All matter is made up of atoms. The central body of an atom is called a nucleus. The nucleus consists of protons (positively charged particles) and neutrons (particles with no electrical charge). Electrons (negatively charged particles) are located outside the atom’s nucleus.
Greek scientists always believed that the atom could not be divided. The word, "atom," comes from a Greek word which means "cannot be divided." But one form of a naturally occurring atom (uranium), is different. It can be mined, reconstituted to form a substance called "yellowcake," and then enriched (by chemical process) to create a type of uranium isotope called U-235.
Uranium 235 is the only type of uranium, or naturally occurring substance, which can break apart. It can split because the nucleus of U-235 is unstable. When it breaks apart, the atom’s neutrons are released. When those "on-the-loose" neutrons hit other U235 atoms, they also split.
The
splitting of those atoms is called "fission." Fission (in a chain reaction) releases more neutrons plus
heat.
The discovery of fission by German scientists, in 1938, led Einstein and, especially, his colleague Leo Szilard (a Hungarian scientist working in America) to worry that Germany could create a new type of bomb. Concerned, and at Szilard’s urging, Einstein
wrote a letter to President Roosevelt on August 2, 1939. In it he observed, among other things:
In the course of the last four months it has been made probable - through the work of Joliot in France as well as Fermi and Szilard in America - that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.
Beyond the theoretical idea of creating power, from splitting atoms, was the very real concern that Germany could develop a nuclear bomb:
This new phenomenon would also lead to the construction of bombs, and it is conceivable - though much less certain - that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air.
Einstein also did something else in this historically significant letter. He suggested that the U.S. government get involved, to some extent, in the process:
In view of this situation you may think it desirable to have some permanent contact maintained between the Administration and the group of physicists working on chain reactions in America.
As the old adage goes: "Once you invite the government in, it’s pretty hard to get them out."