Some chemists believe that satisfaction of the octet rule should be the top criterion for choosing the dominant Lewis structure of a molecule or ion. Other chemists believe that achieving the best formal charges should be the top criterion. Consider the dihydrogen phosphate ion, $\mathrm{H}_{2} \mathrm{PO}_{4}^{-},\( in which the \)\mathrm{H}$ atoms are bonded to \(\mathrm{O}\) atoms. (a) What is the predicted dominant Lewis structure if satisfying the octet rule is the top criterion? (b) What is the predicted dominant Lewis structure if achieving the best formal charges is the top criterion?

In the dihydrogen phosphate ion, \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\), the central atom is phosphorus (P). This is because it has the lowest electronegativity among the other atoms (excluding H).

In order to draw the Lewis structure, we must first count the total number of valence electrons for this ion. Here is the number of valence electrons for each atom: - Hydrogen (H): 1 valence electron x 2 atoms = 2 electrons - Phosphorus (P): 5 valence electrons - Oxygen (O): 6 valence electrons x 4 atoms = 24 electrons There is also an extra electron added to the species due to the negative charge, so we have a total of 32 electrons.

In this step, we will draw the Lewis structure by satisfying the octet rule. 1. Connect the atoms: Connect the 4 oxygen atoms to the central phosphorus atom. 2. Distribute the remaining electrons: Distribute the remaining electrons to each atom (except hydrogen) to complete their octet. 3. Place the hydrogens: Hydrogen will form a single bond with two of the oxygen atoms. Following these steps, we get the Lewis structure as shown below: P with single bonds to 4 O atoms. Each of the 2 O atoms that form a single bond with H has an extra lone pair, while the other 2 O atoms have 3 lone pairs of electrons. The formal charges are: P = +1, H-bonded O = 0, Non-H-bonded O = -1 - this maintains the overall charge of -1. This Lewis structure satisfies the octet rule.

In this step, we will draw the Lewis structure by achieving the best formal charges. 1. Connect the atoms: Connect the 4 oxygen atoms to the central phosphorus atom. 2. Distribute the remaining electrons: Distribute the remaining electrons to each atom (except hydrogen) to complete their octet. 3. Place the hydrogens: Hydrogen will form a single bond with two of the oxygen atoms. 4. Adjust the bonds: Convert one of the single bonds between the P and O that has no hydrogen bonded to form a double bond. This will let P to have a formal charge of 0 and satisfy the best formal charges rule. The Lewis structure we get is the following: P with a double bond to one O atom, and single bonds to the other 3 O atoms. The O atom with the double bond has 2 lone pairs; the other 2 single-bonded O atoms have 3 lone pairs each, and the H-bonded O atoms have an extra lone pair of electrons. This structure has formal charges: P = 0, O with the double bond = 0, H-bonded O = 0, Non-H-bonded O = -1 - this maintains the overall charge of -1. The Lewis structure with the best formal charges has a double bond between P and one of the O atoms. a) The predicted dominant Lewis structure satisfying the octet rule has 4 single bonds between phosphorus and oxygen atoms, with hydrogens bonded to two of the oxygen atoms. b) The predicted dominant Lewis structure achieving the best formal charges has a double bond between phosphorus and one oxygen atom, with single bonds to the other 3 oxygen atoms, and hydrogens bonded to two of the oxygen atoms.