Mendeleev dared to swap two elements out of atomic-weight order so their properties would match the column, and he was right: what fixes an element’s place is electron configuration, not weight. The properties of all 118 elements repeat as a sawtooth in which every tooth is one period: first ionization energy I₁ climbs across a period and peaks on the noble-gas column, atomic radius runs the other way and is largest at the alkali metal opening each period, while electronegativity tops out at fluorine. A few superheavy elements at the end of the table still have blank entries, because only a handful of their atoms have ever existed inside an accelerator before decaying almost at once.
Mendeleev dared to swap two elements out of atomic-weight order so their properties would match the column, and he was right: what fixes an element’s place is electron configuration, not weight. The properties of all 118 elements repeat as a sawtooth in which every tooth is one period: first ionization energy I₁ climbs across a period and peaks on the noble-gas column, atomic radius runs the other way and is largest at the alkali metal opening each period, while electronegativity tops out at fluorine. A few superheavy elements at the end of the table still have blank entries, because only a handful of their atoms have ever existed inside an accelerator before decaying almost at once.
Related topics in Matter & Atom