Why does nitric acid have a different molecular structure from phosphoric and arsenic acids? I remember that this really bugged me as an undergrad, and upon revisiting some inorganic chemistry notes I realized I still don't have a satisfactory answer for it. The structure of nitric acid, $\ce{HNO3}$, is quite different from the structure of phosphoric acid, $\ce{H3PO4}$ and arsenic acid, $\ce{H3AsO4}$:



Something tells me there should be an easy, intuitive argument explaining this, perhaps something about p-orbitals being more diffuse in phosphorus and arsenic? I find it kind of difficult to analyze this as a sort of "trend", since other groups don't really seem to have well defined oxyacids for the first two or three elements. 

There is no oxyacid for water, but sulphuric and selenic acids are similar in structure; there is no oxyacid for fluorine either, but the oxyacids of chlorine and bromine are also analogous. In the carbon group something kind of strange also happens, with silicic acid (I believe) not really being a well-defined compound but a plethora of different things with different relative stabilities.
Phosphorus and arsenic are much bigger atoms with lower electronegativity so they can fit more atoms around them and oxygen can bond more easily.

Under extreme pressure it is possible to make the orthonitrate ion from nitrate and oxide. It rapidly decomposes in contact with water or carbon dioxide.