Halogen Group (Group 17) Trends
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1. All elements are diatomic and molecular and the boiling and melting points increase as a result of the increasing van der Waals interactions between diatomic molecules for the heavier elements.
2. The elements are typical non-metals in their physical and chemical properties. They form anionic compounds based on X- (X = halogen), which is associated with a complete octet.
- The ionic compounds MX become progressively less ionic as the relative atomic mass of X increases, because of the decreasing electronegativity of the halogens. Iodine has the greatest tendency to form cationic species, e.g., I2+, I5+, because it has the lowest ionization energy. The cation Br2+ is known in Br2+Sb3F16-, and Br5+ has been reported.
3. The atoms also form strong covalent bonds with other non-metals. The mean bond enthalpies for E-X bonds are particularly large for fluorine and therefore a wide range of molecular fluorides are known; fluorine is particularly effective at bringing out the highest valencies of the non-metals and highest oxidation states of the metals.
4. The oxidizing ability of the halogens decreases markedly down the group: F2 > Cl2 > Br2 > I2, and only iodine is oxidized by nitric acid.
5. The stabilities of the hydrogen halides decrease down the group, but their acid strengths increase.
6. Only H-F forms strong hydrogen bonds and this is reflected in the boiling and melting points of the hydrogen halides.
7. The halogens form many interhalogen compounds with the less electronegative halogen surrounded by the more electronegative halogens. Neutral, anionic, and cationic interhalogen compounds are known. ICl and IBr are widely used in organic synthesis and are commercially available.
- The most extensive series of compounds exists for iodine, e.g., IF7, IF5, ICl4-, ICl2-.Fluorine does not form any interhalogen compounds where it occupies the central position within the molecule.
8. Oxygen fluorides are extremely strong and reactive oxidants and have been explored as potential rocket fuels; the oxides become less reactive down the column and more numerous. Iodine forms a particularly wide range of oxides.
9. The perhalates, EO4-, are only known for Cl, Br, and I. They exhibit an alternation in their oxidizing abilities, and the perbromates are particularly strong oxidizing agents.
10. In the highest oxidation state (+7) the relative oxidizing ability is:
- Br> I > Cl
- and results in the formation of the corresponding +5 oxoanions,
- ClO4- + 2e- = ClO3- E° = 1.20 V
- BrO4- + 2e- = BrO3- E° = 1.85 V
- IO4- + 2e- = IO3- E° = 1.63 V
- The hypohalite ions disproportionate according to the equation:
- 2XO- = 2X- + XO3-
- The equilibrium constants are 1027 for ClO-/Cl- :(the reaction is slow at room temperature), 1015 for BrO-/Br-, and 1020for IO-/I-. HOF has been prepared from ice + F2 but is very reactive, decomposing to HF + O2.