Question

Consider the phosphate ion, PO₄³⁻. Draw Lewis structures and assign formal charges for structures with:

A) all single bonds
B) three single bonds and one double bond (note this gives rise to four equivalent resonance structures).

Which of the following statements could chemists use to argue about which is the dominant structure? Select all the correct statements.
- Structure A may be dominant because all P and O atoms have an octet.
- Structure B may be dominant because some data shows average bond length to be less than that for a single bond.

Answer 1

The dominant structure of the phosphate ion, PO₄³⁻, among the provided options is difficult to ascertain definitively. However, Structure A might be favored due to its fulfillment of octets for all atoms, suggesting greater stability.

Step-by-step explanation:

Determining the dominant structure of the phosphate ion, PO₄³⁻, involves analyzing resonance structures and their stability. In Structure A with all single bonds, each oxygen atom has two lone pairs and one single bond to phosphorus, following the octet rule for all atoms. Formal charges for Structure A are as follows: phosphorus (+3), three oxygens (-1), and one oxygen (0). This structure seems plausible due to octet satisfaction on all atoms, indicating stability.

On the other hand, Structure B involves three single bonds and one double bond, yielding four equivalent resonance structures. In these structures, one oxygen has a double bond with phosphorus while the other three maintain single bonds. The formal charges for these structures fluctuate: phosphorus (+3), three oxygens (-1), and one oxygen (0). While these structures exhibit resonance, the presence of a double bond might suggest a slightly greater stability due to the sharing of electron density.

However, the argument for the dominance of Structure A rests on the octet rule being fulfilled for all atoms. This principle prioritizes stability, making Structure A a feasible choice as the dominant structure for the phosphate ion, although the concept of resonance implies that the actual ion might exist as a hybrid of the contributing structures. Experimental data often supports the notion that resonance structures contribute unequally to the actual structure, with some structures having a more significant role in the overall electronic distribution. Nonetheless, the fulfillment of the octet rule is a substantial factor in arguing for the dominance of Structure A.