Tips for the Teacher
Language of Chemistry
Bromine nomenclature

Bromide                             Br
Hydrobromic acid              HBr(aq)
Bromine                             Br2
Hypobromous acid             HOBr
Hypobromite                      BrO
Bromous acid                     HBrO2
Bromite                              BrO2
Bromic acid                       HBrO3
Bromate                             BrO3
Perbromic acid                  HBrO4
Perbromate                        BrO4
Bromine dioxide                BrO2
Dibromine oxide                Br2O

Chlorine nomenclature
Chloride                             Cl
Hydrochloric acid              HCl(aq)*
Chlorine                             Cl2
Hypochlorous acid             HOCl
Hypochlorite                      ClO
Chlorous acid                     HClO2
Chlorite                              ClO2
Chloric acid                       HClO3
Chlorate                             ClO3
Perchloric acid                  HClO4
Perchlorate                        ClO4
Chlorine dioxide               ClO2
Dichlorine oxide               Cl2O

*Gaseous HCl should be called hydrogen chloride.
See overhead transparency master for structure of oxoanions.

Fluorine nomenclature
(Given its high electronegativity, it is limited to –1 and 0 oxidation states)
Fluoride                              F
Hydrofluoric acid               HF(aq)*
Fluorine                              F2
Oxygen difluoride               OF2

*Gaseous HF should be called hydrogen fluoride

Iodine nomenclature
Iodide                                 I
Hydroiodic acid                 HI aq)
Iodine                                 I2
Hypoiodous acid                HIO
Hypoiodite                         IO
Iodous acid                        HIO
Iodite                                 IO2
Iodic acid                          HIO3
Iodate                                 IO3
Periodic acid                     HIO4
Periodate                            IO4
Iodine dioxide                    IO2
Diiodine oxide                   I2O

Fluorocarbon – A (CF2 ) n polynuclear species

Freon – Organic chlorofluorocarbon compound; i.e., a hydrocarbon with chloro and fluoro substitution in place of some or all hydrogens of the parent hydrocarbon

General Tips
1. Emphasize to students that bleach and ammonia should never be mixed.  Chloramine, the result of reaction between chlorine or hypochlorites with ammonia, is very toxic. This caution is also important regarding household cleaning materials, where again, products containing ammonia or ammonia derivatives should not be mixed with products containing chlorine or chlorine derivatives.

2. The mix of laboratory activities and demonstrations you select in teaching about halogens depends on fume hood facilities available to you and your students. Safety considerations are vitally important with halogens.

3. Check regarding any respiratory problems or color blindness of students before undertaking this module. Students with respiratory problems should be allowed to observe the laboratory activities, but should probably not be allowed to come close enough to conduct the activities themselves. Color-blind students should be told the observed colors in cases where a color change is undetectable by them.

Pattern Recognition
1. Periodic Table and oxidizing ability . This module allows students to develop relationships between the oxidizing ability of elements and their Periodic Table positions. From Laboratory Activity 2, students learn that Cl2 > Br2 > I2 in terms of oxidizing ability. The extension to F2 as the strongest is straightforward.  Extension to the left on the Periodic Table is hinted at in questions provided with the laboratory activity—you can expand upon this, if you wish. Sulfur (relative to chlorine) provides a simple example beyond oxygen.

2. Periodic Table and halide acid strength . Although the hydrohalic acids HCl, HBr, and HI are essentially completely ionized in aqueous solution, HF is a weaker acid. The small size of fluorine and hydrogen atoms can be used to justify the greater attraction between the two. This can be extended to the left in the Periodic Table as well—for example, water is a much weaker acid than is hydrogen sulfide.

3. Periodic Table and atomic and ionic sizes . As just noted, fluoride is the smallest halide ion and fluorine is the smallest halogen molecule. The sizes of halogens can be related to their physical properties, e.g., electrons on larger halogen molecules are less tightly held by the nuclei and interact with neighboring halogen molecules (London dispersion forces) more strongly.  Halogens become less volatile as the group is traversed from top to bottom, from gases (F2 and Cl2 ), to liquid (Br2 ), and to solids (I2 and At2 ) at room temperature.

Common Student Misconceptions
1. “HF is a strong acid.”

This incorrect notion arises, in part, since hydrofluoric acid is remarkably
reactive, particularly with glass (silica):

SiO2(s) + 6HF(aq) ---> SiF62–(aq) + 2H3O+(aq)
However, HF is a weak acid in terms of its low ionization in water.

2. “Chlorine hysteria.”

Many individuals and some environmental groups have what could be
termed “chlorine hysteria”—they seem unable to ascertain the important
difference between chlorine and chlorides.

3. “All silver halides are insoluble in water.”

Actually, silver fluoride, AgF, has very high water solubility (greater than
10 mol/L at room temperature).

4. “The light in a halogen lamp is caused by a halogen.”

A halogen lamp is actually a tungsten filament lamp with iodine added to
trap tungsten atoms as they vaporize from the filament. The lamp body is
intentionally small; this insures the lamp is hot enough to keep the tungsten
iodide in the vapor state until it hits the very hot filament. There it
decomposes back to tungsten and iodine, allowing the tungsten to return to the filament.

Problem Solving
1. Identification of a laboratory unknown (see Laboratory Activity 2 )

2. Risk assessment (see Decision Making)

3. Solving the environmental pollution problem—this can be handled in a
fashion analogous to the fluoridation example described in Decision Making.

Decision Making

This module’s Group Activities includes a section on water fluoridation as a preventive measure to help prevent tooth decay. A useful (although time consuming) event that will help students become more responsible citizens is to stage a public hearing (debate) on a hypothetical ballot question asking voters to decide whether this should be done. Divide the class into two groups, establish hearing procedures, appoint a moderator (yourself?) and challenge each side to get the facts and support their side.

A key to the public hearing is to explore whether the risk of fluoridation is outweighed
by its benefits. Students will be forced to decide and then support a position
regarding this issue.

This hearing could be held before a larger audience and be videotaped—or whatever
else you think will help instill a sense of “reality.” You need to encourage, however,
sound scientific arguments and respect for alternative beliefs. The opportunity to
practice tolerance and understanding of other positions is one of many benefits of this
type of activity.

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