Re: why does water start a fire with aluminum and iodine

Interesting Question.  Aluminum tends to form a +3 ion or Al+3 by losing 
three electrons.  Iodine will gladly pick up those electrons, but only one 
Iodine atom can gain one electron.  This means that you need actually three 
times as many iodine atoms as aluminum atoms.  Since Iodine has an atomic 
mass of 127 and aluminum has a mass of 26, this means if you massed the two 
reactants out, you should mass out 15 times the weight of iodine as 
aluminum.  As to the water.  As you know, oxygen, which is one of the 
elements in water is a strong oxidizer, more so than iodine.  In this 
reaction, the few drops of water provide the initial oxidizing agent to 
cause the Aluminum to "loose" its electrons, because there is a limited 
amount of water and the iodine is present, and it is energetically 
favorable for the aluminum and iodine to form an ionic compound (AlI3) the 
reaction takes place.  The energy required to get this reaction started is 
called the ionization energy, and the energy released, as you noted by the 
description of the fire and smoke is called the lattice energy.  While 
atoms don't really "loose" their electrons, they will give them to another 
atom that wants them more if they can get something out ot the deal.  What 
they get is amore stable electronic configuration and Iodine gets to 
complete its octet and they both get to move to a lower energy state.  This 
last idea is called entropy.