Question

Draw structures for (a) \((\mathrm{HF})_{2}\) and (b) \(\mathrm{HF}_{2}^{-}\)

Short answer

(HF)_2 has hydrogen-bonded HF molecules, HF_2^- is linear with an extra electron.

Step-by-step solution

Understand the Composition

For both (a) \((\mathrm{HF})_{2}\) and (b) \(\mathrm{HF}_{2}^{-}\), identify the components. In \((\mathrm{HF})_{2}\), there are two HF molecules, suggesting a dimer structure. In \(\mathrm{HF}_{2}^{-}\), there are two fluorides and one hydrogen, suggesting a linear ion structure.

Explore Hydrogen Bonding in (HF)_2

In \((\mathrm{HF})_{2}\), recognize that one HF acts as a hydrogen bond donor and the other as an acceptor. Draw two HF molecules with one hydrogen bond forming between the hydrogen of one HF and the fluorine of another HF.

Analyze Electron Configuration in HF_2^-

In \(\mathrm{HF}_{2}^{-}\), there are extra electrons because of the negative charge. The molecule consists of F-H-F with a single hydrogen trapped between two fluorines. The negative charge results in a linear structure due to resonance or delocalized electrons.

Draw the Structure for (HF)_2

For \((\mathrm{HF})_{2}\), draw two HF molecules parallelly, where one H is hydrogen-bonded with the F of the adjacent HF molecule: H--F...H--F.

Draw the Structure for HF_2^-

For \(\mathrm{HF}_{2}^{-}\), draw a single hydrogen sandwiched linearly between two fluorine atoms: F--H--F. The extra electron explains the negative charge, causing this configuration.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of attraction between molecules that occurs when a hydrogen atom, covalently bonded to a more electronegative atom like fluorine, forms an additional bond with another electronegative atom nearby. This is especially evident in the molecule (HF)₂, where hydrogen bonds help form a stable dimer. In this structure:

• One HF molecule acts as the hydrogen bond donor, providing the hydrogen atom.
• Another HF molecule acts as the acceptor, with its fluorine atom participating in the bond.

The hydrogen bond is represented by a dotted line connecting the hydrogen of one molecule to the fluorine of the next, forming a chain: H--F...H--F. This bonding type results in unique properties, like higher boiling points, due to these strong intermolecular forces.

Electron Configuration

The electron configuration refers to the arrangement of electrons around the nuclei of atoms in a molecule. For the ion HF₂^-, understanding electron configuration is crucial as it influences molecular geometry and bonding.

• The negative charge on HF₂^- suggests extra electrons are present.
• These additional electrons create a stable environment, balancing out forces between multiple atoms, such as fluorines.

This ion is linear due to the involvement of these extra electrons in forming resonance structures, which are arrangements that exhibit electrons in distributed positions, further stabilizing the molecule.

Resonance

Resonance occurs when a molecule can be represented by two or more valid Lewis structures. For HF₂^-, resonance helps explain why the molecule has its particular geometry.

• Resonance in HF₂^- involves the sharing of electrons between the two fluorine atoms.
• These electrons can be envisioned as moving back and forth to create equivalent resonance forms.

Thus, the molecule does not have purely separate electron positions but rather an electron cloud that contributes to an average structure. This resonance facilitates linearity and reinforces the stability of the ion, compensating for any added electrons due to the negative charge.

Negative Charge

A negative charge in a molecular structure, like HF₂^-, indicates an excess of electrons. This influences:

• Molecular formation
• Stability
• Structure

In HF₂^-, this negative charge stems from additional electrons that help construct the linear configuration, where F-H-F forms a symmetrical bridge between fluorine atoms thanks to resonance.

The negative charge affects the electron cloud surrounding the structure, ensuring all atoms achieve a more complete electron shell, contributing to stability. This characteristic leads to important features like increased reactivity, which can affect chemical dynamics and interactions with other substances.