 Mendeleev’s periodic system was briefly called into question with the discovery of the inert gas argon in 1894, which had to be placed outside the existing system after chlorine. But William Ramsay (1852–1916) suspected there might be a whole group of chemically inert substances separating the electronegative halogen group 17 (to which chlorine belongs) and the electropositive alkali metals, and by 1898 he had discovered the other noble gases, which became group 18 on the modern Table.