The current layout also makes it harder to understand one of the key concepts underlying the structure of the periodic table, namely the order of the filling of electron shells. In Mendeleev’s table, ...

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.

They have the same number of electrons in their outer shell, so similar chemical properties. Sample exam questions - atomic structure and the periodic table - AQA Understanding how to approach ...

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 ...