basic building blocks of matter. While the lucrative transformations dreamt of by alchemists are as yet impossible, nuclear physicists regularly create nuclei of atoms from other nuclei: transmutation. In this, as in so much else, science is following in the footsteps of Nature, where such nuclear changes have been occurring spontaneously since the beginning of the Universe.
Any change made to the nucleus of an atom instantly affects the structure of the atom itself. As soon as the nucleus has emitted an alpha particle, say, or an electron or positron, the delicate balance of electric charges that exists in the atom is broken, with the nucleus. In order to regain stability, the atom is forced to either release some of the orbiting electrons, or attract in more from outside.
Once these exchanges have finished, the atom's basic chemical nature has changed. This phenomenon was first noticed by Ernest Rutherford (an omnipresent figure in the early history of radioactivity!), in 1900. Rutherford's experiment was to observe the emanation of a radioactive gas, later known as radon, in the disintegration of metallic radium.
In 1901, along with his pupil Frederick Soddy (1877-1956), he was able to show that both alpha- and beta- emissions are indicative of a fundamental change in the nature of the nuclei and atoms involved. As well as providing evidence for the controversial 'solar system' model of the atom by proving the existence of a nucleus, he was thus able to conclusively disprove one of science longest-held dogmas: the indivisibility of matter. The age-old belief that 'the atom could not be split'. Soddy later recalled that when, in the sheer exultation of the moment, he triumphantly claimed to have witnessed 'transmutation', Rutherford warned him: "For Christ's sake, Soddy, don't call it transmutation. They'll have our heads off as alchemists."
Today, such 'transmutation' is basic currency for all physicists, and can take place on relatively large scales. At the heart of nuclear reactors, where uranium is transformed into plutonium, a one Gigawatt reactor can produce 200 kilograms a year; admittedly, a modest result compared to the amount of energy involved. While there is nothing to theoretically prevent the eventual transformation of lead into gold, a metal detector and a goldpan are still your best bets