Element(s) of the Day

1. Fluorine, F
2. Chlorine, Cl
3. Bromine, Br
4. Iodine, I

We treat these elements together because they are chemically quite similar

• occur, in the elemental form, as diatomic molecules
  • F₂, Cl₂, Br₂, I₂
• react quite vigorously with most metals to form ionic compounds
• The halogens all form Ions with -1 charge
  • that is, they efficiently steal 1 electron
  • with Na metal we get NaF, NaCl, NaBr and NaI
  • with Mg metal we get MgF₂, MgCl₂, MgBr₂ and MgI₂
• They tend to be found together
  • sea water is a major resource for Cl, Br and I
  • salt deposits are also source of Br- and I-
• Partly, nominated for element of the day
  • to point out the nature of our periodic table

• smaller molecules are often gases
• F₂ and Cl₂ are gases (open, room temperature)
• Br₂ is a liquid (easy to form vapor or gas)
• I₂ is a solid (easy to form vapor)

• F₂ is colorless
• Cl₂ is a slightly green-yellow gas
• Br₂ is a brown liquid, brown gas
• I₂ is a black solid, purple vapor and solutions

Uses:

• note the spelling fl-U-O-ine
• not fl-OU-r... as in flour for baking bread
• also applies to fl-U-O-rescent lamps, fl-U-O-rescence of minerals, fl-U-O-ride toothpaste
• familiar in dentistry
  • traces of Fluoride ion (not F₂) are added to water
  • very low concentrations (part per million)
  • reduce tooth decay
  • react with surface of tooth
    • forms CaF₂ (insoluble, tough)
  • -- at much higher concentrations F- is poisonous
    • important to recognize both species and amounts
    • most valuable / essential species are toxic at high levels
• fluorides are one of the few compounds that attack glass
  • can use to etch patterns into glass

• much more familiar,
  • Usually as the chloride-- salt
  • essential to life
  • important for salting roads
• Chlorine gas is powerful and toxic
  • used in WW-I gas warfare
    • terrible lung damage, often death
  • kills germs, too
    • disinfects water at few ppm level
    • chlorinate water supply
    • treat pools
    • treat sewage
  • often done indirectly (more safely)
    • can use Sodium Hypochlorite (Chlorox-TM) NaOCl
    • slowly releases Cl₂
• Important in many products
  • polyvinyl chloride -- one of the major plastics

• a little less reactive than Chlorine
• also good disinfectant (pools-- less eye irritation)
• Often made from sea water
  • very low concentration of Br-
  • bubble cheaper Cl₂ through it
  • get nearly 100% 2Br- + Cl₂ --> 2Cl- + Br₂
  • still dissolved and very dilute
    • but Br₂ will dissolve in water insoluble liquids
    • can collect and concentrate

• even less reactive than Bromine (but still reactive)
• good disinfectant
  • as a solid, easier to handle
  • dissolve in alcohol: Tincture of Iodine
  • combine with other species-- Betadine
  • tablets for campers-- disinfect water for drinking
• good source: Kelp (sea weeds)
  • traces in the oceans
  • plants tend to concentrate it
  • harvest plants and recover the iodine
  • can also find salt deposits much richer in Br and I
• Medical necessity (thyroid gland)
  • low Iodine diet --> goiter (enlarged thyroid)
  • salt is generally Iodized (small amount of naI added)
• very specific reaction: I₂ + I^- ---> I₃-
  • I₃- + starch ----> intense blue color
  • can use as a test for starch

• One point: there are a wider range of Cl, Br and I compounds
  • ClO₃- (chlorate ion) used in explosives
  • IO₃- (iodate) and IO₄- (periodate)

demonstration

• 1. simple iodine clock
• 2. oscillating iodine clock
  • see special web site clock reactions for more details (optional)