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Twin: Beta Decay

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Beta decay emits an electron as a neutron converts to a proton, leaving mass number unchanged while raising atomic number by one.

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Iodine-131 undergoes beta decay to form xenon-131, mass number unchanged but atomic number up by one.

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The emitted electron originates from the nucleus, converting a neutron into a proton, not from the atom's electron cloud — easy to conflate.