Answer: Phosphorus 

Certain elements have been known (and in use by humans) for thousands of years. It’s impossible to pin down who discovered elements like lead, copper, gold, and silver as they are found in relatively pure form without much refinement.

The first unique element to be discovered and isolated in the modern age, however, can be identified. In the 17th century a bankrupt German merchant, Hennig Brand, trying to create the fabled Philosopher’s Stone (that could turn base metals into gold) stumbled across the element Phosphorus (courtesy of his experiments in distilling human urine). Despite the interesting nature of the glowing white substance he had distilled, it got him no closer to his goal of turning lead and such into gold. Brand kept his discovery secret until 1680 when scientist Robert Boyle revealed that he had discovered it (or, rather, rediscovered it as the case was).

Phosphorus became, upon discovery, the 14th known element.