Draw the Lewis Structure for AlCl3. Answer the following questions: What is the electron geometry? What is the molecular geometry? What type(s) of bonds are present in the molecule? Describe the molecule in terms of polarity. What is the strongest intermolecular force present?
The Correct Answer and Explanation is:
Lewis Structure for AlCl₃ (Aluminum Chloride):
In AlCl₃, aluminum (Al) has 3 valence electrons, and each chlorine (Cl) has 7 valence electrons. The total number of valence electrons is:
- Al: 1 × 3 = 3
- Cl: 3 × 7 = 21
- Total = 24 valence electrons
Each Cl atom forms a single covalent bond with Al, using one of Al’s electrons and one from each Cl. That accounts for 6 electrons (3 bonds × 2 e⁻ each = 6 e⁻). The remaining 18 electrons are placed as lone pairs on the Cl atoms (6 electrons per Cl atom as lone pairs).
In this structure, Al has only 6 valence electrons, not a full octet. This is acceptable for Al because it’s a Group 13 element, which commonly forms compounds where it has only six electrons.
Answer Summary:
- Electron Geometry: Trigonal planar
- Molecular Geometry: Trigonal planar
- Bond Types: Polar covalent bonds
- Polarity: The molecule is nonpolar overall
- Strongest Intermolecular Force: London dispersion forces
Explanation
Aluminum chloride (AlCl₃) is an interesting molecule because it defies the usual octet rule. Aluminum, with three valence electrons, bonds with three chlorine atoms, each contributing one electron. This creates three Al–Cl bonds, each being polar covalent, since Cl is more electronegative than Al. Despite these individual bond polarities, the molecule adopts a trigonal planar geometry, with 120° bond angles. This shape results in the symmetrical distribution of charge, and as a result, the dipole moments cancel out, making AlCl₃ nonpolar overall.
The electron geometry is trigonal planar, the same as the molecular geometry, because there are no lone pairs on the central aluminum atom to distort the shape. This geometry is determined by VSEPR theory, which predicts that the three regions of electron density (the three Al–Cl bonds) will arrange themselves as far apart as possible in a plane.
In terms of bonding, all three Al–Cl bonds are polar covalent, but because the molecule is symmetrical, these polarities do not lead to a net molecular dipole. Therefore, AlCl₃ is a nonpolar molecule, even though it contains polar bonds.
The strongest intermolecular force in AlCl₃ in its molecular form (typically at low temperatures or in the gas phase) is London dispersion forces. These are weak forces that arise due to temporary dipoles in molecules. Since AlCl₃ is nonpolar, it does not experience dipole-dipole interactions or hydrogen bonding. However, in the solid state or at high temperatures, AlCl₃ can form dimers (Al₂Cl₆), which change its properties significantly, but in its simple molecular form, London dispersion is dominant.
