Atomic and Physical Properties of Group 7 Elements

The following topics will explore the trends in some atomic and physical properties of Group 7 elements (the halogens) – fluorine, chlorine, bromine, and iodine. We will cover trends in atomic radius, electronegativity, electron affinity, melting and boiling points, and solubility. Additionally, we will discuss the bond enthalpies (strengths) of halogen-halogen bonds (e.g., Cl-Cl) and hydrogen-halogen bonds (e.g., H-Cl).

Trends in Atomic Radius of group 7 (Halogen) Elements

Increasing Atomic Radius: As you move down the group, the atomic radius increases. This is due to the addition of electron shells, resulting in a greater shielding effect which reduces the effective nuclear charge felt by the outermost electrons.

Trends in Electronegativity

Decreasing Electronegativity: Halogens exhibit high electronegativity, meaning they have a strong tendency to attract electrons and form negative ions (X⁻). However, electronegativity values decrease from fluorine to iodine. This is because the increasing atomic radius and electron shielding reduce the nucleus’s pull on bonding electrons.

Trends in First Electron Affinity

Decreasing Electron Affinity: The electron affinity of halogens decreases from chlorine to iodine. Fluorine, despite being highly electronegative, has a slightly lower electron affinity than chlorine. This is due to the small size of the fluorine atom, which causes significant repulsion between the added electron and the electrons already present.

Trends in Melting and Boiling Points

Increasing Melting and Boiling Points: The melting and boiling points of halogens increase from fluorine to iodine. This indicates that the intermolecular forces (Van der Waals forces) become stronger as the size and mass of the molecules increase. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid.

Bond Enthalpies (Bond Energies or Bond Strengths)

Definition:

• Bond Enthalpy: Bond enthalpy is the energy required to break one mole of a covalent bond in the gas phase to produce individual atoms.

Examples:
Cl_2(g) → 2Cl_(g)
Br_2(g) → 2Br_(g)

Note: These reactions involve the gaseous state.

Bond Enthalpy in Halogens (X₂)

Trend:
The bond enthalpies of Cl-Cl, Br-Br, and I-I bonds decrease as expected down the group. However, the F-F bond deviates from this trend and is weaker than expected.

Reason of deviation in F-F bond:

1. Short Bond Length: The very short bond length in F₂ results in significant electron-electron repulsion within the molecule, weakening the bond.
2. Bond Strength Comparison: X-X bonds in Cl₂, Br₂, and I₂ are stronger than the F-F bond due to the involvement of d-orbitals in bonding for the heavier halogens, which helps stabilize the bond.

Bond Enthalpies in Hydrogen Halides (HX)

Trend:
When a halogen atom bonds with a hydrogen atom, the bond strength decreases as the halogen atom gets larger and more distant from the nucleus. This results in weaker attraction and weaker bonds, consistent with the observed trends.

Example:
H-Cl → H_(g) + Cl_(g)

Summary:

1. Atomic Radius: Increases down the group due to added electron shells.
2. Electronegativity: Decreases down the group as atomic radius and shielding increase.
3. Electron Affinity: Generally decreases down the group due to increased atomic size and shielding.
4. Melting and Boiling Points: Increase down the group due to stronger Van der Waals forces.
5. Bond Enthalpies: Cl-Cl, Br-Br, and I-I follow expected trends, while F-F is weaker than expected due to short bond length and high electron repulsion.