Different shells can hold different maximum numbers of electrons. Electrons occupy shells starting with the innermost one. They begin to occupy the next shell when a shell becomes full.

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 ...

The ions formed have a full outer electron shell, so are very stable. See the study guide on groups in the periodic table for more information.

The periodic table organizes chemical elements based on atomic weight, electron configurations, and chemical properties, ...