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Why is natural potassium radioactive?

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A periodic table element box for potassium shows atomic number 19, symbol K, atomic weight 39.0983, electron configuration [Ar]4s1, and name potassium. Icons indicate it is an alkali metal, solid at room temperature, body-centered cubic crystal structure, and has strongly basic higher-valence oxides. A key explains the symbols for alkali metals, solid state, body-centered cubic structure, and strong basicity. — potassium Properties of potassium.

Natural potassium is radioactive because it contains a small fraction—approximately 0.01 percent—of the radioactive isotope potassium-40 ( $^{40} K$ ).12

While most potassium atoms are stable potassium-39, the potassium-40 isotope possesses an extra neutron in its nucleus. This additional neutron renders the nucleus unstable, causing it to decay over time into other elements, such as calcium-40 or argon-40, while emitting radiation in the process. This natural radioactivity is a standard, low-level phenomenon found in many foods and even within the human body itself.2