Electrons in atoms occupy energy levels, also called electron shells, outside the nucleus. Different shells can hold different maximum numbers of electrons. The electrons in an atom occupy the ...

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 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, ...