An atom bomb works by initiating a nuclear chain reaction, which releases a huge amount of energy relative to conventional explosives. Per unit volume, an atom bomb may be millions or billions of times more powerful than TNT. The first atomic explosion occurred on 16 July 1945 at the Alamogordo Test Range in New Mexico, during a test called Trinity. It was developed during the top secret Manhattan Project, which was directed by General Leslie R. Groves of the US Army.

Nuclear reactions occur when neutrons are fired at closely packed atoms with heavy nuclei (uranium or plutonium isotopes). These heavy nuclei break apart into lighter nuclei when hit by a neutron, in turn generating more neutrons which bombard other nuclei, creating a chain reaction. This process is known as fission. (Another process known as fusion releases energy by fusing together nuclei rather than breaking them apart.) By breaking down the nuclei themselves rather than releasing energy through a conventional chemical reaction, atom bombs can release more than 80 terajoules of energy per kilogram (TJ/kg).

In the earliest bombs, the chain reaction was initiated simply by firing two half-spheres of high purity uranium isotope at one another in a small chamber. In updated designs, a uranium or plutonium bomb core is surrounded by high-explosive lenses designed to compress the core upon detonation. The compressed core goes critical, initiating a chain reaction that persists until many of the heavy nuclei have been broken apart.

The atom bomb and its cousin the hydrogen bomb have probably been the most powerful weapons in the world since their creation many decades ago. Large bombs can destroy entire cities. Thousands of atom bombs have been detonated, though only two have been used in warfare - both used by the US against Japan during World War II. There are seven countries that openly declare possessing nuclear weapons; the United States, Russia, the United Kingdom, France, the People's Republic of China, India and Pakistan. The world currently possesses enough nuclear capability to make the human race go extinct many times over.