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S1, Ep 2: Naming with Roman Numerals

Caroline Christian, Ph.D.

Naming ionic compounds is always confusing, when there are so many things to remember.  Memorize all the polyatomic ions? What are the charges of the ions in groups IA, IIA, IIIA?  One of the hardest things to remember is when to use the roman numerals in parentheses?  In the following periodic table, the elements in yellow are the ones to be concerned with, or the transition metals.

The metals in this large block in the middle of the table (starting with Sc) are the only ones that need the roman numerals.  They only need roman numerals because some of them can have more than one charge; take for example iron, it can be +2 or +3 in a compound. The only way to know the charge of the metal in the name of the compound is to include the roman numerals.  Notice that I said name of the compound, how about for the formula for the compound? The formula of the compound does not need to designate which charge of the metal ion you need, because the compound will do this for you with how many anions are needed to neutralize the metals charge. Don't know what ions or anions are?  Look at the previous blog post!  Take for example, FeCl2 and FeCl3. They are both iron chloride compounds; the only difference is the charge on the iron ion.  In the first compound listed, there are 2 Cl- ions needed to neutralize the charge of the Fe ion.  The charge on a chloride ion is -1, and an ionic compound must have an overall charge of 0.  So the Fe ion must be +2 charged (Fe2+) , and the name Iron(II) chloride results. For the second compound listed, it takes 3 chloride (Cl-) ions to neutralize the charge of the Fe ion; so it needs to be +3 charged, and the name Iron(III) chloride ensues.

01/21/2018