Summary

This chapter traced the classification of elements from Dobereiner's triads and Newlands' octaves through Mendeleev's mass-based periodic table to Moseley's modern periodic law — properties are a periodic function of atomic number, not atomic mass — which underlies the long-form periodic table of 7 periods and 18 groups, divided into the $s$-, $p$-, $d$- and $f$-blocks by which subshell receives the atom's last electron.

Every periodic trend studied follows from the same two underlying ideas working in opposite directions: effective nuclear charge, which rises across a period and pulls the same outer shell inward, and the addition of a new principal shell, which happens once per period going down a group and dominates over the rising nuclear charge. From these two ideas: atomic radius shrinks across a period and grows down a group; ionic radius follows the same pattern, with the added rules that a cation is always smaller than its parent atom and an anion always larger, extending cleanly to isoelectronic-species comparisons; van der Waals' radius is always the largest of the three radii for a given atom, since it measures only a loose non-bonded contact. …