The Chemistry of the Atomic Bomb (Level 4)

Level 4: Comparison between Atomic and Hydrogen Bombs

For atoms lighter than Nickel-62, the stability of the nucleus increases as atomic mass (nucleon number) increases. Hydrogen bombs derive their energy from nuclear fusion when two light atomic nuclei fuse to form a heavier, more stable nucleus. (Refer to Level 4 of Physics for more information on nuclear fusion) 

As you have learnt in level of chemistry, the nucleus of an atom contains the positively charged protons and electrically neutral neutrons. This thus makes nuclei positively charged. Extremely high temperatures are required for the positively charged nuclei to overcome their mutual repulsion and gain enough kinetic energy to fuse. Deuterium (hydrogen-2) and tritium (hydrogen-3) are isotopes of hydrogen, and carry weak positive charges due to them only having 1 positively charged proton, making it easier to overcome their mutual repulsion and fuse into helium. 

As seen from the diagram above, hydrogen bombs rely on a fission reaction to compress the fusion fuel lithium-6 deuteride of chemical formula ⁶Li²H. Neutrons produced in the fission reaction bombard lithium-6 deuteride to produce tritium. The deuterium and tritium then fuse to produce helium-4 which has a more stable nucleus.

	Atomic Bombs	Hydrogen Bombs
Mechanism	Nuclear Fission: splitting of heavy isotopes into smaller atoms	Nuclear fusion (fusion of nuclei of lighter atoms into larger, more stable ones) caused by fission
Cost	Expensive: rare isotopes used require enrichment to obtain a supercritical mass.	Expensive: needs both fission and fusion components.
Energy needed	Less	More as a high density & high temperature environment is required for nuclear fusion
Energy produced	Less: about 20 kilotons of TNT	More: about 10 megatons of TNT

The Chemistry of the Atomic Bomb (Level 2 and 3)

Level 2: Isotopes in Atomic Bombs

For atoms heavier than Nickel-62, the stability of the nucleus decreases as atomic mass (nucleon number) increases. Energy given off by atomic bombs arise from nuclear fission in which heavier isotopes such as uranium or plutonium split into more tightly bound stable elements.

The atomic bombs ‘Little Boy’ and ‘Fat Man’ were used by the USA in WWII. Little Boy used Uranium-235 and Fat Man used Plutonium-239. 

Level 3: Comparison between Little Boy and Fat Man

Little Boy

Little Boy used the gun-type assembly method. In the diagram on the left, the red rings represent 80% Uranium-235. When the explosive (orange section) is detonated, the uranium ‘bullet’ in front of the explosive is accelerated towards the other uranium section. Each section of uranium is sub-critical, that is of an insufficient mass for an explosion to occur. (For more information of critical mass, refer to Level 3 of Physics) When the uranium sections are 25cm from each other, free neutrons may hit the uranium, resulting in the formation of the highly unstable Uranium-236 which is deformed elastically. It then splits into the highly radioactive fission products of Barium-144, Krypton-89 and 3 neutrons which collide with more uranium to cause a chain reaction. This could cause a pre-donation.

To prevent pre-detonation, the speed of the ‘bullet’ would have to be very high which requires a long and heavy barrel. The gun-type method is unsuitable for Plutonium-239 because it contains about 20% Plutonium-240 which makes pre-donation inevitable. 

In contrast, Fat Man used the more complicated implosion assembly method. As seen from the figure of the left, the center contains a sub-critical mass of plutonium-239. When the explosion lenses are detonated, the density of the plutonium increases until it becomes super-critical (sufficient to start and sustain a chain reaction). This method is much safer as it prevents accidental pre-detonations. However, the complexity of the design leads to higher costs and allows for the creation of a smaller bomb. 

The table below summarises the comparisons

	Little Boy	Fat Man
Method	Gun-type	Implosion: higher complexity, higher costs, safer
Explosiveness	Less explosive: energy generated equivalent to 15 kilotons of TNT (a common explosive)	More explosive: energy generated equivalent to 20 kilotons of TNT
Fissile material	60kg of 80% Uranium-235	8kg of 80% Plutonium-239
Size	Larger and heavier	Smaller and lighter

The Chemistry of the Atomic Bomb (Level 1)

Level 1: Elements and Isotopes

In Year 1, you would have learnt that an element is a substance that cannot be broken down into simpler substances by chemical methods and that an atom is the smallest unit of an element.

Atoms consist of subatomic particles: protons, neutrons and electrons. Protons and neutrons are contained in a nucleus and electrons surround the nucleus. In normal atoms, there are the same number of protons and electrons. The term 'nucleons' refer to both protons and neutrons. The table below compares these 3 subatomic particles.

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These elements are arranged in a periodic table according to their proton number. Isotopes are atoms of the same element that have the same number of protons but different number of neutrons and are commonly expressed as [element]-[nucleon number]. For example, Carbon-12 means that carbon has 12 nucleons, that means a total of 12 protons and neutrons. As all isotopes of carbon have the same number of 6 protons, Carbon-12 has 6 neutrons. 

Q: Do ALL elements have naturally-occurring Isotopes?

Ans: A few elements do not have isotopes. Examples include Beryllium, Fluorine and Sodium.

Q: Do all elements have the SAME number of Isotopes?

Ans: NO. Hydrogen has 3 naturally-occurring isotopes: protium (hydrogen-1), deuterium (hydrogen-2), and tritium (hydrogen-3). In contrast, Tin has 10 naturally-occurring isotopes.