Group 17 Elements: The Halogens
Halogen elements are a group of highly reactive nonmetals found in Group 17 (Group VIIA) of the periodic table. "halogen" means "salt-former," as these elements readily react with metals to form salts. Due to their reactivity and ability to form compounds with many elements, halogens play crucial roles in various industrial, medical, and chemical applications.
1.0What Are Halogens?
Six periodic table elements are in Group 17 (Group VIIA). These elements share specific properties.
The halogen group comprises fluorine (F), chlorine (Cl), bromine (Br), and iodine (I) and occasionally includes astatine (At) and tennessine (Ts). While Ts and At show some metallic properties and don’t form negative ions like other halogens, they are often excluded from discussions about typical halogen behaviour.
Therefore, the focus is typically on four elements: fluorine, chlorine, bromine, and iodine.
These elements are called halogens (from the Greek "hals," meaning "salt," and "gennan," meaning "to form or generate") because they are the salt formers.
In their elemental states, halogens form diatomic molecules (X₂) with nonpolar covalent single bonds. They are highly reactive and rarely found uncombined. Fluorine is the most reactive, while astatine is the least.
Halogens combine with Group 1 elements to form salts that exist as halide anions with a -1 charge (e.g., Cl⁻ is "chloride"). They act as oxidizing agents and are involved in oxidation-reduction reactions. In organic compounds, halogens typically form single bonds with carbon or nitrogen. When substituting a covalently-bonded hydrogen atom, they use the prefixes halo-, fluoro-, chloro-, bromo-, or iodo-.
2.0General Characteristics of Halogens
The important characteristics of the halogen family are listed below:
Density Order: F > Cl > Br > I
Electron Affinity Order : Cl > F > Br > I
3.0Chemical Properties of Halogens
Oxidizing Power
- Strong oxidizers; power decreases down the group.
- Cl2: Oxidizes Br - to Br2 and I- to I2.
- Br2: Oxidizes I- to I2.
Reaction with Hydrogen
- Form acidic hydrogen halides; reactivity and acidity decrease down the group.
- H2 + F2 → 2HF (dark)
- H2 + Cl2→ 2HCl (sunlight)
- H2 + Br2 → 2HBr
- H2 + I2 → 2HI
Reaction with Oxygen
- Form unstable oxides (X2O to X2O7).
Reaction with Metals
- Form ionic metal halides.
- 2Na (s) + Cl2 (g) → 2NaCl (s)
Reaction with Other Halogens
- Form interhalogens (XYn, n = 1, 3, 5, 7).
4.0Compounds of Halogens
- Hydracids (HX)
Halogens combine with hydrogen to form hydracids (HX).
- Reaction: H2 + X2 → 2HX
- HF and HCl: Obtained by heating fluorides/chlorides with concentrated H2SO4.
- CaF2 + H2SO4 → CaSO4 + 2HF
- 2 NaCl + H2SO4 → Na2SO4 + 2HCl
- HBr and HI: These cannot be prepared using the above method as they reduce concentrated H2SO4. Instead, they are prepared by hydrolysis of phosphorus halides.
- PX3 + 3H2O → H3PO3 + 3HX (X = Br, I)
Properties:
- Boiling Point: HF > HI > HBr > HCl (due to hydrogen bonding in HF)
- Acid Strength, Reducing Character, Bond Length: HI > HBr > HCl > HF
- Thermal Stability, Dipole Moment, Bond Strength, Stability: HI < HBr < HCl < HF
- Etching of Glass: HF reacts with silica in glass.
- Oxyacids of Halogens
Halogens form oxyacids with general formulas HOX, HXO2, HXO3, and HXO4
Trends:
- Stability, Acid Strength, Oxidizing Nature: HXO4 > HXO3 > HXO2 > HOX
- Conjugate Base Stability: Increases with more oxygen, making acids stronger.
Important Acids :
- Hypohalous Acids (HOX): Formed by halogen disproportionation.
Acid strength: HOCl > HOBr > HOI.
- Halic Acids: Strong oxidizers (HClO3, HBrO3, HIO3).
Uses: NaClO3 (weed killer), KClO3 (fireworks).
- Perhalic Acids: Obtained via halite oxidation. Strong oxidizers.
- Oxides of Halogens
Halogens form various types of oxides but do not combine directly with oxygen. Indirect methods prepare these oxides.
- Halogen oxides are powerful oxidizing agents.
- They decompose explosively when heated or struck
Interhalogen Compounds
Interhalogen compounds are molecules composed solely of halogen atoms (fluorine, chlorine, bromine, iodine, or astatine) without any atoms from other element groups.
- Created when different halogen atoms react with each other.
- Exhibit characteristics intermediate between the two parent halogens, with some unique differences. E.g. ClF (Chlorine Monofluoride), BrCl (Bromine Chloride), ICl (Iodine Chloride), IF7 (Iodine Heptafluoride)
- Exhibit intermediate properties of parent halogens.
Preparation:
- Direct Combination:
- Pure halogens react under specific conditions.:
- I2 + 3Cl2 → 2ICl3
Polyhalide Ions:
- Ions with more than two halogen atoms, e.g., I3−.
Iodine Cations:
- Forms I+ and I3+ e.g., (CH3COO)3I , I(ClO4)3.
5.0Important Uses of Halogens
- Fluoro derivatives are used as solvents, lubricants, refrigerants, fire extinguishers, fungicides, germicides, dyes, and plastics. They are crucial in forming uranium hexafluoride (UF6) for separating U-235, preparing Teflon (polytetrafluoroethylene), and manufacturing tetraethyl lead (TEL), an antiknock agent in fuels.
- Chlorine is a key bleaching agent and disinfectant. It produces chloroform, carbon tetrachloride, DDT, antiknock fuel compounds, bleaching powder, and phosgene. Additionally, it is involved in making tear gas and mustard gas.
- Sodium bromide (NaBr) and potassium bromide (KBr) are sedatives in medicine. Silver bromide (AgBr) is utilized in photography for film production, while ethylene bromide enhances the efficiency of tetraethyl lead (TEL), an antiknock agent used in fuels.
- Tincture of iodine acts as an antiseptic for wounds, while iodex and iodoform are employed in various medicinal preparations for their antiseptic properties. Potassium iodide (KI) and iodine (I₂) also treat goitre.