Nuclear Reactions and Decay

Nuclear reactions such as fusion, fission, and radioactive decay involve either combining or splitting
the nuclei of atoms. Since the binding energy per nucleon is greatest for intermediate-sized atoms,
when small atoms combine or large atoms split a great amount of energy is released.

FUSION

Fusion occurs when small nuclei combine into a larger nucleus. As an example, many stars including
the sun power themselves by fusing four hydrogen nuclei to make one helium nucleus. By this
method, the sun produces 4 × 10^26 J every second. Here on Earth, researchers are trying to find ways
to use fusion as an alternative energy source.

FISSION

Fission is a process in which a large nucleus splits into smaller nuclei. Spontaneous fission rarely
occurs. However, by the absorption of a low-energy neutron, fission can be induced in certain nuclei.
Of special interest are those fission reactions that release more neutrons, since these other neutrons
will cause other atoms to undergo fission. This in turn releases more neutrons, creating a chain
reaction. Such induced fission reactions power commercial nuclear electric-generating plants.

Example: A fission reaction occurs when uranium-235 (U-235) absorbs a low-energy neutron,
briefly forming an excited state of U-236 which then splits into xenon-140, strontium-
94, and x more neutrons. In isotopic notation form the reactions are:

How many neutrons are produced in the last reaction?
Solution: The question is asking “What is x in the equation above?” By treating each arrow as an
equal sign, the problem is simply asking to balance the last “equation.” The mass
numbers (A) on either side of each arrow must be equal. Since 235 + 1 = 236, the first