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Why did Carl Wilhelm Scheele not recognize chlorine as a chemical element?

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A labeled diagram of the chlorine element from the periodic table shows its atomic number 17, symbol Cl, electron configuration, and atomic weight range. The box is shaded green to indicate chlorine is a halogen, with icons for orthorhombic crystal structure, gas state at room temperature, and strong acidity. A key explains the icons and colors used. — chlorine Properties of chlorine.

Carl Wilhelm Scheele did not recognize chlorine as a chemical element because he interpreted his 1774 discovery through the lens of the prevailing phlogiston theory.31

Under this dominant 18th-century framework, chemists believed that combustion involved the release of a hypothetical substance called phlogiston. When Scheele produced the greenish-yellow gas by treating manganese dioxide with hydrochloric acid, he concluded that the gas was a compound—specifically, "dephlogisticated muriatic acid"—rather than a distinct element.231

Scheele was a brilliant experimentalist who focused on laboratory discovery rather than theoretical development, and he remained committed to the phlogiston theory throughout his career. It was not until 1810 that the English chemist Humphry Davy demonstrated that the gas could not be decomposed and was, in fact, a chemical element, which he subsequently named chlorine.341