1 (10 points, 2 points each part) For the cation, As³⁺

a) Give the formula for the oxide, explaining the method by which you obtained it. Formula is As₂O₃, obtained using the "criss-cross" approach.

b) Indicate whether the oxide is basic, amphoteric, or acidic, and explain your choice. Oxides of 3+ cations, which are weakly or moderately acidic, are usually amphoteric.

c) Calculate coordination numbers for the cation and anion in the oxide. We base this on the radius ratio:

r (As³⁺) = 72 pm; r (O^2-) = 126 pm. Radius ratio = 72/126 = 0.57. This suggests a TCN of 6 for As³⁺. Because As³⁺ has one lone pair, its CN will be 5. The CN for O^2- would then be 5x2/3 = 10/3. Because this comes out larger than 2 in a purely covalent compound, which is unreasonable, we have to drop down to the next lower TCN for As³⁺, which is 4. This leads to CN 3 for As³⁺, and CN 2 for O^2-. This is OK.

d) Indicate the extent of formula unit linking in the oxide. The CN(O^2-) of 2 indicates that all oxygens are bridging between two As³⁺ ions; there are no terminal oxygens. The structure is expected to be polymeric.

e) Propose a reasonable structure for the oxide. Your final structure should be consistent with all results above, and with the usual rules governing Lewis structures. State the stereochemistry at every atom that is bonded to 2 or more other atoms. A TCN of 4 (i.e., 4 electron groups around arsenic) is consistent with tetrahedral electron group distribution, with one of the 4 positions occupied by a lone pair. All 3 oxygen atoms will bridge to other arsenics. The structure is shown on page 96 of Wulfsberg.

The stereochemistry at arsenic is trigonal pyramidal; the stereochemistry at oxygen is bent.

2 (10 points) For each oxide below, state whether the oxide is basic, amphoteric, or acidic, and predict its solubility in water (i.e., soluble or insoluble). Please briefly justify your answers.