Draw the Lewis structures for each of the following molecules. Identify those that do not obey the octet rule and state the type of exception from the octet rule. ( a) AlH3 b) NO2 c) PF5 d) NH3
The correct answer and explanation is:
Lewis Structures and Octet Rule Analysis
(a) AlH₃ (Aluminum Hydride)
- Lewis Structure:
H | H–Al–H - Octet Rule: Violated
- Exception Type: Incomplete Octet
- Explanation: Aluminum (Al) has only 6 valence electrons in this molecule. Being in Group 13, it is an exception to the octet rule and can be stable with fewer than 8 electrons.
(b) NO₂ (Nitrogen Dioxide)
- Lewis Structure:
O=N–O (with one single bond and one double bond, and one unpaired electron on N) - Octet Rule: Violated
- Exception Type: Odd-Electron Molecule (Free Radical)
- Explanation: Nitrogen has 5 valence electrons and shares with two oxygens. After bonding, one electron remains unpaired, making NO₂ a free radical. This is a common case of odd-electron species that cannot have a full octet on all atoms.
(c) PF₅ (Phosphorus Pentafluoride)
- Lewis Structure:
F | F–P–F | \ F F - Octet Rule: Violated
- Exception Type: Expanded Octet
- Explanation: Phosphorus (P), in Period 3, can use d-orbitals to expand its valence shell beyond 8 electrons. In PF₅, it forms five bonds, resulting in 10 electrons around phosphorus.
(d) NH₃ (Ammonia)
- Lewis Structure:
.. N /|\ H H H - Octet Rule: Obeys
- Explanation: Nitrogen has 5 valence electrons and forms three covalent bonds with hydrogen atoms, using 3 electrons. The remaining 2 electrons form a lone pair, giving nitrogen a full octet.
300-Word Explanation
The octet rule states that atoms tend to form bonds until they are surrounded by 8 electrons, similar to noble gases. However, not all molecules follow this rule strictly due to various exceptions.
In AlH₃, aluminum forms three single bonds with hydrogen. Each bond contributes 2 electrons, giving aluminum only 6 valence electrons. Group 13 elements like aluminum are stable with incomplete octets, making AlH₃ an example of an incomplete octet.
NO₂ presents another classic exception. Nitrogen typically needs 8 electrons, but in NO₂, it forms one single bond and one double bond with two oxygens and has one unpaired electron left over. This makes it a free radical, where an atom has an odd number of electrons and cannot satisfy the octet rule.
In PF₅, phosphorus forms five single bonds with fluorine atoms. This adds up to 10 electrons around the central phosphorus atom. Phosphorus can do this because it’s in Period 3 of the periodic table and can access d-orbitals to expand its octet, making PF₅ an expanded octet exception.
Finally, NH₃ (ammonia) follows the octet rule. Nitrogen forms three bonds with hydrogen atoms and retains one lone pair, giving it exactly 8 electrons. This is a typical octet-compliant molecule.
In conclusion, while the octet rule is a useful guideline, many molecules (especially those involving elements in Period 3 or having an odd number of electrons) can be stable while violating it. Recognizing these exceptions is key in understanding real-world molecular structures.