How the Manhattan Project Worked

The Discovery of Nuclear Fission

To get to the Manhattan Project and the bombings of Hiroshima and Nagasaki, it helps to understand the advancements made in physics leading up to World War II. Between 1919 and the early 1930s, scientists were piecing together the important parts of the atom's structure. In 1919, at Manchester University in England, New Zealand physicist Ernest Rutherford discovered protons, positively charged particles located in the nucleus of the atom that, along with negatively charged electrons orbiting around the center, make up the atom.

There was one problem -- physicists couldn't explain why several elements weighed different amounts. This remained a mystery until 1932, when James Chadwick, one of Rutherford's colleagues, discovered the neutron, a third subatomic particle. With no charge, neutrons share space with protons in the atom's nucleus. While the number of protons and electrons is always the same for any given element -- carbon, for instance, always has 6 protons and 6 electrons -- there can be different numbers of neutrons. This explained why carbon could weigh different amounts, even though it was essentially the same element. These different weights of atoms are known as isotopes.


Around this time, scientists began using particle accelerators to bombard the nuclei of atoms in the hopes of splitting atoms and creating energy. Initially, they achieved very little success -- early particle accelerators shot out protons and alpha particles, both positively charged. Even at high speeds, these particles were easily repelled by the positively charged nuclei, and figures such as Rutherford, Albert Einstein and Niels Bohr felt that harnessing atomic power was close to impossible.

This changed when Italian physicist Enrico Fermi thought to use neutrons for bombardment in 1934. Since neutrons have no charge, they can hit an atom's nucleus without being repelled. He successfully bombarded several elements and created new, radioactive ones in the process. What Fermi had done, without recognizing it, was discover the process of nuclear fission. Two German scientists, Otto Hahn and Fritz Strassmann, were the first to officially acknowledge this process in 1938 when they successfully split uranium atoms into two or more parts.

Pellets of natural uranium oxide fuel used for nuclear power.
Fritz Goro/Time Life Pictures/Getty Images

­Uranium, the heaviest natural element on Earth, was involved in many of these early processes and became a subject of great interest in physics for a few reasons. Uranium is the heaviest natural element with 92 protons. Hydrogen, in contrast, is extremely light and only has one proton. The interesting part about uranium, however, isn't so much the number of protons -- it's the unusually high number of neutrons in its isotopes. One isotope of uranium, uranium-235, has 143 neutrons and undergoes induced fission very easily.

When a uranium atom splits, it's essentially losing mass. According to Einstein's famous equation E = mc², where E is energy, m is mass and c is the speed of light, matter can be converted into energy. The more matter you have, the more energy you're able to create. Uranium is heavy since it has so many protons and neutrons, so when it's split into two or more parts it has more matter to lose. This loss of mass, as tiny as an atom may be, is equivalent to the creation of a great deal of energy.

On top of this, extra neutrons break off from the pieces of a split uranium atom. Since a pound of uranium contains trillions of atoms, the chances of a stray neutron hitting another atom of uranium are very high. This caught the attention of the physics world -- a controlled chain reaction could create safe nuclear power, while an uncontrolled reaction had the potential to devastate.

On the next page, we'll talk about the U.S. decision to build a nuclear bomb.