During most of the 19th Century physicists believed that atoms were the smallest particles of matter and thus couldn’t be further subdivided. With such characteristics, an atom of one element obviously couldn’t become an atom of another element – a process they labeled “alchemy”. J. J. Thomson forced a change in this concept of the atom in 1897 when he showed that atoms actually consisted of smaller charged particles( negatively charged electrons, positively charged protons and neutrons with a neutral charge). This concept made possible his subsequent discovery that atoms could actually lose protons allowing an atom of one element to become an element of another element – a process called fission. Subsequent investigations indicated that Thomson’s particles were themselves composed of even smaller “particles”, or maybe “wavelets”. Thomson initially conceived of all of these particles as being distributed evenly within the atom. Ernest Rutherford introduced the concept that the protons and neutrons were concentrated in a nucleus with the electrons orbiting the nucleus like planets orbiting the sun. Physicists no longer compare electrons to the planets, but the basic concept of a nucleus surrounded by electrons is still use. Many physicists still believe that electrons go around the nucleus. This concept made possible a major new concept developed by Niels Bohr. During the 19th Century physicists believed that the absorption of specific wavelengths of light by atoms resulted in them becoming hotter because there wasn’t any other aspect of the atoms that could change. In 1913 Bohr reported that the process of absorbing specific wavelengths of light actually involved a change in the energy level of the electrons in atoms. Rather than continuously absorbing radiation, the electrons would switch to a higher energy state when they absorbed radiation and then back to a lower level when they emitted that radiation. Electrons had to emit radiation before absorbing any new radiation. Bohr provided the concept of electrons being in “shells”. The concept of atoms as particles didn’t adequately explain how atoms could combine to form molecules. The concept of atoms comprised of smaller particles provides a mechanism for atoms to join together. The “shells” Bohr suggested needed to have a certain number of electrons. Sharing electrons allowed atoms to fill those “shells” with one electron in effect becoming part of two different atoms. Having moving electrons in atoms would complicate such sharing. How would electrons move around multiple atoms? Another problem of moving electrons would be keeping them from colliding or being thrown out of the atom by the repulsive effect of their negative charges. If electrons maintained fixed positions within atoms they could easily be shared by two atoms. Fixed positions could explain the need for specific numbers of electrons. The electrons negative charges would repel each other. The positive charge of the nucleus would tend to draw them to the nucleus. Electrons could form a sphere around the nucleus. The electrons could not move closer to the nucleus because the repulsive force of their charges would prevent them from getting close enough to each other to move closer to the nucleus. The force keeping the electrons in position might prevent a determination of their exact position by creating an energy field. Fixed positions would allow atoms to share electrons by positioning themselves so that an electron could hold a fixed position in both atoms.