It all started before World War II, when a German physicist discovered that an atom could be split, or fissioned, which would result in an enormous amount of energy being released. By 1940, five countries, United States, Britain, France, Soviet Union, and Japan, were all working to find a way to sustain a chain reaction for the possibility of using it for a weapon. Once World War II started, the United States was the only country that could financially fund a project.
Even with Albert Einstein writing a letter to President Roosevelt explaining the possibilities of nuclear energy and warning him of the military implications, the project didn’t officially become a reality until the bombing of Pearl Harbor by the Japanese on December 7, 1941.  Soon after the bombing the Manhattan Project was formed and given the overall responsibility to design and build the atomic bomb.
The main elements that are used to build the atomic bombs are: uranium-235 and plutonium-239.  Both uranium and plutonium are fissionable and can be used to produce the atomic explosion needed for the bomb. Two sites were constructed under the Manhattan Project for the development of the uranium and the plutonium. At Oak Ridge, Tennessee, a diffusion and electromagnetic process plant was built to separate the uranium-235 from the more abundant form, uranium-238. Hartford, Washington is the location of the nuclear reactors, which produced the plutonium. 
A third location was constructed by the Manhattan project at Los Alamos, in northern New Mexico.  This was the site were the project was to design and build the nuclear bomb. Many of the greatest scientists of the time were brought together at Los Alamos to develop the theory of sustainable fission and to construct the nuclear device.
From the beginning the scientists developed two designs of the nuclear bombs, one using the uranium-235, and the other using the plutonium. The uranium bomb was less complex and the scientist had enough confidence in the design that they felt they didn’t need to test it, but the plutonium bomb was more complex. For the plutonium bomb to work, they had to compress the plutonium into a critical mass, allowing it to sustain a chain reaction. In order to compress the plutonium, it needed to be “surrounded by a lense-shaped charge ” of TNT. The TNT explodes at the same time, directed inward, thus compressing the plutonium on all sides.
For an atomic explosion to occur, “a chain reaction picks up speed as atoms split, releasing neutrons plus great amounts of energy. The escaping neutrons strike and slip more atoms, thus releasing still more neutron and energy. In a nuclear explosion this all occurs in a millionth of a second with billions of atoms split .”
For the plutonium bomb to be tested, the project leaders for the testing site looked at a list of eight sites in California, Texas, New Mexico, and Colorado. Trinity, New Mexico was eventually chosen as the test site, in large part due to it already being in the governments control, and because its location is close to Los Alamos. The diagram below is the Trinity Site map.
At one minute past midnight on Friday, July 13, 1945, the material for the bomb left Los Alamos and was taken to the Trinity Site. Later that morning the assembly of the plutonium core began. That afternoon the assembled plutonium was taken to ground zero, the location where the bomb was to be detonated.
The bomb was assembled under a 100-foot tall steel tower. The photo below is of the bomb on top of the tower. The first attempt to insert the plutonium core into the device, it got stuck. But after “letting the temperature of the plutonium core and casing equalize, the core slid smoothly into place .”
The following morning the bomb was raised to the top of the steel tower and placed in a shelter. By the end of the day the detonators were attached and ready for the test.
The test was scheduled for 4 a.m. July 16, but due to rain and lightning, the test was postponed. The rain and wind would increase the danger by blowing the radioactive fallout, and interfering with the testing equipment. By 4:45, the rain had stopped and the wind had calmed, thus allowing the testing to proceed. At 5:10 the countdown began.
The physicists all hid in bunkers six miles from ground zero, and put on sunglasses and sunscreen. “At 5:29:45, the bomb was detonated, unleashing an explosion that could be seen 250 miles away. The heat that was release was four times greater than the interior of the sun, and created a pressure of 100 billion atmospheres. The flash lit the surrounding mountains brighter than the noonday sun, and the roar broke windows up to 120 miles away .” Scientists later estimated that the energy released from the bomb, equivalent to 18,000 tons of TNT, was “equal to all the bombs dropped on London by the Germans during the Blitz .” The photo to the right is of the explosion 0.025 seconds after detonation.
As a direct result of the atomic bomb that was detonated at Trinity Site, a second bomb was dropped on Hiroshima on August 6, 1945. This bomb, named “Little Boy”, exploded with the power of 16,000 tons of TNT. Three days after the bombing of Hiroshima, on August 9, 1945 a third bomb was dropped on Nagasaki. This bomb exploded with the power of 22,000 tons of TNT. It is estimated that “from these two bombs, the number of deaths range from 115,000 to 340,000 .” These estimates try to take into account the deaths do to cancer caused by radiation. If the “latter number of deaths is correct, these two bombs killed more Japanese than all the Americans killed during all of World War II .”
Following World War II, in 1949, the Soviet Union exploded a copy of the original bomb tested at Trinity Site. From that time on, the nuclear arms race began, and people throughout the world live in an era with the possibility of nuclear annihilation.
Today, the nuclear warheads that have been developed are far more powerful than the original. One of the “modern warheads, called the Trident, carries up to eight warheads, with each warhead containing 14 to 24 times the power of Nagasaki’s “Fat Man” .”
1 kiloton TNT is equal to 4.187*1012 J
18,000 tons TNT is equal to: (18 kiloton TNT) * (4.187*1012 J/kiloton TNT) = 7.54*1013 J
There is 200 MeV / Fission: (200MeV/Fission) * (1.6*10-13 J/MeV) = 3.2*10-11 J/Fission = 3.3*1010 Fission/J
(7.54*1013 J) * (3.3*1010 Fission/J) = 2.49*1024 Fission
It takes one atom for every fission, thus there are 2.49*1024 atoms
[(2.49*1024 atoms) * (239g/mole)] / (6.02*1023 atoms/mole) = 988.55 grams of plutonium-239
* This amount of plutonium is the minimum required, assuming that every atom is fissioned.
 Public Affairs Office White Sands Military Range, “Einstein’s Letter to Roosevelt”, http://www.wsmr.army.mil/paopage/Pages/Einstn.htm
 Public Affairs Office White Sands Military Range,” Trinity Site Pamphlet”, http://www.wsmr.army.mil/paopage/Pages/trinph.htm