In the previous post, the point of discussion was monatomic ions, particularly predicting the charges on monatomic ions, and naming them, too. This post will add a level of complexity to things, because we’ll discuss polyatomic ions.
As the name suggests, a polyatomic ion is one in which two or more atoms are chemically bonded together, and because they are ionic, carry a net charge. When discussing polyatomic ions, you will often encounter oxyanions, which are anions consisting of oxygen along with another element, that is referred to as the characteristic or central element. Examples of oxyanions are chlorite (ClO2-) and chlorate (ClO3-). From this, you can see that oxyanions tend to have a stem name from the central element, and have a suffix –ite that indicates the lesser number of oxygen atoms in the oxyanion, as well as a suffixe –ate that indicates the greater number of oxygen atoms in the oxyanion. Another example of this is the nitrite ion, NO2-, and the nitrate ion, NO3-. Keep in mind that these suffixes actually don’t indicate how many oxygen atoms are in the oxyanion. The suffixes only tell you the relative number (Ebbing and Gammon 2009).
Sometimes, there are cases in which more than two oxyanions of a substance exist. In such a case, as with oxyanions of chlorine, a couple of which were listed above, a different approach is taken to naming. There are actually four different oxyanions of chlorine, ClO-, ClO2-, ClO3-, and ClO4-. In a case like this, the prefixes hypo— and per—are used, where hypo—would be added to the oxyanion with the least number of oxygen atoms in the case of chlorine, making ClO- hypochlorite ion. For the oxyanion with the second to least number of oxygen atoms in the case of chlorine, ClO4- is known as perchlorate ion. Next, the two oxyanions that have greatest number of oxygen atoms are named using the suffix –ate and the prefix per—. ClO3- is given the name chlorate ion and ClO4- is given the name perchlorate ion (Ebbing and Gammon 2009).
Some polyatomic ions are actually oxyanions with hydrogen atoms attached. Because an H+ ion is provided, these types of oxyanions are often referred to as acid anions. An example of this is monohydrogen phosphate ion, written as HPO42-. This ion is very similar to the phosphate ion, PO43-, with a hydrogen bonded, making the charge less negative. Dihydrogen phosphate is another example, which contains two hydrogen ions, as indicated by the prefix di—, meaning two. Note also that the prefix mono— means one (Ebbing and Gammon 2009).
You will also probably encounter in your study of polyatomic ions, thiosulfate ion, S2O32-. Thio—is a prefix, and it indicates that an oxygen atom in the root ion name has been replaced by a sulfur atom. The root ion is SO42-, so you can see how this works. And if you’ve been paying attention, you will know why the charge on the polyatomic ion does not change (Ebbing and Gammon 2009).
Overall, most polyatomic ions that you will encounter are anions. Cations are much less common as polyatomic ions. Two notable examples are the mercury(I) ion that is also known as the mercurous ion, and the ammonium ion. It is uncommon for a metal ion to be polyatomic, but mercury(I) ion is one case of a polyatomic metal cation, written Hg22+. Remember that the number in the parentheses indicates the charge on a particular atom of an ion. Ammonium is another polyatomic cation that you will become familiar with in your studies of chemistry, and it is composed only of nonmetal atoms, which is another uncommon feature of polyatomic ions. It is written as NH4+ (Ebbing and Gammon 2009).
Though this post was just a brief introduction to polyatomic ions, you should now have an understanding of some important things to know about these ions. For a future post, I am considering creating a diagram with some common monatomic ions as well as polyatomic ions. This would not really be for the purpose of memorization, but for reference. Also, look forward to some upcoming MCAT questions. We are closing in on chapter two of general chemistry and that is why I have been focusing heavily on it as opposed to other subjects. Mentally, I am preparing myself to soon do a much-needed chapter review. Good luck and happy studying! I hope to have you around for the next post.
A prospective medical student, looking to help others succeed.