Expert: Dr. Henry Boyter, Jr. - 10/30/2005

Question
Dear Dr. Boyter


I have a quick question. I'm in a begining level chem class, so I don't know much yet. Anyway, I do know that an ion forms when at atom looses or gains electrons, and that the ideal form is that of a noble gass (eight valence electrons) and that atoms either loose or gain electons depending on how easily thay can get to that eight, for example, halogens such as flourine tend to gain just one electron because it is easier than loosing all the seven that they already have. OI also know that it is extremely rare for an atom to loose or gain more than three electrons, so my question is, how do you predict the ind of ion an element such as aluminum, which would have to loose or gain more than three electrons to get eight valence electrons, will form? Well, thank you so much for taking the time to help me out, it is really really appreciated!

Sincerely,
Amanda

Answer
Aluminum is like B, it looses three in most cases.  You will find the +/-3 rule is a general rule not an absolute.  For example N can be gain three or lose five.  Other elements such as transition metals can have multiple valance levels.  The short answer is that you will learn the rules and/or infer them from context.  For example, it should be obvious that the N in NCl5 will be N(+5).