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The incredible process of altering an atom's nucleus, fundamentally changing its identity, is known as a nuclear reaction. Unlike chemical reactions that involve the exchange or sharing of electrons, nuclear reactions involve the protons and neutrons deep within an atom's core. The outcomes of these interactions are profound, leading to the creation of different elements or the release of vast amounts of energy.

Nuclear reactions can happen naturally, like when cosmic rays strike the atmosphere, or be artificially induced in a lab or nuclear reactor. The artificial methods use particles like neutrons or protons to bombard atoms, resulting in the alteration of their nuclei and thus transmuting the elements into new forms. We harness these reactions for various purposes, from power generation in nuclear reactors to the creation of medical isotopes.

Alpha decay is a type of radioactive decay and an illustrative example of natural transmutation. Think of it like an unstable atom shedding weight to become more stable. In this process, the atom emits an alpha particle — which is essentially a helium nucleus made up of two protons and two neutrons.

This expulsion transforms the atom into a new element with a mass number that is four less and an atomic number two fewer than before. For example, take Uranium-238. Through alpha decay, it loses an alpha particle to become Thorium-234, shifting from one point to another on the periodic table. Alpha decay is a process that takes us on a journey through the natural transmutation of elements, showcasing how even atoms are not eternal but ever-transforming.

Nuclear fission is a powerful process, integral to artificial transmutation, where an atom's nucleus splits into lighter nuclei, releasing energy and additional neutrons in the process. It's like splitting a droplet of water into smaller droplets, except the process releases an immense amount of energy.

In a nuclear reactor, this is carefully controlled to generate electricity. For example, uranium-235 absorbs a neutron, becoming uranium-236 — an excited state that cannot hold itself together. It then breaks into smaller elements such as krypton and barium. This action gatekeepers a chain reaction: the additional neutrons produced can go on to split more nuclei. The result is a steady stream of energy output that can be harnessed, but the reaction requires meticulous management to prevent meltdowns, as in the case of nuclear accidents.