This question and the one that follows will ask questions about the three structures drawn in this video: I3-, POCl3, and BF3.

Draw all three structures and match them to the reason they are allowed to break the octet rule. (hint: you can use the same answer more than once)
1: I_3^-1
2: POCl_3
3: BF_3

Question

This question and the one that follows will ask questions about the three structures drawn in this video: I3-, POCl3, and BF3.

Draw all three structures and match them to the reason they are allowed to break the octet rule. (hint: you can use the same answer more than once)
1: I_3^-1
2: POCl_3
3: BF_3

BlackTom AI · Accepted Answer

To tackle this matching question, I’ll first restate the problem and list the options clearly so we can reason through each one.
Question context: We have three structures (I3−, POCl3, and BF3) and we need to match each to the reason they can break the octet rule. The provided answer options to choose from are:
- Option A: One or more atoms is allowed to have less than 8 valence electrons
- Option B: n≥3 for one or more atoms
- Option C: The total number of electrons is odd

Now, evaluate each option one by one.
Option A: One or more atoms is allowed to have less than 8 valence electrons
- This is a classic description of a deficient octet. BF3 is the standard example where boron ends up with only six valence electrons around it, which is why it is considered to violate the octet rule. Therefore, this option correctly explains the octet deficiency observed in BF3.
- Why this is not a misfit: The concept of a deficient octet applies specifically to elements like boron in BF3, where the central atom does not achieve eight electrons. Since BF3 is the molecule that truly exemplifies an atom with less than 8 valence electrons, this option aligns well with that structure.

Option B: n≥3 for one or more atoms
- The phrase n≥3 suggests that at least one atom in the structure forms bonds to three or more other atoms (i.e., coordination number 3 or higher). In POCl3, the phosphorus atom is bonded to four substituents (one oxygen and three chlorines), giving it a coordination number of 4, which satisfies n≥3. This supports the idea that POCl3 can accommodate more than the typical octet through bonding to multiple substituents, enabling an expanded valence with more than eight electrons around the central atom.
- For I3−, the situation is more nuanced: it’s a linear I–I–I arrangement involving three iodine atoms with bonding that can be described using three-center electron-deficient bonding concepts. Some treatments describe such bonding as involving hypervalent or multi-centered bonding, which can be framed in terms of expanded bonding interactions around iodine that go beyond a simple two-coordinate scenario. In that sense, the idea of having three or more atoms participating in the bonding around the central region is applicable. However, note that the strict interpretation of n≥3 for a single central atom may feel stretched for I3− if one enforces a simple two- or three-neighbor picture. Still, the broader meaning of “three or more neighbors” can be invoked in hypercoordinate or multi-centered bonding contexts.
- Overall assessment: As a general statement, n≥3 for one or more atoms can plausibly account for POCl3’s expanded octet (and can be stretched to describe the multi-centered bonding in I3−). This makes Option B a reasonable match for the structures that do not follow a standard octet by simple two-center-two-electron bonds.

Option C: The total number of electrons is odd
- If a molecule has an odd total number of valence electrons, it cannot satisfy the octet rule for all atoms, but that criterion is not the distinguishing feature of the three structures here. I3− actually has an even number of total valence electrons when you count all I atoms and the extra electron from the negative charge. BF3 has an even count as well, since boron (3) plus three fluoride (18) totals 24 electrons in the closed-shell accounting. POCl3 likewise sums to an even number of valence electrons when you tally all bonds and lone pairs. Therefore, the “odd total electrons” reasoning does not correctly explain why any of these particular structures break the octet rule, making this option inappropriate.

Putting it together:
- Option A correctly captures the octet deficiency seen in BF3.
- Option B provides a reasonable rationale for structures that can display expanded octets or multi-centered bonding, notably POCl3 and, in a broader interpretation, I3−, where bonding can involve more than a simple octet around certain atoms.
- Option C does not align with the electron count rationale for these three structures, since their total valence electron counts are not odd in the way this option implies.

Answer alignment with the given choices: The two statements that correctly describe the reasons these structures break or extend the octet rule are Option A and Option B. The third statement, Option C, does not apply to these cases.

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