Argon is an inert gas in group VIII A of the periodic table with atomic number 18, an atomic weight of 39.948, and a density of 1.40 Mg/m 3. In the solid state it forms an fcc crystal with a melting ...

After the second level has 8 electrons, the next electron goes into the third level. 3rd Period: Sodium to argon—Electrons go into the third energy level. After the third energy level has 8 electrons, ...

In 1869, Mendeleev's first periodic table did not include them, as closed-shell elements remained undiscovered until Lord Rayleigh and William Ramsey isolated argon in 1894. Remarkably ...