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Britannica Sources used in this answer

Why does sodium not occur as a free element in Earth’s crust?

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A labeled diagram shows the periodic table entry for sodium, highlighting its symbol Na, atomic number 11, atomic weight, electron configuration, physical state as a solid, body-centered cubic crystal structure, and strongly basic oxide properties.
sodium

Sodium does not occur as a free element in Earth’s crust because it is extremely reactive. As an alkali metal, sodium readily reacts with atmospheric oxygen and water vapor. Because of this high reactivity, it is always found in nature combined with other elements in the form of compounds, such as silicate materials and various halide deposits like sodium chloride (common salt).123