Atomic Theory

Early studies of radioactivity revealed that certain atomic nuclei were naturally radioactive. Some scientists wondered that if particles could come out of the nucleus, would it also be possible to force particles into the nucleus? In 1932, Cockcroft and Walton succeeded in building a particle accelerator, a device that could make streams of charged particles move faster and faster. These fast particles, protons for example, were then aimed at a thin plate of a lighter element like lithium (Li). If a lithium atom nucleus "captures" a proton, the nucleus becomes unstable and breaks apart into two alpha particles. This technique of inducing radioactivity by bombardment with accelerated particles is still the most used method of studying nuclear structure and subatomic particles. Today, accelerators race the particles in straight lines or, to save land space, in ringed paths several miles in diameter.

The spontaneous rearrangement of the atomic nucleus always results in a release of energy in the form of kinetic motion in fast-moving neutrons. When a large nucleus falls apart to form smaller atoms, the process is called fission. When lighter atoms are forced together to produce a heavier atom, the process is called fusion. In either case, fast neutrons are released. These can transfer their kinetic energy to the surroundings, heating it. This heat can be used to boil water, producing steam to run a turbine that turns an electric generator. Fusion is the process occurring in the center of the Sun and other stars. So much energy can be released quickly that the process has also been used for the hydrogen bomb. However, fusion is not yet controlled enough for running a power plant. Research continues to find a controlled method of using fusion energy.

On the other hand, fission reactions have also been used for very powerful weapons. The first atomic bomb was detonated in 1945. Since then, however, fission energy has also been controlled enough to operate the many nuclear power plants around the world.

While an atom is the smallest part of an element which still is that element, atoms are not the smallest particles that exist. Even the protons and neutrons in the atomic nucleus are believed to made of even smaller particles called quarks. Current research in atomic physics focuses on describing the internal structure of atoms. By using particle accelerators, scientists are trying to characterize quarks which may combine in a number of ways to produce other types of subatomic particles.

No one has ever seen a single atom even with the best optical microscopes. Special types of microscopes called scanning tunneling microscopes and atomic force microscopes make use of the forces produced by the electrons to obtain images of the electron clouds. These clouds indicate how atoms are arranged but we cannot "see" through the cloud to the nucleus. Because of the limitations of size, we will never see an atom with our own eyes. Everything we know about atoms must be deduced from larger-scale experiments. As a result, the description of atoms is still called a theory. However, this theory explains atomic experiments so well that we usually think of the existence of atoms as a fact.