Atom Bomb

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The sudden release of energy produced by splitting the nuclei of the fissile elements constituting the bomb’s core is the reason behind the immense destructive power of atomic weapons. Two types of atomic bombs were developed by USA during the second world war. The first one was named ‘Little Boy’, it was a gun-type weapon with a uranium core. It was dropped on Hiroshima. The second nuclear weapon was dropped on the city of Nagasaki, it was called the ‘Fat Man’. The nuclear bomb was an implosion-type device with a plutonium core. 



Atomic scientists selected isotopes uranium-235 and plutonium-239 as they readily undergo fission. When a neutron strikes the nucleus of either isotope, it splits the nucleus into fragments, releasing a tremendous amount of energy – this is called fission. It becomes a self-sustaining process. The neutrons created by splitting of atoms strike surrounding nuclei and produce more fission. It is called chain reaction and results in an atomic explosion.

A uranium-235 atom fissions into two new atoms by absorbing a single neutron. As a result three new neutrons and some binding energy is released. Out of this two neutrons are lost or absorbed by uranium-238 atoms and won’t continue the reaction. The remaining neutron collides with an atom of uranium-235, which fissions releasing two neutrons and some binding energy. A nuclear chain reaction happens, the neutrons further collide with uranium-235 atoms, each of which fission to release between one and three neutrons. The process goes on.


A critical mass of fissionable material is required to detonate an atomic weapon. For producing a chain reaction, you need enough U-235 or PU-239, so as to ensure that neutrons released by fission strike another nucleus. More the fissionable material, higher the chances for chain reaction to occur. The amount of material at which a neutron produced by a fission process will, on average, create another fission event is defined as the critical mass. 

Difference between the bombs

While Little Boy detonated due to a fission chain reaction involving U-235 isotope of Uranium, the Fat Man used plutonium’s PU-239 form. Nuclear weapons Little Boy and Fat Man used different elements and totally distinct methods of construction. 

Little Boy

little boy

Little Boy was powered by uranium isotope U-235. Uranium naturally exists as uranium-238. Only 0.7 percent is found as U-235 isotope. The isotope mostly captures the neutron to become U-239 when a neutron bombardsU-238, failing to fission, and hence preventing a chain reaction which would detonate a bomb. So the primary challenge was to figure out the most efficient manner to separate and purify uranium-235 from the overly abundant uranium-238. Scientists were simultaneously investigating four separate methods of separating and purifying the uranium-235. Gaseous diffusion, centrifuge, electromagnetic separation, and liquid thermal diffusion were the four methods.

A gun-type design was used for constructing Little Boy. The critical mass set off a fission chain reaction for eventually detonating the bomb. It was necessary for U-235 to combine with one another quickly, for avoiding spontaneous fission of the atoms, which would have caused the bomb to fizzle (fail to explode).

Fat Man

fat boy

The ‘Fat Man’ was powered by plutonium. It did not use the gun-type design which allowed Little Boy to explode effectively. The design for the plutonium bomb was constructed by physicist Seth Neddermeyer. Conventional explosives around a central plutonium mass were used to quickly squeeze and consolidate the plutonium. This will increase the pressure and density of the substance. The increased density helped plutonium to reach its critical mass, facilitating the chain reaction. The explosives were ignited releasing a shock wave which compressed the inner plutonium leading to nuclear explosion.

Atomic Glossary

Atom: An atom is the building blocks of matter. It has a small, dense nucleus and is surrounded by a cloud of electrons. 

Nucleus: The center of the atom is called the nucleus. It consists of positively charged protons and neutrons (no charge). While the number of protons determines which chemical element the atom is, the number of neutrons determines which isotope of that element the atom is.

Isotope: The isotopes of an element will have the same number of protons in their nuclei but will have different numbers of neutrons. 

Fission: It is the process by which an atom’s nucleus splits into smaller particles, hence resulting in the release of neutrons and a lot of energy.

E=mc2 : It explains how a tiny amount of matter contains a tremendous amount of energy. The equation was made popular by Albert Einstein

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