Consider the \(\mathrm{PO}_{4}^{3}\) phosphate anion. (a) Draw a dot diagram that obeys the octet rule for all of the atoms. (b) Suppose the actual bond order for the \(\mathrm{P}-\mathrm{O}\) bonds was \(1.25 .\) Draw a dot diagram that explains this, using the concept of expanded octet and resonance. Also state whether expanding the octet as you have shown would be allowed, and why.

To draw a dot diagram for the phosphate anion, \(\mathrm{PO}_{4}^{3-}\), we need to first determine the total number of valence electrons for this species. Phosphorus (P) has 5 valence electrons, oxygen (O) has 6 valence electrons, and there are 4 oxygen atoms in the molecule. Additionally, the 3 negative charges on the anion are accounted as 3 extra valence electrons. Total valence electrons: \(5 + (4 \times 6) + 3 = 32\) Now, placing phosphorus in the center and arranging the four oxygen atoms around it, we can distribute the valence electrons. 1. Form single bonds between phosphorus and each oxygen atom. This uses 2 valence electrons for each bond, totaling 8 electrons. 2. Complete the octets of all oxygen atoms by adding 6 more electron pairs to each oxygen atom. This uses 24 more electrons. The dot diagram for \(\mathrm{PO}_{4}^{3-}\) that obeys the octet rule is: O : .. | P O .. = .. .. | : : : -- O -- : .. .. | . Each atom has a complete octet of valence electrons. The \(\mathrm{P}-\mathrm{O}\) bond order in this structure is 1.

To draw a dot diagram that explains a bond order of 1.25 for the \(\mathrm{P}-\mathrm{O}\) bonds, we need to consider resonance structures and the concept of expanded octet. 1. Write a structure with the following modifications: * One of the oxygen atoms has a double bond with phosphorus. * No lone pair on that oxygen atom. The new structure will look like this: O : .. | P O .. (= '| || ' : : ' '-- O --':' .. '.'|' : 2. Indicate resonance structures by drawing arrows and moving the double bond around each of the other oxygen atoms. The actual structure will have an average of each resonance structure. The number of resonance structures is 4, and their sum represents the actual structure of the molecule. There are 5 total bonds in each resonance structure, but 4 resonance structures represent the 4 \(\mathrm{P}-\mathrm{O}\) bonds in the true structure, so the bond order can be calculated as follows: Bond order: \(\frac{5}{4} = 1.25\) Expanding the octet as described above would be allowed in this case, as the phosphorus atom can accommodate more than 8 valence electrons due to its vacant d orbitals (phosphorus is in the 3rd period of the periodic table and has access to 3d orbitals).