All elements, except helium, neon and argon, form oxygen compounds, there are only eight forms of oxygen compounds. In the periodic table, they are often represented by general formulas located under each group in ascending order of the degree of oxidation of the elements: R2O, RO, R2O3, RO2, R2O5, RO3, R2O7, RO4, where the symbol R denotes an element of this group. The formulas of higher oxides apply to all elements of the group, except in exceptional cases when the elements do not exhibit an oxidation state equal to the group number (for example, fluorine).
Oxides of the composition R2O exhibit strong basic properties, and their basicity increases with increasing ordinal number, oxides of the composition RO (with the exception of BeO) exhibit basic properties. Oxides of the composition RO2, R2O5, RO3, R2O7 exhibit acidic properties, and their acidity increases with increasing ordinal number.
The elements of the main subgroups, starting from group IV, form gaseous hydrogen compounds. There are four forms of such compounds. They are placed under the elements of the main subgroups and are represented by general formulas in the sequence RH4, RH3, RH2, RH.
RH4 compounds have a neutral character; RH3 — weakly basic; RH2 — slightly acidic; RH — strongly acidic.
Recall that a period is a horizontal series of elements arranged in ascending order of ordinal (atomic) numbers.