7.6 Halogens and Noble Gases

Halogens and noble gases are the last two groups on the periodic table, with vastly different properties. Halogens are highly reactive, forming compounds with most elements, while noble gases are typically inert due to their full outer electron shells.

These groups showcase the extremes of chemical reactivity. Halogens readily form ionic and covalent compounds, with applications in water treatment and materials science. Noble gases, though mostly unreactive, find uses in lighting, cryogenics, and specialized chemical reactions.

Halogen Properties and Reactivity

Halogen Characteristics and Trends

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• Halogens (Group 17) are highly reactive nonmetals that form diatomic molecules (F2, Cl2, Br2, I2) in their elemental state
• Electronegativity, atomic radius, and ionization energy decrease down the group, while melting point and boiling point increase
  • Fluorine has the highest electronegativity (4.0) and smallest atomic radius (42 pm), while iodine has the lowest electronegativity (2.5) and largest atomic radius (133 pm)
  • Melting points range from -220°C (F2) to 114°C (I2), and boiling points range from -188°C (F2) to 184°C (I2)
• Reactivity of halogens decreases down the group due to increasing atomic size and decreasing electronegativity
  • Fluorine is the most reactive, readily combining with most elements, while iodine is the least reactive
  • This trend is evident in the displacement reactions, where a more reactive halogen displaces a less reactive one from its compounds (F2 > Cl2 > Br2 > I2)

Halogen Compounds and Reactions

• Halogens readily form ionic compounds with metals, typically having a -1 oxidation state
  • Examples include sodium chloride (NaCl), potassium bromide (KBr), and calcium fluoride (CaF2)
  • These compounds have high melting points and are soluble in water, forming electrolytic solutions
• Halogens can also form covalent compounds with other nonmetals
  • Examples include hydrogen chloride (HCl), carbon tetrachloride (CCl4), and sulfur hexafluoride (SF6)
  • These compounds have lower melting points and varying solubilities in water
• In aqueous solutions, halogens can act as oxidizing agents, with the strength of oxidation decreasing from fluorine to iodine
  • Fluorine is the strongest oxidizing agent, capable of oxidizing water to oxygen gas
  • Chlorine is commonly used as a disinfectant in water treatment due to its oxidizing properties

Preparation and Applications of Halogen Compounds

Hydrogen Halides and Their Uses

• Hydrogen halides (HF, HCl, HBr, HI) can be prepared by the reaction of a halogen with hydrogen gas or by the reaction of a metal halide with a strong acid
  • Example: H2(g) + Cl2(g) -> 2HCl(g)
  • Example: NaCl(s) + H2SO4(aq) -> NaHSO4(aq) + HCl(g)
• Hydrogen halides are used in various applications
  • Hydrofluoric acid (HF) is used for etching glass and in the production of fluoropolymers (Teflon)
  • Hydrochloric acid (HCl) is used for cleaning and pickling metals, as well as in the production of vinyl chloride for PVC plastics
  • Hydrobromic acid (HBr) and hydroiodic acid (HI) are less common but have applications in organic synthesis and as reducing agents

Interhalogen Compounds and Halogen Oxides

• Interhalogen compounds (e.g., ICl, BrF3, IF5) are formed by the reaction of two different halogens and exhibit unique geometries and properties
  • Example: I2(s) + 3F2(g) -> 2IF3(g)
  • These compounds have varying oxidation states and can act as fluorinating agents or oxidizers
• Halogen oxides (e.g., Cl2O, Br2O) and oxyacids (e.g., HClO, HBrO3) are important in water treatment and as oxidizing agents
  • Chlorine dioxide (ClO2) is used for water disinfection and bleaching
  • Hypochlorous acid (HClO) is the active ingredient in household bleach and is used for sanitization
• Organic halogen compounds (e.g., chloroform, CFCs) have various applications but some are known to have adverse environmental effects
  • Chloroform (CHCl3) was historically used as an anesthetic but is now primarily used as a solvent and precursor to other compounds
  • Chlorofluorocarbons (CFCs) were widely used as refrigerants and aerosol propellants but have been phased out due to their role in ozone depletion

Unique Properties of Noble Gases

Physical Properties and Inertness

• Noble gases (Group 18) are monatomic gases with completely filled outer electron shells, making them highly stable and unreactive under standard conditions
  • This stability is due to the octet rule, where atoms are most stable with eight electrons in their outer shell
  • Examples of noble gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn)
• They have very low boiling points and melting points due to weak intermolecular forces (London dispersion forces)
  • Helium has the lowest boiling point of all elements at -269°C, while radon has the highest at -62°C
  • These low boiling points make noble gases useful as cryogenic liquids and in low-temperature applications

Applications of Noble Gases

• Noble gases have high ionization energies, making them excellent electrical insulators and useful in applications such as neon lighting and plasma displays
  • Neon signs are filled with neon or other noble gases, which emit characteristic colors when an electric current is passed through them
  • Plasma displays use a mixture of noble gases (typically xenon and neon) to create color images
• Helium has the lowest boiling point of all elements and is used in cryogenics
  • Liquid helium is used to cool superconducting magnets in MRI machines and particle accelerators
  • Helium is also used as a protective atmosphere for welding and in helium-filled balloons
• Heavier noble gases like xenon and krypton are used in high-intensity lamps and lasers
  • Xenon arc lamps are used in projectors and for UV curing of inks and coatings
  • Krypton lasers are used in various applications, including laser light shows and photolithography for semiconductor manufacturing

Reactivity of Halogen vs Noble Gas Compounds

Reactivity of Halogen Compounds

• Halogen compounds can participate in a variety of chemical reactions, including oxidation-reduction, nucleophilic substitution, and elimination reactions
• In redox reactions, halogens and their compounds can act as oxidizing agents, accepting electrons from other species
  • Example: 2Fe2+(aq) + Cl2(g) -> 2Fe3+(aq) + 2Cl-(aq)
  • The strength of oxidation decreases from fluorine to iodine compounds, with fluorine being the strongest oxidizer
• In nucleophilic substitution reactions, a halogen atom in an organic compound can be replaced by a nucleophile
  • Example: CH3Cl + OH- -> CH3OH + Cl-
  • This reaction is important in the synthesis of alcohols from alkyl halides and in the production of various pharmaceuticals and chemicals
• Elimination reactions involving halogen compounds can lead to the formation of alkenes or alkynes, depending on the reaction conditions and substrate
  • Example: CH2BrCH2Br + 2NaOH -> HC≡CH + 2NaBr + 2H2O
  • These reactions are useful in the synthesis of unsaturated hydrocarbons and in the production of polymers

Reactivity of Noble Gas Compounds

• Despite their general inertness, some noble gases (xenon and krypton) can form compounds under extreme conditions
  • Xenon fluorides (XeF2, XeF4, XeF6) and krypton fluoride (KrF2) are examples of such compounds
  • These compounds are formed by direct reaction of the noble gas with fluorine under high pressure and temperature
• Noble gas compounds exhibit unique reactivities due to the presence of highly electronegative ligands (e.g., fluorine) and the high oxidation states of the noble gas atoms
  • Xenon can form compounds with oxidation states ranging from +2 to +8, while krypton can form compounds with oxidation states of +2 and +4
• Xenon fluorides can act as strong fluorinating agents and oxidizers in organic synthesis
  • Example: C6H6 + XeF2 -> C6H5F + Xe + HF
  • They can also form stable salts with metal cations (e.g., XeF+, XeF3+), which have potential applications in chemical synthesis and as oxidizing agents