![]() By making the maximum number of bonds, it releases most energy and is the most energetically stable. Six is the maximum number of water molecules it is possible to fit around an aluminum ion (and most other metal ions). You might wonder why it chooses to use six orbitals rather than four or eight or whatever. These six hybrid orbitals accept lone pairs from six water molecules. The aluminum reorganizes (hybridizes) six of these (the 3s, three 3p, and two 3d) to produce six new orbitals all with the same energy. That means that all the 3-level orbitals are now empty. When it forms an Al 3 ion it loses the 3-level electrons to leave 1s 22s 22p 6. ![]() The electron configuration of aluminum is 1s 22s 22p 63s 23p x 1. Figure: Water has two lone pairs of electrons The bonding in this (and the similar ions formed by the great majority of other metals) is coordinate (dative covalent) using lone pairs on the water molecules. It's called the hexaaquaaluminum complex ion with as six ("hexa") water molecules ("aqua") wrapped around an aluminum ion. ![]() Six water molecules bond to the aluminum to give an ion with the formula Al(H 2O) 6 3 . Ions with water molecules attached are described as hydrated ions.Īlthough aluminum chloride is a covalent compound, when it dissolves in water, ions are produced. In many cases, the attractions are so great that formal bonds are made, and this is true of almost all positive metal ions. Water molecules are strongly attracted to ions in solution - the water molecules clustering around the positive or negative ions. There's nothing special about those two particular lone pairs - they just happen to be the ones pointing in the right direction.Įnergy is released when the two coordinate bonds are formed, and so the dimer is more energetically stable than two separate AlCl 3 molecules. The uninteresting electrons on the chlorines have been faded in color to make the coordinate bonds show up better. Each chlorine atom has 3 lone pairs, but only the two important ones are shown in the line diagram. The bonding between the two molecules is coordinate, using lone pairs on the chlorine atoms. ![]() It exists as a dimer (two molecules joined together). Measurements of the relative formula mass of aluminum chloride show that its formula in the vapor at the sublimation temperature is not AlCl 3, but Al 2Cl 6. There is likely to be a similarity, because aluminum and boron are in the same group of the Periodic Table, as are fluorine and chlorine. The dots-and-crosses diagram shows only the outer electrons.ĪlCl 3, like BF 3, is electron deficient. The implication is that it when it sublimes at this relatively low temperature, it must be covalent. If it simply contained ions it would have a very high melting and boiling point because of the strong attractions between the positive and negative ions. When a hydrogen ion breaks away again, it could be any of the three.Īluminum chloride sublimes (phase transition from solid to gas) at about 180☌. Note that once the coordinate bond has been set up, all the hydrogens attached to the oxygen are exactly equivalent. ![]() When it reacts with something (an alkali, for example), the hydrogen ion simply becomes detached from the water molecule again. \)" represents the water molecule that the hydrogen ion is attached to. ![]()
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