Draw the best Lewis Structure for KrO2 based on overall formal charges. Answer the questions below based on that structure.
How many (total) bonding electron pairs are in the molecule?
How many (total) non-bonding electron pairs are in the molecule?
How many non-bonding electron pairs are on the central atom?
The Correct Answer and Explanation is:
Lewis Structure of KrO₂
The compound KrO₂ contains krypton (Kr) as the central atom and two oxygen (O) atoms. Krypton is a noble gas, but it can expand its octet since it’s in period 4. Oxygen is more electronegative and typically forms two bonds or holds lone pairs to complete its octet. The goal is to draw the Lewis structure that minimizes formal charges and obeys the octet rule as much as possible.
Step-by-Step:
- Valence Electrons:
- Kr: 8 valence electrons
- Each O: 6 valence electrons
- Total = 8 (Kr) + 2 × 6 (O) = 20 valence electrons
- Initial Bonding:
- Place Kr in the center with two O atoms bonded to it.
- Add single bonds between Kr and each O (2 bonds = 4 electrons used).
- Distribute remaining 16 electrons to complete octets on O atoms.
- Adjust for Formal Charges:
- Oxygen with only a single bond has a formal charge of -1.
- To reduce formal charges, form double bonds (Kr=O) with each oxygen.
- Final Structure:
- Two double bonds (Kr=O) between Kr and the two O atoms.
- Each O has two lone pairs; Kr has one lone pair (2 electrons).
This structure minimizes formal charges:
- Kr: 8 valence e⁻ − (4 bonding e⁻/2 + 2 lone e⁻) = 8 − (2 + 2) = 4 → FC = 0
- O: 6 valence e⁻ − (4 bonding e⁻/2 + 4 lone e⁻) = 6 − (2 + 4) = 0
Answers:
- Total bonding electron pairs:
- Each double bond = 2 bonding pairs
- 2 Kr=O bonds → 2 × 2 = 4 bonding pairs
- Total non-bonding electron pairs:
- Each O has 2 lone pairs → 2 × 2 = 4 pairs
- Kr has 1 lone pair
- Total = 4 (O) + 1 (Kr) = 5 non-bonding pairs
- Non-bonding electron pairs on central atom (Kr):
- 1 lone pair on Kr
Explanation
The Lewis structure of KrO₂ requires careful consideration due to the involvement of krypton, a noble gas. Although noble gases are generally inert, krypton can form compounds due to its ability to expand its octet, thanks to its empty d-orbitals. The objective when constructing the Lewis structure is to obey the octet rule (as much as possible) and minimize formal charges across the atoms.
Krypton contributes 8 valence electrons, and each oxygen contributes 6, totaling 20 electrons. Placing Kr in the center and bonding it to each O with a single bond uses 4 electrons. The remaining 16 electrons are first distributed as lone pairs to satisfy the octet for each atom. However, with just single bonds, the oxygens would each have a formal charge of -1 and Kr would carry a positive charge. To resolve this, each oxygen forms a double bond with Kr, reducing formal charges to zero for all atoms and yielding a more stable structure.
In the final structure, Kr is double-bonded to each O atom. Each O has two lone pairs, and Kr retains one lone pair. This accounts for a total of 4 bonding pairs (2 for each double bond) and 5 non-bonding pairs (4 on oxygen, 1 on Kr). Only one lone pair resides on Kr, the central atom.
This structure is consistent with known chemistry of krypton oxides like KrO₂, where Kr exhibits oxidation states beyond 0. The bonding satisfies both octet considerations (where possible) and formal charge minimization, making it the best representation of the molecule.
