Editing Periodic table (section)

=== Further manifestations of periodicity ===
There are some other relationships throughout the periodic table between elements that are not in the same group, such as the [[diagonal relationship]]s between elements that are diagonally adjacent (e.g. lithium and magnesium).<ref name="PTSS2">Scerri, pp. 407–420</ref> Some similarities can also be found between the main groups and the transition metal groups, or between the early actinides and early transition metals, when the elements have the same number of valence electrons. Thus uranium somewhat resembles chromium and tungsten in group 6,<ref name="PTSS2" /> as all three have six valence electrons.<ref name="Jensen" /> Relationships between elements with the same number of valence electrons but different types of valence orbital have been called secondary or isodonor relationships: they usually have the same maximum oxidation states, but not the same minimum oxidation states. For example, chlorine and manganese both have +7 as their maximum oxidation state (e.g. [[dichlorine heptoxide|Cl<sub>2</sub>O<sub>7</sub>]] and [[manganese heptoxide|Mn<sub>2</sub>O<sub>7</sub>]]), but their respective minimum oxidation states are −1 (e.g. [[hydrogen chloride|HCl]]) and −3 (K<sub>2</sub>[Mn(CO)<sub>4</sub>]). Elements with the same number of valence vacancies but different numbers of valence electrons are related by a tertiary or isoacceptor relationship: they usually have similar minimum but not maximum oxidation states. For example, hydrogen and chlorine both have −1 as their minimum oxidation state (in [[hydride]]s and [[chloride]]s), but hydrogen's maximum oxidation state is +1 (e.g. [[water|H<sub>2</sub>O]]) while chlorine's is +7.<ref name=jensenlaw/>

Many other physical properties of the elements exhibit periodic variation in accordance with the periodic law, such as [[melting point]]s, [[boiling point]]s, [[heat of fusion|heats of fusion]], [[heat of vaporisation|heats of vaporization]], [[atomisation energy]], and so on. Similar periodic variations appear for the compounds of the elements, which can be observed by comparing hydrides, oxides, sulfides, halides, and so on.<ref name="Greenwood25" /> Chemical properties are more difficult to describe quantitatively, but likewise exhibit their own periodicities. Examples include the variation in the [[acid]]ic and [[base (chemistry)|basic]] properties of the elements and their compounds, the stabilities of compounds, and methods of isolating the elements.<ref name="Greenwood27" /> Periodicity is and has been used very widely to predict the properties of unknown new elements and new compounds, and is central to modern chemistry.<ref name="Greenwood29bis">Greenwood and Earnshaw, pp. 29–31</ref>