For this problem, I want to find the IUPAC name for PCl3. The first thing I have to decide is whether this is an ionic or molecular compound. And to do that, I'll look at the elements in the formula. I notice that I have phosphorous and chlorine, both of which are non-metals. So that indicates that this is a molecular compound. And for molecular compounds I'm going to use our prefixes to indicate how many I have of each element. So here I have my phosphorus, that's my first element. And I have chlorine and so the ending on that becomes chloride. And so now I have to put my prefixes in, indicating the number. For phosphorous there's only one of them, and so it would be monophosphorus, however, when the first element has a prefix of mono, I omit that, so I don't need to add a prefix there. For our chlorine or the chloride part of our compound, I see that I have three of them, and so I'm going to use the prefix tri, to indicate three of them. So I can write the name as phosphorus trichloride.