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Group 17 Elements: The Halogens

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

Property

Fluorine 

Chlorine

Bromine 

Iodine 

Physical State

Gas

Gas

Volatile liquid

Volatile solid

Intermolecular Forces

Weak

Weak

Increasing

Increasing

Atomicity

Diatomic(F2)

Diatomic (Cl2)

Diatomic (Br2)

Diatomic (I2))

Abundance in Earth's Crust

Highest

High

Moderate

Low

Electronic Configuration

1s² 2s² 2p⁵

[Ne] 3s² 3p⁵

[Ar] 3d¹⁰ 4s² 4p⁵

[Kr] 4d¹⁰ 5s² 5p⁵

Colour

Pale yellow

Yellowish green

Reddish brown

Deep violet

Metallic Character

Nonmetal

Nonmetal

Nonmetal

Nonmetal

Oxidation States

-1

-1 to +6

-1 to +6

-1 to +7

Bond Length (Å)

1.42

1.99

2.28

2.67

Bond Energy (kJ/mol)

158.8

242.6

192.8

151.1

Ionisation Potential

High

High

High

High

Solubility in Water

High

High

Moderate

Low

Solubility in Organic Solvents (CS2, CHCl3, CCl4)

-

Yellow solution

Brown solution

Violet solution

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

  1. 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.
  1. Oxyacids of Halogens

Halogens form oxyacids with general formulas HOX, HXO2, HXO3, and HXO4

Name

Oxidation State

Fluorine

Chlorine

Bromine

Iodine

Salt's Name

Hypohalous Acid

1

HOF

HOCl

HOBr

HOI

Hypohalite

Halous Acid

3

-

HClO2

-

-

Halite

Halic Acid

5

-

HClO3

HBrO3

HIO3

Halate

Perhalic Acid

7

-

HClO4

HBrO4

HIO4

Perhalate

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.
  1. 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

Halogen

Oxide Formula

Oxide Name

Fluorine

OF2

Oxygen Difluoride

F2O

Difluorine Monoxide

Chlorine

Cl2O

Dichlorine Monoxide

ClO2

Chlorine Dioxide

Cl2O7

Dichlorine Heptoxide

Bromine

Br2O

Dibromine Monoxide

BrO3

Bromine Trioxide

Iodine

I2O5

Diiodine Pentoxide

I2O

Diiodine Monoxide

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.

Frequently Asked Questions

Reactivity decreases down the halogen group. Fluorine is the most reactive, followed by chlorine, bromine, iodine, and astatine. This trend is due to the increasing atomic size and decreasing electronegativity as you move down the group.

Halogens share several common properties: They are nonmetals. They have high electronegativity. They form salts when combined with metals. At room temperature, halogens exist in all three states of matter: iodine and astatine are solids, bromine is a liquid, and fluorine and chlorine are gases. They are highly reactive, with reactivity decreasing down the group.

Halogens form compounds by gaining an electron to achieve an entire outer electron shell, forming negative ions (halides). They readily react with metals to form ionic compounds and nonmetals to form covalent compounds.

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