Question

Explain theoretically the lewis structure diagram of the covalent compound ${\mathrm{Al}}_2{\mathrm{Cl}}_6$?

Answer 1

${\mathbf{AlCl}}_{\mathbf{3}}$as dimer;

1. ${\mathrm{AlCl}}_3$ exists as a dimer as the $\mathrm{Al}$ atom in the molecule is electron deficient due to the presence of only six electrons in its valence shell, that is it has an incomplete octet due to which it is quite unstable.
2. So in order to complete its octet and thus attain stability, two ${\mathrm{AlCl}}_3$ molecules dimerize through the formation of coordinate bonds between lone pair of electrons on the $\mathrm{Cl}$ atom in one ${\mathrm{AlCl}}_3$ molecule and the empty p orbital on $\mathrm{Al}$ in another ${\mathrm{AlCl}}_3$ molecule.
3. Thereby forming a dimer which consists of three-center-two-electron bonds called banana bonds between electron-deficient $\mathrm{Al}$ and $\mathrm{Cl}$ in each ${\mathrm{AlCl}}_3$ molecule and the coordinate bonds between bridging $\mathrm{Cl}$ atoms and the electron deficient $\mathrm{Al}$.

Lewis structure of ${\mathbf{Al}}_{\mathbf{2}}{\mathbf{Cl}}_{\mathbf{6}}$:

1. The compound Aluminum chloride ( ${\mathrm{AlCl}}_3$) has six covalent bonds and two coordinate bonds.
2. Chlorine has $17$ electrons, out of this $10$ electrons are occupied in the $1\mathrm{s},2\mathrm{s}\mathrm{and}2\mathrm{p}$ orbitals ($1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^6$).
3. The other seven are distributed in the $3\mathrm{s}$ and the three $3\mathrm{p}$ orbitals.
4. However, by forming (Covalent) bonds these orbitals hybridize into $\mathrm{sp}$-orbitals.
5. Chlorine thus has $7$ electrons in the four ${\mathrm{sp}}^3$orbitals; here three orbitals are filled with two electrons each, and the fourth orbital has filled with only one electron.
6. Aluminum has $13$ electrons, out of this $10$ electrons are occupied in the $1\mathrm{s},2\mathrm{s}\mathrm{and}2\mathrm{p}$ orbitals ($1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^6$).
7. The other three are distributed in the $3\mathrm{s}$ and $3\mathrm{p}$ orbitals $(3{\mathrm{s}}^{2}3{\mathrm{p}}^1)$.
8. In the excited state, the electron from the $3\mathrm{s}$ orbital shift to $3\mathrm{p}$orbitals. .
9. During the formation of ${\mathrm{AlCl}}_3$ $\mathrm{Al}$ will share these three electrons to chlorine and form ${\mathrm{Al}}^{3+}$(Chlorine has a high electronegativity, which means that it attracts electrons from other atoms), each chlorine will accept one electron and form ${\mathrm{Cl}}^{-}$.
10. Here, now $\mathrm{Al}$ has six electrons in its valence shell so the compound is quite unstable and it requires two more electrons to complete the octet.
11. Therefore, ${\mathrm{AlCl}}_3$ exist as a dimer$\left({\mathrm{Al}}_2{\mathrm{Cl}}_6\right)$, in order to complete the octet and attain stability.
12. The Lewis structure of ${\mathrm{Al}}_2{\mathrm{Cl}}_6$can be represented as;
13. Therefore, In the ${\mathrm{Al}}_2{\mathrm{Cl}}_6$ Chlorine atom attracts electrons from the Aluminium atom and attains stability by completing the octet.