Periodic Trends in Elemental Properties

The periodic table's structure was originally built purely from empirically-observed periodicity — elemental properties recur in a similar way within a group and vary gradually across a period, exactly the pattern Mendeleev noticed. The quantum mechanical model of the atom now lets that observed periodicity be explained mechanistically, by correlating it with electronic configuration and with the nuclear charge felt by the valence electrons. Two fundamental, simultaneously-acting forces drive these trends inside any multi-electron atom: the nucleus's attraction pulling the extranuclear electrons inward, and the mutual repulsion between electrons belonging to the same atom pushing them apart. Together, these two forces give rise to two closely-related phenomena, covered next: effect …