From the lewis structure of AlCl4- where should I place the formal charge? next to Al or Cl?
The Correct Answer and Explanation is:
In the Lewis structure of the tetrachloroaluminate ion (AlCl₄⁻), the formal charge should be placed next to the aluminum (Al) atom, not the chlorine atoms. This is because the extra negative charge in the ion is associated with the central atom, which is aluminum, after accounting for the electrons contributed by all atoms in the structure.
Explanation
To understand where to place the formal charge, we must first draw the correct Lewis structure for AlCl₄⁻.
Aluminum (Al) is the central atom and is surrounded by four chlorine (Cl) atoms. Each Cl atom forms a single bond with Al. Since aluminum belongs to group 13, it normally has 3 valence electrons. Each chlorine atom, from group 17, contributes 7 valence electrons. So the total valence electrons are:
- Al: 3 electrons
- Cl: 4 × 7 = 28 electrons
- Extra electron due to the negative charge: 1 electron
- Total = 3 + 28 + 1 = 32 valence electrons
These electrons are used to form single bonds between Al and each Cl atom (4 bonds = 8 electrons), and the remaining 24 electrons are placed as lone pairs on the Cl atoms (each gets 6 electrons). Aluminum ends up with 8 electrons in its valence shell due to the four bonds.
Now let’s calculate formal charges:
- Chlorine: Has 7 valence electrons, 6 are in lone pairs, and it shares 1 electron in the bond.
Formal charge = 7 – (6 + 1) = 0
So each chlorine atom has a formal charge of zero. - Aluminum: Has 3 valence electrons, shares 4 electrons (one from each bond).
Formal charge = 3 – 4 = –1
Hence, the negative formal charge is located on the aluminum atom, not the chlorine atoms.
So in your Lewis structure, place the formal charge (–) next to the aluminum atom to accurately represent the distribution of electrons in the AlCl₄⁻ ion.
