Chemistry - Molecular Structure (14.5 of 59) Basic Shapes - Predict the Shape of AlCl4(-). By Michel van Biezen

Summary:

1. Introduction:

   • Molecule: Aluminum tetrachloride (AlCl4-) is discussed, characterized as an ion due to aluminum's valence electron deficiency.

2. Lewis Structure:

   • Central Atom: Aluminum (Al), with three valence electrons, is placed in the center.
   • Chlorine Atoms: Four chlorine (Cl) atoms, each with seven valence electrons, are bonded to aluminum in a single bond arrangement.
   • Electron Requirement: Since aluminum requires an additional electron to complete bonding with four chlorines, it becomes an ion (AlCl4-).
   • Chlorine's Free Electron Pairs: Each chlorine atom retains three sets of free electron pairs.
   • Absence of Aluminum's Free Electrons: Aluminum lacks free electron pairs, having only four sets of bonded electrons with chlorine atoms.

3. Molecular Structure:

   • VSEPR Notation: The structure is represented as one central atom (Al) surrounded by four atoms (Cl).
   • Tetrahedral Shape: Aluminum tetrachloride adopts a tetrahedral geometry, maximizing the separation between chlorine atoms.
   • Attempted Plane Formation: Initially, a planar structure might seem logical, but the resulting 90-degree bond angles would not suffice.
   • Tetrahedral Arrangement: To achieve maximum separation, the molecule forms a tetrahedron, with aluminum at the center and three chlorines forming a triangular base, while the fourth chlorine sits atop.
   • Pyramid Analogy: The arrangement resembles a three-sided pyramid with an aluminum ion at the center and a triangular base formed by three chlorine atoms, while the fourth chlorine sits atop as the apex.

4. Conclusion:

   • Aluminum tetrachloride (AlCl4-) adopts a tetrahedral molecular shape, resembling a three-sided pyramid with a triangular base and aluminum ion at the center.
   • The tetrahedral geometry ensures maximum separation between chlorine atoms, fulfilling the octet rule for aluminum and achieving stability.