The so-called pyroanions, \(\mathrm{X}_{2} \mathrm{O}_{7}^{n-},\) form a series of structurally similar polyatomic anions for the elements Si, P, and S. (a) Draw the Lewis structures of these anions, and predict the geometry of the anions. What is the maximum number of atoms that can lie in a plane? (b) Each pyroanion in part (a) corresponds to a pyroacid, \(\mathrm{X}_{2} \mathrm{O}_{7} \mathrm{H}_{n} .\) Compare each pyroacid to the acid containing only one atom of the element in its maximum oxidation state. From this comparison, suggest a strategy for the preparation of these pyroacids. (c) What is the chlorine analogue of the pyroanions? For which acid is this species the anhydride?

Start by determining the Lewis structures for the pyroanions of Si, P, and S. All three anions will follow the general structure of \(\mathrm{X}_{2} \mathrm{O}_{7}^{n-}\), where X represents the element in question (Si, P, or S). Using Lewis dot structures, place the more electronegative atoms (Oxygen in this case) around the central atom (X) and start filling up the octets. Your resulting structures should have a single atom of X at the center, surrounded by bonded oxygen atoms. For part two of this step, you need to employ the VSEPR model to predict the geometry based on the number of pairs of electrons around the central atom. In this case, 4 regions of electron density around each X, two bonding and two nonbonding, suggest a tetrahedral molecular geometry. The number of atoms that can lie in a plane here would be three.

When we assign hydrogen atoms to each of the pyroanions, turning them into pyroacids, we see that the compounds all have a higher oxidation state than their corresponding monoatomic acids. This is due to the extra oxygen atoms, which pull more strongly on the hydrogen's electron, resulting in a higher oxidation state for the central atom. From this comparison, we can infer that a strategy for preparing these pyroacids would be to increase the oxidation state of the central atom, perhaps through some sort of oxidation reaction.

The chlorine analogue of the pyroanions would have a similar structure, but with Cl as the central atom, resulting in \(\mathrm{Cl}_{2} \mathrm{O}_{7} .\) The corresponding acid would be perchloric acid, \( \mathrm{HClO}_{4},\) as it is the most common acid with chlorine in its highest possible oxidation state, +7. In this case, perchloric acid is the species for which \( \mathrm{Cl}_{2} \mathrm{O}_{7}\) is the anhydride.