Atoms come together in groups to form three basic types of structures: first, ionic compounds that we have already talked about; second, molecular compounds that we haven’t discussed yet; and third, polyatomic ions
Polyatomic ions are groups of atoms that carry a positive or negative charge; like monatomic ions positive polyatomic ions are called cations and negative ions are called anions
A chart with a list of ions both monatomic and polyatomic can be found on this page below; each ion has a specific charge that is associated with the name it is given
Compounds formed with polyatomic ions must have balanced charge, just like those formed from monatomic ions
This means that in many compounds there will be more than one unit of a given polyatomic ion
This is reflected in the formulas of these compounds by the use of parentheses
In compounds of polyatomic ions the ions are treated as you would treat an element and the subscript after the parentheses tells you how many units of that ion you have
To find out how many atoms of a given element are in the formula containing one of these parenthetical polyatomic ions simply multiply the subscripts inside the parentheses by the subscript written outside them
A helpful thing to know is that you can figure out the charge on a polyatomic ion if you know the charge of the ion it is coupled with; for example, in the formula K2CO3 we know that the ion CO32- has a -2 charge because it required two K+ ions to balance its charge
Here are some examples of ionic compounds containing polyatomic ions:
Ba(SCN)2 NH4Cl Ba(OH)2 K(ClO3) Fe(OH)3 Fe(OH)2 Al(OH)3 Al2(SO4)3
Naming Salts
Any system for naming things is called a nomenclature; we will be talking about the nomenclature of ionic compounds
Monatomic cations (positive ions, remember) are named by adding the word ion after the name of the element; typically, metals form cations
Some examples of cation names are: Mg2+ magnesium ion, Fe2+ iron(II) ion, Fe 3+ iron(III) ion, and Na+ sodium ion
The Roman numerals after the name of the element tells you its charge: some metals can have a number of different charges; this number is not the subscript that it should be given in a formula; Fe2+ is read ‘iron two ion’
Monatomic anions (negative ions) are named by dropping the last syllable of the element’s name and adding the suffix -ide; typically, non-metals form anions
Some examples of anion names are: Cl- chloride, F- fluoride, and N3- nitride
Some metals naturally exhibit several different charges (as you may have noticed); other ions may also form under special conditions, such as the high level of ionization that occurs in the Sun: these ions are not what we will be using since we will be talking about the ions found in typical ionic compounds
Polyatomic ions have their own individual names which (unfortunately) are not so easy to describe as a group; you really just have to do your best to memorize them; a number of common ions are listed in the chart below
To name an ion, given its formula, just say or write the name of each ion in the order it is written; notice that cations always come before anions in formulas
To write the formula of a compound, given just its name is a bit more involved:
Given a name, write down the symbol corresponding to each name, in the order given in the name (for Iron(II) Chloride write Fe Cl)
The next step is to balance the charges; always balance charges in the lowest whole number ratio possible by finding the least common multiple of the charges (for Fe2+ and Cl- the least common multiple is 2; this means that you need two chlorides to balance one iron(II) ion)
Write the number needed of each ion as a subscript after each ion (FeCl2 for our example)
Remember that this works just as well for polyatomic ions as for monatomic ions; just use parentheses around the polyatomic ions to indicate if there is more than one by using a subscript
Also note that you do not write a subscript of ‘1’ for ions that appear only once in the formula
Finally, the formulas of ionic compounds are written in the smallest whole number ratios because that way they describe the compound no matter how much of it is present; this is why NaCl is not also written as Na2Cl2 or Na50Cl50; a formula written in this fashion is called an empirical formula
Measuring Salts: Moles and Masses
Rather than reiterate notes I have already written, see how to calculate
molar masses, and how to do all sorts of calculations with moles, here
Chart of cations and anions from Chemistry by Raymond Chang, McGraw Hill
Publishing, © 2002.