Ap chemistry memorization question?

Question

Hi I am going to be in AP chemistry next year. I haven't had a class that has only focused on chemistry and I'm just finishing up the summer memorization homework.

I have a few questions.

A part of the homework is being able to name compounds. For example name PCl3. Now my first guess would have been phosphorus chloride but I look it up and its phosphorus TRIchloride. Now I have noticed similar equations don't have the "tri" in front of it. For example the equation FeI3 has the name Iron iodide. Why isn't it Iron triiodide. It sounds a little weird but there must be a better explanation as to why it isn't called that other then it just sounds funny.

Another example is with the equation N2F4. I know that it is Dinitrogen Tetrafluoride. But the equation Al2S3 the name is Aluminum Sulfide. Why isnt it Dialuminum trisulfide?

Answer 1

The main difference between all those compounds that you're confused about is that some are molecular compounds while some are ionic. In molecular substances, the name of the substance must indicate the number of each type of atom in one molecule. PCl3 is a molecular compound, and so the number of each type of atom must be included in the name. We end up with monophosphorus trichloride. If the least electronegative element has only 1 atom in each molecule, then you can ignore the "mono-" part. We end up with phosphorus trichloride.

FeI3 is an ionic compound, so we do not need to worry about the number of each type of atom. This is possibly due to the crystal lattice structure ionic substances create as opposed to the individual units molecular substances have. FeI3 is iron (III) iodide.

It's true some things in chemistry must be memorized without any definite rules or guidance. For example, H2O would be dihydrogen monoxide, but everyone simply calls it water.

Answer 2

You''re absolutely right!

While a lot of chemistry is pretty logical, much of it is strictly memorization.

No shortcuts.

Sorry.