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How to Memorize the Charge on Polyatomic Ions

In general chemistry courses, students are presented with ions -- atoms or molecules that have gained or lost one or more electrons, resulting in a positive or negative charge. Many, but not all, monoatomic ions -- ions consisting of a single atom -- are learned quickly and easily since many of their charges correspond to the arrangement of elements on the periodic table. Polyatomic ions -- ions consisting of multiple atoms -- present students the challenge of memorization. However, even long lists of polyatomic ions may be memorized using some techniques to manage the information.

Things You'll Need

List of polyatomic ions

Instructions

- 1
  List the polyatomic ions to be memorized with their names, formulas and charges. Textbooks and professors generally provide a list of polyatomic ions needed for their course and while some of the ions may vary between lists, certain polyatomic ions are universally included such as sulfate, sulfite, nitrate, hydroxide and ammonium ions.
- 2
  Divide the list between cations (positively charged ions) and anions (negatively charged ions). Most of the polyatomic ions will be anions with often only two cations: ammonium and hydronium. Occasionally, additional cations may be provided for memorization and may be added to the cation list as needed.
- 3
  Divide the anions into columns according to their charge. Generally, the anions given will fall into a -1 charge, -2 charge or -3 charge but some lists will include polyatomic anions with greater charges such as pyrophosphate.
- 4
  Group similar ions within each charge list together and match these with similar ions in other columns. For example, sulfate and sulfite both fall within the list of -2 charge anions. The sulfate and sulfite ions may be matched to hydrogen sulfite and hydrogen sulfate ions in the -1 charge anion column as these polyatomic ions have a similar formula but with a hydrogen atom.
- 5
  Learn to distinguish polyatomic ions with the -ite and -ate suffixes. Polyatomic ions with the -ite suffix have fewer oxygen atoms than polyatomic ions with the -ate suffix. This aids distinguishing sulfite/sulfate, nitrite/nitrate and chlorite/chlorate ions.
- 6
  Build on the -ite/-ate relationship to include the naming system for halogen oxoanions. All halogen oxoanions have a -1 charge and are named using a hypo- prefix and -ite suffix if one oxygen atom is present, only the -ite suffix if two oxygen atoms are present, only the -ate suffix if three oxygen atoms are present, and the per- prefix and -ate suffix if four oxygen atoms are present. For example, bromine oxoanions include hypobromite (BrO-), bromite (BRO2-), bromate (BrO3-), and perbromate (BrO4-), all of which have a -1 charge.
- 7
  Notice that the addition of a hydrogen atom to a polyatomic ion reduces the charge as exemplified by hydrogen sulfite which has a -1 charge and sulfite which has a -2 charge.
- 8
  Memorize the polyatomic ions in the groups created using the patterns identified. Some polyatomic ions may fall within their own group on your list such as acetate and permanganate. However, many will be grouped together and the groups are easily learned together.

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