Editing Nonmetal (section)

====Secondary periodicity====
[[File:EN values of chalcogens.png|thumb|upright=0.8|Electronegativity values of the group 16 [[chalcogen]] elements showing a W-shaped alternation or secondary periodicity going down the group|alt=A graph with a vertical electronegativity axis and a horizontal atomic number axis. The five elements plotted are {{abbr|O|oxygen}}, {{abbr|S|sulfur}}, {{abbr|Se|selenium}}, {{abbr|Te|tellurium}} and {{abbr|Po|polonium}}. The electronegativity of {{abbr|Se|selenium}} looks too high, and causes a bump in what otherwise be a smooth curve.]]
An alternation in certain periodic trends, sometimes referred to as [[Periodic table#Atomic radius|secondary periodicity]], becomes evident when descending groups 13 to 15, and to a lesser extent, groups 16 and 17.<ref>[[#Kneen|Kneen, Rogers & Simpson 1972, pp. 226, 360]]; [[#Siekierski|Siekierski & Burgess 2002, pp. 52, 101, 111, 124, 194]]</ref>{{efn|The net result is an even-odd difference between periods (except in the [[s-block]]): elements in even periods have smaller atomic radii and prefer to lose fewer electrons, while elements in odd periods (except the first) differ in the opposite direction. Many properties in the p-block then show a zigzag rather than a smooth trend along the group. For example, phosphorus and antimony in odd periods of group 15 readily reach the +5 oxidation state, whereas nitrogen, arsenic, and bismuth in even periods prefer to stay at +3.<ref>[[#Scerri2020|Scerri 2020, pp. 407–420]]</ref>}} Immediately after the first row of [[Block (periodic table)#d-block|d-block]] metals, from scandium to zinc, the 3d electrons in the [[Block (periodic table)#p-block|p-block]] elements—specifically, gallium (a metal), germanium, arsenic, selenium, and bromine—prove less effective at [[shielding effect|shielding]] the increasing positive nuclear charge.

The Soviet chemist {{Interlanguage link|Shchukarev|2=ru|3=Щукарев, Сергей Александрович|preserve=1}} gives two more tangible examples:<ref>[[#Shchukarev|Shchukarev 1977, p. 229]]</ref>
:<span style="font-size:95%">"The toxicity of some arsenic compounds, and the absence of this property in analogous compounds of phosphorus [P] and antimony [Sb]; and the ability of [[selenic acid]] [{{chem2|H2SeO4}}] to bring metallic gold [Au] into solution, and the absence of this property in sulfuric [[sulfuric acid|[{{chem2|H2SO4}}]]] and [[telluric acid|[{{chem2|H2TeO4}}]]] acids."</span>