Radioactivity was discovered in 1896, quickly spurring research in countries burgeoning during the industrial revolution. Scientists, including Marie Curie in France and Ernest Rutherford in England, restlessly searched for radioactive minerals and conducted lab experiments on their properties.
Before the 20th century, the most commonly accepted model of the atom placed electrons inside a positively charged fluid. The negative electrons floated in the fluid and occasionally escaped as radioactivity.
Around the turn of the century, that idea had to be changed. Ernest Rutherford and his students H. Geiger and E. Marsden made a fundamental discovery revealing the structure of the nucleus. Their experiments indicated that there was a tightly packed and positively charged core in the atom and the electrons orbited around it.
In 1913, Neils Bohr came up with a model that fit much of the experimental observations. Bohr theorized that electrons orbited the nucleus in set orbits, instantaneously jumping from orbit to orbit when the atom lost or gained energy.
The secrets of the nucleus were harder to penetrate. Rutherford suggested that the nucleus had a positively charged proton and a neutral particle he called a neutron. In 1932, Chadwick in England found the neutron. That same year, Kurchatov shifted his field of research to nuclear physics.
In 1938 and 1939, Hahn Strassmann observed a fission reaction by bombarding uranium nuclei with neutrons, producing barium atoms. The added neutrons create an unstable uranium nucleus, causing it to divide and emit extra energy.
The potential for a new energy source was quickly realized. The next year, physicists worked on producing a chain reaction of uranium. By the end of the year, Szilard and Einstein sent a letter to President Roosevelt warning him of the potential for a new weapon.
Before the first atomic bomb was built using fissionable material, both the Americans and Soviets were working on a more powerful fusion bomb.
Fusion occurs when nuclei fuse together -- a reaction that includes emission of energy. However, this reaction can only occur under extremely high temperatures, reaching the temperature of the interior of stars. Nuclei have a strong repulsive force against each other because of their positive charge, so they need to be very energetic to overcome the repulsion.
Because fusion uses elements such as hydrogen isotopes, and it results in non-radioactive products such as helium, fusion power holds great promise. However, because of the incredibly high temperatures required, a form of containment needs to be developed before a fusion reactor can be built. Sakharov's Tokamak is one potential reactor, using powerful magnetic fields.