Draw and Explain the Lewis Structure of the Most Important Resonance Form of the Ion AsO4^3-

The arsenate ion, AsO₄^3-, has multiple resonance forms, but the most significant one can be represented clearly through its Lewis structure. In this structure, arsenic (As) is located at the center, surrounded by four oxygen (O) atoms. Here’s a step-by-step explanation of how to draw it:

1. Count the Valence Electrons: Arsenic has 5 valence electrons, and each oxygen has 6. Since there are four oxygen atoms, the total from oxygen is 24. Adding arsenic’s 5 gives us a total of 29 valence electrons. However, because the ion has a charge of 3-, we must add 3 more electrons, bringing the total to 32 valence electrons.
2. Central Atom Placement: Place arsenic in the center of the structure since it’s the least electronegative compared to oxygen.
3. Draw Single Bonds: Connect arsenic to each oxygen atom with a single bond. This uses 8 electrons (4 bonds), leaving us with 24 electrons to distribute.
4. Fill the Octets of Oxygen: Place 6 electrons (3 lone pairs) around each of the four oxygen atoms. This uses 24 of our remaining electrons, completing the octet for each oxygen.
5. Assign Multiple Bonds: To account for the negative charge and to ensure arsenic can expand its octet (which it can due to being in period 4), we can convert one lone pair from two of the oxygen atoms into double bonds with arsenic. This is critical because the negative charge is distributed among the oxygens and helps stabilize the structure.

In the most stable resonance form, we typically show two of the O atoms with double bonds to As, and the other two with single bonds. This leads to a formal charge of 0 on As and the double-bonded O, while the single-bonded O carries a -1 formal charge.

Formal Charges:

• As: 5 – (0 + 0) = 5 (valence electrons – (lone pair electrons + 1/2 bonding electrons)) → 0
• For double-bonded Os: 6 – (4 + 0) = 6 – 4 = 0
• For single-bonded Os: 6 – (6 + 1) = 5 – 6 = -1

The formal charges add up to -3, consistent with the overall charge of the ion.

Oxidation Numbers:

• Arsenic (As): +5
• Each double-bonded oxygen: -2
• Each single-bonded oxygen: -2

In summary, the important resonance form of the arsenate ion emphasizes the distribution of charges among the oxygen atoms, while arsenic exposes its oxidation state through the bonds it forms, providing stability to the overall structure.