1. The Incomplete Octet of Aluminum
* In AlCl3, aluminum only has 6 electrons in its valence shell, making it electron deficient. This is because it has 3 bonds with chlorine, each contributing one electron.
* Aluminum's electron configuration is [Ne] 3s² 3p¹. It doesn't want to be electron deficient.
2. The Lone Pair on Nitrogen
* Nitrogen in ammonia (NH3) has a lone pair of electrons that it can donate. Its electron configuration is [He] 2s² 2p³.
3. Formation of the Coordinate Covalent Bond
* The key is the coordinate covalent bond. This type of bond forms when one atom provides both electrons for the shared pair.
* Nitrogen donates its lone pair to aluminum. This results in a dative bond (also known as a coordinate covalent bond) between the nitrogen and aluminum atoms.
* This donation of electrons allows aluminum to achieve a full octet.
4. Why It Works
* Electron Deficiency: The aluminum atom in AlCl3 is electron deficient, making it a good electron acceptor.
* Lone Pair Availability: Nitrogen in ammonia has a readily available lone pair to donate.
* Stability: The formation of the coordinate covalent bond leads to a more stable structure for both AlCl3 and NH3.
In Summary:
The interaction between AlCl3 and NH3 isn't about filling an already complete octet on aluminum. It's about aluminum, which is electron deficient, accepting a lone pair from nitrogen to achieve a stable octet.
