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Ionic Nomenclature

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Objectives


  • Name any given ionic compound given its chemical formula
  • Provide the chemical formula for any ionic compound given its name

An ionic compound is composed of an anion and cation whose charges together must be neutral. When naming an ionic compound, we must determine the names of the anion and cation that have bonded. Given the formula, we determine the elements (or polyatomic ions) that make up the compound in question. We then write the anion name after that of the cation, and we have the formula. See examples 1 & 2

When an element in an ionic compound is a transition metal, we must take additional measures to ensure that the name we provide the compound is not ambiguous. Many transition metals have multiple ionized forms, where they have different charges. For example, Nickel can have either a 2+ charge, or 3+ charge. To make sure that the name we provide an ionic compound reflects the actual charge Nickel has in, say, NiBr, we must either use the latin system of suffixes(not discussed here) or the preferred Roman numeral extension. We determine the charge of the transition metal by finding the sum of the other charges in the compound, which must be balanced by that of our metal. This is a Roman numeral appended to the name of the metal. See example 3


Given the name of an ionic compound, we must break the name down into anion and cation to find its formula. Generally the name will be two words, the cation first, then the anion. We find the charges of each so that we can find the ratio of their balance. Once we have the charges, we can swap their number (3+ -> 3) and use them as subscripts to create neutrality. We must reduce the subscripts much as we would fractions to ensure the simplest form. Once again, cation before anion. See example 4

Examples


1. NaCl

  • Na = Sodium, whose ion is Sodium with a + charge; this is our cation
  • Cl = Chlorine, whose ion is Chloride with a - charge; this is our anion

So we put cation before anion and get Sodium Chloride (this, by the way, is table salt)


2. Ca(C2H3O2)2

  • Ca = Calcium, whose ion is Calcium with a 2+ charge; this is our cation
  • C2H3O2 = Acetate, a polyatomic ion with a - charge. There appear to be two for every Ca; this is our anion.

Again, we put cation before anion and get Calcium Acetate . Note how we disregard the number of acetate ions per calcium. Because this is the simplest form, there is no need to include that number in the name.


3. Ni(BrO3)3

  • Ni = Nickel, but we don't know the charge yet. It could be 2+ or 3+.
  • BrO3 = Bromate, polyatomic ion with - charge. There are 3, leading to a total charge of 3-.

Because the charge of the ion without nickel is 3-, nickel mist have a charge of 3+ to balance that. Therefore, we name the compound Nickel (III) Bromate or Nickelic Bromate.


4. Potassium Cyanide

  • Potassium = K+
  • Cyanide = Cn-

So swapping the charges, KCN.

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