The formula of monomeric oxyanions, , is dictated by the oxidation state of the element A and its position in the periodic table. Elements of the first row are limited to a maximum coordination number of 4. However, none of the first row elements has a monomeric oxyanion with that coordination number. Instead, carbonate () and nitrate () have a trigonal planar structure with π bonding between the central atom and the oxygen atoms. This π bonding is favoured by the similarity in size of the central atom and oxygen.
The oxyanions of second-row elements in the group oxidation state are tetrahedral. Tetrahedral  units are found in olivine minerals, , but the anion does not have a separate existence as the oxygen atoms are surrounded tetrahedrally by cations in the solid state. Phosphate (), sulfate (), and perchlorate () ions can be found as such in various salts. Many oxyanions of elements in lower oxidation state obey the octet rule and this can be used to rationalize the formulae adopted. For example, chlorine(V) has two valence electrons so it can accommodate three electron pairs from bonds with oxide ions. The charge on the ion is +5 − 3 × 2 = −1, and so the formula is . The structure of the ion is predicted by VSEPR theory to be pyramidal, with three bonding electron pairs and one lone pair. In a similar way,
The oxyanion of chlorine(III) has the formula , and is bent with two lone pairs and two bonding pairs.
In the third and subsequent rows of the periodic table, 6-coordination is possible, but isolated octahedral oxyanions are not known because they would carry too high an electrical charge. Thus molybdenum(VI) does not form , but forms the tetrahedral molybdate anion, . MoO units are found in condensed molybdates. Fully protonated oxyanions with an octahedral structure are found in such species as  and . In addition, orthoperiodate can be only partially deprotonated, with 
: having pK=11.60.