Category: Best Practices in Teaching Chemistry
Title: How to Teach Polyatomic Ions in Chemistry
Presenter: Professor John T. Taylor
Florida State College at Jacksonville
The presenter has examined how the textbooks and faculty peers in high school, introductory and general college chemistry courses approach teaching the names and formulas of polyatomic ions. A survey of over 50 texts at the three levels show all textbooks introduce polyatomic ions, some devote a separate section in the compound chapter, while others may have only one paragraph, but all have a listing of common polyatomic ions in a table. Chemistry faculty approach the subject in three ways. Some give the students a list (or the table from the textbook) and require them to memorize the formulas and the charge. Others provide the students with a list, and allow the students to use the list on exercises and tests. Some only allow the students to use the list on only one test, and expect the students to learn through rote usage the formulas and charges and do not allow the list on future tests. A search of Google reflects 137,000 hits for polyatomic ions. Most are static lists/tables. Many web sites go the next step to draw the Lewis Dot Structure of polyatomic ions. The presenter has developed two mnemonics that allow students to look at a periodic chart and write the formula and charge of polyatomic ions without memorization. A web site has been developed for student to attempt a self discovery of the mnemonics, but has been under revision since last summer. The web site is:
Two decades ago there was a movement in the chemical education division to perform a general education reform similar to the projects of calculus reform a few years earlier. The topics and the total content were examined by three processes, one was zero based content, one was to add and remove content, and the third was through laboratory based discovery methods. The presenter began a research project to examine how the textbooks and his peers in high school, introductory and general college chemistry courses approach the names and formulas of polyatomic ions. My question was “Which polyatomic ions do students need to know to survive a chemistry course?”
The presenter surveyed over 50 texts at the three levels. All textbooks introduce polyatomic ions, some devote a separate section in the compound chapter, while others may have one paragraph, but all have a listing of common polyatomic ions in a table.
Chemistry faculty address the subject in three ways.
1. Some give the students a list (or the table in the textbook) and require them to memorize the formulas and the charge.
2. The others provide the students with a list, but allow the students to use the list on exercises and tests.
3. Some only allow the students to use the list on the next test, and expect the students to learn through rote usage the formulas and charges, but do not allow the list on future tests.
P.S. Also, some faculty provide a crutch by allowing the students to write notes, problem solving formulas, and also polyatomic ions on a single piece of paper to use during a test.
A search of Google reflects 137,000 hits for polyatomic ions. Most are static lists/tables. Many web sites go the next step to draw the Lewis Dot Structure of polyatomic ions.
The presenter uses the apparent guideline from the ACS examinations. There are no tables of polyatomic ions on any of the introductory or general chemistry level examinations. Students need to know the formulas and the charges of common polyatomic ions.
The Presenter’s dilemma about polyatomic ions began when he was a high school chemistry student. On a Monday the teacher provided each student with a list of 50 polyatomic ions and explained that there would be an exam the next day on the ions. “Here memorize this list!” There was no instruction as she began teaching students to write chemical formulas of ionic compounds. During the week before, monoatomic anions and cations had been introduced via a table to memorize and there was a test of ions on the previous Friday. Covalent bonding had not been introduced nor had atomic theory or the periodic chart element’s ionic charge based on the position of the Roman Numeral A Group Elements..
That night the young presenter spent hours trying to memorize the list using flash index cards he made. He could not remember which oxyanions had three oxygen atoms and which had four, but saw the –ite cards had the same charge but one less oxygen. The charges made no sense, but all the anions except ammonium (and hydronium) had negative charges, most were -3, -2, or -1. The young presenter was getting the actual numeric charges mixed up. There were too many ions. About midnight, his text opened to the page with the periodic table. He got the idea to arrange the –ate file cards into a table resembling the periodic table. He noticed a pattern for the formula, while the charges were later explained by understanding atomic theory and covalent bonding. During the presenter’s first year of teaching high school chemistry the two algorithms were finally developed such that a student may look at a periodic chart and write formula and charge of over 30 polyatomic ions from two rules.
Here is the summary of the problem:
Knowing dot structures of polyatomic ions, and some keen observations the student can boil it down to six questions:
1. What is the formula for the –ate polyatomic ion?
2. What is the charge on –ate polyatomic ion?
3. What happens when you attach hydrogen atom(s) to the polyatomic 2- and 3- oxyanions
4. What does –ite mean?
5. How do the hypo- and per- prefixes apply to polyatomic ions?
6. What are the two –ide polyatomic anions and two -ium positive cations?
The following web site demonstrates almost a complete listing of all polyatomic ions found the CRC Handbook of Chemistry and Physics:
Another web site has been initially developed for a student to discover the two algorithms. The URL address is:
By the time this conference is held, the presenter may have participated in the Summit Conference on ChemEd DL/ChemEd Research, July 11-15, 2011 in Washington D.C where he hopes to farther develop this site for posting on the ChemEd Server . Otherwise, this web site will be improved for better navigation, instruction and possibly a Microsoft Agent.
The presenter hopes to enlist other chemistry faculty in testing this process in their Fall 2011 and Spring 2011 classes as part of a chemical education research project. During the Fall and Spring terms this academic year, he is testing classes he teaches. He is presenting ne class with the traditional method listed above (Here memorize this table); while the other groups go through the discovery exercise.
Each student will be required to complete the Interactive online exercises before the second exam which includes nomenclature:
Recognition of Polyatomic Names from the given formula and charge:
Recognition of the Polyatomic Formula and charge given the Name of the ion:
(for the control group the above web sites will be modified to remove the algorithm images)
Before the Second Exam, he will administer a pop quiz on 30 polyatomic ions and formulas/charges. Then just before the final exam each term he plans to give the same pop quiz of 30 polyatomic ions. Each set of students will have a very plain periodic chart to use.