GROUP VII ELEMENTS

Group VII Elements

Overview

  • Group 7 elements are called halogens.

  • Elements include: fluorine, chlorine, bromine, iodine, and astatine.

  • Each has seven electrons in its outer shell.

  • All halogens are non-metals and exist as diatomic molecules (e.g., Cl₂).

Objectives

At the end of this lesson students will be able to:

  • Explain variations in physical properties in terms of structure and bonding.

  • Discuss relative reactivities as oxidizing agents.

  • Describe reactions with hydrogen and thermal stabilities of hydrides.

  • Examine reactions of halide ions with AgNO3 and concentrated sulfuric acid.

  • Explain chlorine reactions with sodium hydroxide in hot and cold conditions.

Physical Properties of Halogens

General Trends

  • State at Room Temperature:

    • Gaseous: F₂ and Cl₂

    • Liquid: Br₂

    • Solid: I₂

  • Atomic Radius:

    • Increases down the group as additional filled energy shells are added.

  • Melting and Boiling Points:

    • Increases down the group due to stronger attractive forces and van der Waals dispersion forces.

  • Volatility:

    • Decreases down the group due to stronger intermolecular forces.

  • Density:

    • Increases due to closer packing of atoms.

  • Electronegativity:

    • Decreases down the group due to increased atomic radius affecting attraction to bonding electrons.

Bonding Types

Ionic Bonding

  • Halogens form ionic bonds with electropositive metals, gaining an electron and becoming halide ions (X⁻).

  • Example:

    • X + e⁻ → X⁻

    • I₂(aq) + I⁻(aq) → I₃⁻(aq) (Polyhalide ions)

Covalent Bonding

  • Halogens can share electrons with other atoms to form covalent bonds.

  • Example:

    • H₂(g) + Cl₂(g) → 2HCl(g)

Reactivities of Halogens as Oxidizing Agents

Definition

  • Oxidizing agent: an element that can accept or gain electrons.

Reactivity Trends

  • Reactivity decreases down the group, with fluorine being the most powerful oxidizing agent.

  • Standard electrode potentials are more positive for stronger oxidizing agents.

Examples of Reactions

  • Fluorine can oxidize water:

    • 2F₂ + 2H₂O → 4HF + O₂

  • Chlorine can oxidize bromide ions:

    • Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)

  • Bromine oxidizes iodide ions:

    • Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)

Reactions of Halogens with Hydrogen

Reaction Conditions

  • Fluorine: Explosive even at low temperatures.

    • H₂(g) + F₂(g) → 2HF(g)

  • Chlorine: Slow in the dark, explosive in sunlight.

    • H₂(g) + Cl₂(g) → 2HCl(g)

  • Bromine: High temperatures required;

    • H₂(g) + Br₂(g) → 2HBr(g)

  • Iodine: Slow and reversible; yields low amounts.

    • H₂(g) + I₂(g) ⇋ 2HI(g)

Relative Thermal Stabilities of Hydrides

General Characteristics

  • Hydrides: Compounds of hydrogen with another element.

  • All hydrogen halides are colorless gases except HF.

  • Boiling points increase with molecular weight but HF has the highest boiling point due to strong hydrogen bonding.

Bond Strength Trends

  • H-X bond length increases down the group, leading to weaker bonds and decreased thermal stability.

  • The strength of hydrohalic acids increases down the group, with HI being the strongest.

Reaction of Halide Ions

Key Reactions

  • Most metal halides are soluble except Pb²⁺, AgCl, AgBr, and AgI.

  • Reactions can test for halides' presence:

    • Ex: Pb²⁺ + 2Cl⁻ → PbCl₂(s)

Specific Reactions

  • With Concentrated Sulfuric Acid:

    • HCl oxidized to Cl₂, HBr to Br₂, HI to I₂.

Questions for Review

  1. What are the physical states and colors of chlorine, bromine, and iodine at room temperature?

  2. Describe reactions between:

    • (a) bromine and iodide ions;

    • (b) bromine and chloride ions.

  3. Describe halogen reactions with hydrogen with balanced equations.

  4. What is the difference between the reaction of chlorine with cold and hot NaOH?

  5. Explain thermal stability trends of Group VII hydrides.

Group VII Elements

Overview

Group VII elements, known as halogens, include fluorine, chlorine, bromine, iodine, and astatine. These elements all possess seven electrons in their outer shell, making them distinct in their chemical characteristics. All halogens are non-metals and primarily exist as diatomic molecules, such as Cl₂.

Objectives

By the end of this lesson, students will be able to explain variations in physical properties in terms of structure and bonding, discuss the relative reactivities of halogens as oxidizing agents, describe their reactions with hydrogen and the thermal stabilities of their hydrides, examine reactions of halide ions with AgNO₃ and concentrated sulfuric acid, and explain the reactions of chlorine with sodium hydroxide in both hot and cold conditions.

Physical Properties of Halogens

General Trends

In terms of physical states at room temperature, fluorine (F₂) and chlorine (Cl₂) are gases, bromine (Br₂) is a liquid, and iodine (I₂) is solid. The atomic radius of halogens increases down the group due to the addition of filled energy shells. Consequently, melting and boiling points increase down the group, resulting in stronger attractive and van der Waals dispersion forces. However, volatility decreases as one moves down the group due to stronger intermolecular forces, while density increases due to closer packing of atoms. Electronegativity also decreases down the group as the increasing atomic radius affects the attraction to bonding electrons.

Bonding Types

Halogens can form ionic bonds with electropositive metals by gaining an electron and becoming halide ions (X⁻), represented by the equation X + e⁻ → X⁻. An example of a polyhalide ion formation is I₂(aq) + I⁻(aq) → I₃⁻(aq). Additionally, halogens can share electrons to form covalent bonds, as illustrated in the reaction H₂(g) + Cl₂(g) → 2HCl(g).

Reactivities of Halogens as Oxidizing Agents

An oxidizing agent is defined as an element that can accept or gain electrons. The reactivity of halogens as oxidizing agents decreases down the group, with fluorine being the most powerful oxidizing agent. Standard electrode potentials become more positive for stronger oxidizing agents. For instance, fluorine can oxidize water according to the reaction 2F₂ + 2H₂O → 4HF + O₂. Chlorine can oxidize bromide ions in the reaction Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq), and bromine can oxidize iodide ions as shown in Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq).

Reactions of Halogens with Hydrogen

The reactions of halogens with hydrogen vary based on the halogen involved. Fluorine reacts explosively even at low temperatures (H₂(g) + F₂(g) → 2HF(g)), while chlorine's reaction is slow in the dark and explosive in sunlight (H₂(g) + Cl₂(g) → 2HCl(g)). Bromine requires high temperatures for its reaction (H₂(g) + Br₂(g) → 2HBr(g)), whereas iodine reacts slowly and reversibly, producing low amounts of HI (H₂(g) + I₂(g) ⇋ 2HI(g)).

Relative Thermal Stabilities of Hydrides

Hydrides are compounds formed between hydrogen and another element. Most hydrogen halides are colorless gases, with the exception of HF, and their boiling points tend to increase with molecular weight. Notably, HF possesses the highest boiling point due to strong hydrogen bonding. Bonds in hydrohalic acids weaken down the group as H-X bond length increases, which leads to decreased thermal stability, with HI being the strongest acid in this series.

Reaction of Halide Ions

Most metal halides are soluble, with notable exceptions such as Pb²⁺, AgCl, AgBr, and AgI. Certain reactions can be employed to test for the presence of halides; for example, Pb²⁺ + 2Cl⁻ → PbCl₂(s). When reacting with concentrated sulfuric acid, hydrochloric acid (HCl) is oxidized to chlorine (Cl₂), hydrobromic acid (HBr) to bromine (Br₂), and hydroiodic acid (HI) to iodine (I₂).