Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 8.24 Trends (mp, bp, electronegativity, reactivity)-Study Notes - New Syllabus

8.24 Trends in Group 7 (Halogens)

Group 7 elements (halogens) include \( \mathrm{F_2, Cl_2, Br_2, I_2} \). Their properties show clear trends down the group due to changes in atomic size, intermolecular forces, and electron attraction.

(i) Melting & Boiling Points + Physical State

Trend

• Melting and boiling points increase down the group.
• Physical state changes:
  • \( \mathrm{F_2, Cl_2} \) → gases
  • \( \mathrm{Br_2} \) → liquid
  • \( \mathrm{I_2} \) → solid

Explanation

• Halogens exist as diatomic molecules (\( \mathrm{X_2} \)).
• Intermolecular forces = London dispersion forces.
• Down the group:
  • Number of electrons increases
  • Electron cloud becomes more polarisable
  • Stronger intermolecular forces
• More energy needed to separate molecules → higher m.p./b.p.

(ii) Electronegativity

Trend

• Electronegativity decreases down the group.

Explanation

• Atomic radius increases.
• Increased shielding effect.
• Nuclear attraction for bonding electrons decreases.
• Therefore, ability to attract electrons decreases.

Key Fact

• Fluorine is the most electronegative element.

(iii) Reactivity

Trend

• Reactivity decreases down the group.

Explanation

• Halogens react by gaining an electron:

\( \mathrm{X_2 + 2e^- \rightarrow 2X^-} \)

• Down the group:
  • Atomic radius increases
  • Shielding increases
  • Attraction for incoming electron decreases
• Therefore, less ability to gain electron → lower reactivity

Displacement Reactions (Evidence)

• More reactive halogen displaces less reactive one:

\( \mathrm{Cl_2 + 2Br^- \rightarrow 2Cl^- + Br_2} \)

Summary

• m.p./b.p. increase due to stronger intermolecular forces.
• Electronegativity decreases due to shielding.
• Reactivity decreases due to reduced electron attraction.

Therefore, trends in Group 7 are controlled by increasing atomic size and shielding down the group.