Draw a Lewis structure for the tetrachloraluminate ion,[AlCl4]

Draw a Lewis structure for the tetrachloraluminate ion,[AlCl4]

. This ion can be considered as a combination of
and
. What kind of bond joins
and
in

The Correct Answer and Explanation is:

Lewis Structure of the Tetrachloroaluminate Ion [AlCl4]−[ \text{AlCl}_4 ]^-[AlCl4​]−

The tetrachloroaluminate ion [AlCl4]−[ \text{AlCl}_4 ]^-[AlCl4​]− is formed by combining an aluminum ion Al3+\text{Al}^{3+}Al3+ and four chloride ions Cl−\text{Cl}^-Cl−.

Steps to Draw the Lewis Structure:

1. Count total valence electrons:
   • Aluminum (Al) has 3 valence electrons.
   • Each chloride (Cl) atom has 7 valence electrons, and there are four Cl atoms.
   • The ion has an overall charge of -1, which means one extra electron is added.
   So, total valence electrons =
   3 (from Al)+4×7 (from Cl)+1 (extra)=323 \text{ (from Al)} + 4 \times 7 \text{ (from Cl)} + 1 \text{ (extra)} = 323 (from Al)+4×7 (from Cl)+1 (extra)=32 electrons
2. Place the central atom and connect ligands:
   • Put Al in the center since it is less electronegative.
   • Connect each of the four Cl atoms to Al using a single bond (4 bonds = 8 electrons).
3. Distribute remaining electrons to complete octets:
   • Each Cl needs 6 more electrons to complete an octet (6 × 4 = 24 electrons).
   • We have used all 32 electrons: 8 for bonds and 24 for lone pairs.
4. Check the octet rule:
   • Each Cl has 8 electrons (2 from bond + 6 lone electrons).
   • Al has 8 electrons from the four single bonds (it can accommodate only 6 in neutral AlCl₃, but here the ion is stable with 8 due to negative charge delocalization).

Final structure:

• Al is at the center with single bonds to four Cl atoms.
• Each Cl has three lone pairs.
• The overall ion has a charge of -1, placed outside the brackets.

Nature of Bond Between Al and Cl:

The bond between aluminum and chloride in [AlCl4]−[ \text{AlCl}_4 ]^-[AlCl4​]− is a coordinate covalent bond or dative bond. Aluminum starts with only 3 valence electrons and lacks a complete octet. When each chloride ion donates one of its lone pairs to form a bond with Al, the bonds are coordinate covalent in nature.

Thus, [AlCl4]−[ \text{AlCl}_4 ]^-[AlCl4​]− is a stable complex ion where the Al–Cl bonds are covalent, but they originate from lone pair donation, making them coordinate covalent bonds.