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Phosphorus is well-known for forming a variety of oxoanions. These anions consist of phosphorus atoms covalently bonded to oxygen atoms, creating unique structural motifs. Oxoanions of phosphorus vary greatly in size and complexity, from simple phosphate \[ \text{(PO}_4^{3-}) \] to more complex entities like \[ [\text{P}_4\text{O}_{13}]^{n-} \]. These structures arise from phosphorus's ability to expand its coordination number beyond four, unlike many other elements. The richness in forms of phosphorus oxoanions is due to strong P-O bonds and the multiple ways in which phosphorus and oxygen can combine to form stable structures. Each variation in oxoanion not only affects its molecular geometry but also its chemical properties and reactivity.

Understanding oxoanion structure is essential to predicting their properties. Oxoanions like \([\text{P}_4\text{O}_{13}]^{n-}\) exhibit fascinating geometries. Typically, these anions have phosphorus atoms forming tetrahedral structures surrounded by oxygen atoms. Multiple \(\text{PO}_4^{3-} \) groups can be linked through P-O-P oxygen bridges. These bridges create complex architectures with features like rings or chains. Each phosphorus atom usually bonds to four oxygen atoms, highlighting its tetrahedral arrangement. In some structures, oxygens may double bond to phosphorus (P=O), while others link as OH groups, accentuating their capability to form varied and complex networks.

In chemistry, the term 'ionizable hydrogen atoms' refers to hydrogen atoms attached to an oxygen in compounds forming OH groups that can be released as protons. In phosphorus oxoanions, determining how many ionizable hydrogen atoms exist means assessing potential protonation sites. Fully protonated phosphorus oxoacids, such as \([\text{H}_7\text{P}_4\text{O}_{13}]\), indicate each terminal oxygen can acquire a hydrogen ion. Therefore, the number of ionizable hydrogens is equivalent to the number of negative charges on the anion before protonation. For \([\text{P}_4\text{O}_{13}]^{7-}\), this suggests seven possible ionizable protons, whereas for \([\text{P}_4\text{O}_{12}]^{4-}\), only four hydrogen atoms can ionize upon full protonation.

Predicting the charge of a phosphorus oxoanion involves analyzing the connectivity of phosphorus and oxygen atoms. Each phosphate unit in \(\text{PO}_4^{3-} \) carries a \(-3\) charge inherently. In polyoxoanions like \([\text{P}_4\text{O}_{13}]^{n-}\), the presence of bridging oxygens alters typical charges. Normally, free or terminal oxygens (-2 charge potential) are key in predicting the anion's net charge. Mixed linkages like P-O-P bridges impact the final anion charge due to tone down net negativity. Thus, calculating how many oxygens are shared and the unique arrangement leads to identifying the overall charge. For \([\text{P}_4\text{O}_{13}]^{7-}\), net charges consolidate around seven due to these complexities, while \([\text{P}_4\text{O}_{12}]^{4-}\) demonstrates four due to the oxygen's closer configuration and more intense bridging.