Final answer:
The noble gases discovered by Sir William Ramsay and Lord Rayleigh include Argon, Helium, Neon, Krypton, Xenon, and Radon. H. G. J. Moseley's work later contributed to the understanding that the periodic table should be organized by atomic number, leading to further advancements and additions to the table.
Step-by-step explanation:
The group of elements discovered by Sir William Ramsay and Lord Rayleigh (John William Strutt) is known as the noble gases. These elements, which were missing from Mendeleev's original periodic table, include Argon, Helium, Neon, Krypton, Xenon, and Radon. They were identified between 1894 and 1898. Ramsay initially struggled with the placement of noble gases in the periodic table due to their unique properties.
It was later understood, thanks to the work of H. G. J. Moseley, that the proper arrangement of elements in the periodic table was based on atomic number rather than atomic mass. Subsequently, other modifications to the table were proposed, including by Glenn T. Seaborg, who suggested adding the lanthanide and actinide series after the discovery of new radioactive elements such as Americium and Curium during the Manhattan Project.