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Why is anhydrous aluminum chloride a covalently bonded dimer

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Why is Anhydrous Aluminum Chloride a Covalently Bonded Dimer The fast technique for identification of the chemical bond type of a substance is by identifying the nature of the component particles. When a compound is formed between a metal and a nonmetal, it generally has an ionic bond, where electrons are transferred from metal to nonmetal in accordance with the octet rule. On the other hand, when nonmetals formed a compound, they share electrons forming a covalent bond. Taking anhydrous aluminum chloride, AlCl3 for instance, Aluminum is a metal and Chlorine is a nonmetal, by nature they should have formed an ionic compound. This basic concept however is not true with anhydrous aluminum chloride. Instead it has both an ionic and molecular character. In fact its ionic character is very weak such that at relatively low temperature of 180oC, it sublimes (directly changing from solid to gas phase). If the pressure is raised to just over 2 atmospheres, it melts instead at a temperature of 192C.These characteristics are indirect evidence of weak intermolecular forces between the molecules and therefore, the compound is covalent. In the vapor phase, AlCl3 does not exist. In its place is a dimer (two molecules joined together), Al2Cl6 as shown and measured by its molecular mass. The molecular structure is formed when "one chlorine atom on each AlCl3 unit donates both electrons from one of its lone pairs to form a new shared electron pair with the aluminum atom from the other unit. This completes a stable octet for the aluminum. And the two chlorines involved become bridging atoms that join the two AlCl3 units into a single Al2Cl6 molecule. The two linking bonds are what we call co-ordinate bonds" (Christie, 2001). Once a co-ordinate bond is formed it cannot be distinguished from a normal covalent bond.
Why is anhydrous aluminum chloride a covalently bonded dimer One major reason is stability. "Energy is released when the two co-ordinate bonds are formed, and so the dimer is more stable than two separate AlCl3 molecules" (http://www6.grafton.k12.wi.us/ghs/teacher/mstaude/CovalentBonds.htm). Another reason is the size of the highly charged aluminum ion, Al 3+. The Al 3+ ion has a large charge to size ratio. It attracts electrons so strongly that aluminium chloride is a covalent molecule. This large charge is due to higher ionic potentials caused by the inability of the d electrons to shield the nucleus very effectively. Electronegativity also is a factor of the occurrence of the covalent character. Electronegativity increases as you go across the period and, by the time you get to aluminum, there isn’t enough electronegativity difference between aluminum and chlorine for there to be a simple ionic bond (Clark, 2003). These, among others explains why at room temperature solid aluminum chloride has an ionic lattice with a lot of covalent character and why its dimmers becomes coordinately, covalently bonded.

Works Cited
Christie, John.(2001). Re: are there coordinate ionic bonds Posted September 17 20:48:54 2001 Retrieved: December 6, 2007 at http://www.madsci.org/posts/archives/2001-10/1002424202.Ch.r.html
Clark, Jim. (2003). Properties of the period 3 chlorides. Retrieved: December 6, 2007 at
http://www.chemguide.co.uk/index.html

http://www6.grafton.k12.wi.us/ghs/teacher/mstaude/CovalentBonds.htm