Argon was not discovered until 1894 by Sir William Ramsay and Lord Rayleigh, even though it was suspected to be present in air by Henry Cavendish in 1785.
Ammonia has been used as drinking water purifier, rocket fuel, and industrial food processing additive, and is the number one ingredient in fertilizers.
Arsenic compounds have been known for thousands of years (since at least the days of Ancient Rome and Greece). They were used by poisoners as well as physicians.