Which element was discovered by Joseph Priestley?

Joseph Priestley discovered oxygen, independently isolating the gas in 1774. In an experiment near Calne in Wiltshire, England, he focused sunlight on red mercuric oxide and collected a gas that supported combustion and respiration unusually well.

Priestley called the gas dephlogisticated air because he interpreted his results using the then-popular phlogiston theory. He observed that a candle burned more brightly in it and that a mouse survived longer. He published his findings in 1775, giving his work priority in print.

The discovery has a complicated history. Swedish chemist Carl Wilhelm Scheele had produced oxygen earlier, around 1771–1772, but published later. Antoine Lavoisier learned of Priestley’s experiment and correctly recognized oxygen as a distinct chemical element, explaining its role in combustion and respiration and helping overturn phlogiston theory.

Thus, Priestley is a standard and fair answer to this quiz question, even though he was not the only scientist involved in oxygen’s discovery. Priestley identified the gas but did not understand its modern elemental nature.