In many ways this blog follows on from my last one 'Training students to think critically'. Recently Natalie Brink, who teaches at Shekou International School in China, asked a very sensible and valid question to a Facebook group for IB chemistry teachers. Natalie (who has given me her permission to quote her) asked,

"The IB periodic table of ions shows phosphorus as a 5+ charge, when I have always taught this as a 3– charge. What do you all do? Or am I reading this periodic table incorrectly?"

What surprised me was that none of the eleven responses she received from IB teachers actually addressed the question correctly. Essentially the responses said that in binary ionic compounds with metals phosphorus forms the 3– ion but noted that in IB chemistry we almost never come across phosphide ions or compounds such as Na₃P or AlP.  Almost all the common compounds of phosphorus covered in the IB, such as phosphates, phosphorus(V) fluoride and phosphorus(V) oxide, contain phosphorus with an oxidation number (sic) of +5 hence the reason for including the ionic radius of P⁵⁺ rather than P^3– in the IB periodic table.

What seems to be happening is that many teachers are completely confusing ionic charge with oxidation state. The IB table quite clearly states that the value of 38 x 10^–12 m given is for the radius of the P⁵⁺ ion. Looking critically at compounds such as PO₄^3-, PF₅ and P₄O₁₀ it should immediately be obvious that none of them contain P⁵⁺ ions. The bonds within the species between phosphorus and the other atoms such as oxygen or fluorine are all covalent.

Even metals do not form 5+ ions in their compounds. As the charge on positive ions increases their size gets smaller so that the charge density increases.  Small highly charged ions attract the electron cloud of anions more strongly (effectively 'pulling" the electrons towards them) leading to covalent bonding, especially with larger anions. So that aluminium fluoride (melting point 1290 ^oC) is ionic but aluminium chloride (melting point 192 ^oC) is covalent. This explains why aluminium chloride is acidic in aqueous solution as the surrounding water ligands are strongly attracted to the densely charged Al³⁺ ion releasing H⁺ ions in the process.

I think Natalie was right to ask her question and the inclusion of the ionic radius for the P⁵⁺ ion in the IB periodic table is both confusing and unnecessary.  It is worth asking why this confusion between ionic charge and oxidation states (or numbers) has come about. There may be at least three contributing factors.

• Teachers and students may forget that the whole concept of oxidation states is based on the false concept that compounds are assumed to be ionic (see How useful are oxidation states?). 

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• The IB syllabus actually states under Topic 13.1"Explanation of the ability of transition metals to form variable oxidation states from successive ionization energies". Vanadium for example forms common oxidation states of +2, +3, +4 and +5. It is tempting to explain the oxidation state of +5 in V₂O₅ by looking at the energy to remove the first five electrons from vanadium and then seeing a 'jump' to remove the sixth one. However this is completely misleading as vanadium does not form the 5+ ion in V₂O₅ as vanadium(V) oxide is covalent.  Similarly with chromium and manganese compounds with higher oxidation states. In the manganate(VII) ion, MnO₄^– there are no Mn⁷⁺ ions present. In fact manganese only really forms metallic ions (Mn²⁺ and Mn³⁺) in Mn(II) and Mn(III) compounds and possibly the Mn⁴⁺ ion in manganese(IV) oxide, MnO₂ (melting point 535 °C). Compounds containing metals with an oxidation state higher than +4 always contains the metal atoms covalently bonded to other atoms. If the complex does contain a charge, such as the dichromate(VI) ion, Cr₂O₇^2-, or the manganate(VII) ion, MnO₄^– then the charge is delocalised over the whole covalently bonded complex.

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• The hydrolysis of salts of complex ions, such as [Al(H₂O)₆]³⁺ and [Fe(H₂O)₆]³⁺, to explain why they are acidic in aqueous solution, is no longer in the current 2014 Guide whereas it was on the syllabus in previous guides. Perhaps the whole concept of charge density and its ramifications has become less understood. Looking at a table of charge densities, the charge density of Al³⁺ is 15 times greater than that of Na⁺ whereas the charge density of P⁵⁺ is 56 times greater than Na⁺ (and almost four times greater than Al³⁺) so the P⁵⁺ ion will not be present in P(V) compounds.  

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