Nomenclature and Formula Writing

Ionic Compounds

The total charge for an ionic compound must be zero.  As a result, the number of cations and anions must balance so there is no unbalanced charge.  Consider the compound formed by magnesium and hydroxide:

Magnesium, an alkali earth metal, has a charge of +2.  Hydroxide has a charge of -1.  The formulas for each are Mg+2 and OH-1 respectively.  Two hydroxide ions will be necessary to balance the charge of the magnesium.  In other words, Mg+2 + OH-1+ OH-1 = 0.  For a proper formula, the two hydroxides are indicated by placing parentheses around the OH and a subscript 2 on the outside:


The name for this compound comes from joining the names of the cation and anion together, magnesium hydroxide.  Further examples:
iron(III) nitrate ammonium phosphate aluminum sulfate
Fe+3 NO3- NH4+ PO4-3 Al+3 SO4-2
×1 ×3 ×3 ×1 ×2 ×3
+3 -3 +3 -3 +6 -6
Sum = 0 (+3 + (-3)) Sum = 0 (+3 + (-3)) Sum = 0 (+6 + (-6))
Fe(NO3)3 (NH4)3PO4 Al2(SO4)3
iron(III) nitrate ammonium phosphate aluminum sulfate

Note that the number of ions needed to write the formula is determined in the cells colored gray. For more information on assigning oxidation numbers, check the module on oxidation-reduction reactions.

Guidelines for Naming Different Compound Types

•In nearly all cases, the more positive element/ion is given first in the formula.
•Organic formulas are often written in a manner to simplify drawing the structural formula.  As a result, many organic anions are written first.
•Anions that do not contain oxygen have the ending -ide.  Oxide and hydroxide are notable exceptions.
•Covalent compounds are made of two nonmetals.
•Covalent compounds use the prefixes to indicate the number of each element in the compound.
•Inorganic acids have formulas that begin with H.
•Organic acids have formulas that contain COOH
•Acids derived from ions that end in -ide add the prefix hydro- and use the suffix -ic.
•Acids derived from polyatomic ions that end in -ate use the suffix -ic.
•Acids derived from polyatomic ions that end in -ite use the suffix -ous.
•Ionic compounds are named by naming the cation first and the anion second.
•If the cation in an ionic compound is a polyatomic ion, Group 1 through 3, Group 13, Zn, Cd, or Ag ‐ no Roman numeral is needed to indicate charge.
•Ammonia is the compound NH3, ammonium is the polyatomic ion NH4+.

Mercury Ions

Mercury has two oxidation states, or charges.  They are mercury(I) and mercury(II).  While this may imply that the corresponding symbols for these ions are Hg2+ respectively, this is not the case.  The mercury(I) ion is found as Hg22+ which at first glance seems impossible.  However, the "2+" charge for a mercury(I) ion is possible because there are two mercury ions linked together.  Therefore, the total charge of two mercury ions is +2, making the value for each ion a +1.

Polyatomic Ions

There are some tricks for certain polyatomic ion "families."  Take a look at the examples below:
per- root -ate periodate IO4-    
root -ate iodate IO3- sulfate SO42-
root -ite iodite IO2- sulfite SO32-
hypo- root -ite hypoiodite IO-    

The above examples should show you the relationship between certain prefix/suffix combinations and the number of oxygens in the corresponding polyatomic ion.  The charge does not change, only the number of oxygens.  As is the case with sulfate (and many others), not all four ions exist, although it is possible to predict the name and charge.

There is another rule for ions that begin with the prefix "bi" or the word "hydrogen."  Firstly, in this context the two are synonyms of each other.  This means that bicarbonate and hydrogen carbonate refer to the same ion.  The relationship for the ions carbonate and sulfate are shown below: 
carbonate CO32- bicarbonate/hydrogen carbonate HCO3-  
sulfate SO42- bisulfate/hydrogen sulfate HSO4-