Question

(a) Is the molecule \(\mathrm{BF}_{3}\) polar or nonpolar? (b) If you react \(\mathrm{BF}_{3}\) to make the ion \(\mathrm{BF}_{3}^{2-}\), is this ion planar? (c) Does the molecule \(\mathrm{BF}_{2} \mathrm{Cl}\) have a dipole moment?

Short answer

(a) The molecule $\mathrm{BF}_{3}$ is nonpolar. (b) The ion $\mathrm{BF}_{3}^{2-}$ is not planar, as it has a tetrahedral electron domain geometry. (c) The molecule $\mathrm{BF}_{2}\mathrm{Cl}$ has a dipole moment.

Step-by-step solution

1. Determine the geometry of BF3

To determine whether BF3 is polar or nonpolar, we first need to establish its molecular geometry. This can be done by using the VSEPR (Valence Shell Electron Pair Repulsion) theory. In this case, boron is the central atom surrounded by three fluorine atoms. Boron has 3 valence electrons, while fluorine has 7 valence electrons. Thus, the BF3 molecule has 3 pairs of bonding electron domains. The molecular geometry of BF3 is trigonal planar, resulting in a 120-degree bond angle.

2. Identify electronegativity and polarity of BF3

Now we look at the electronegativity of the atoms in the BF3 molecule. Fluorine is among the most electronegative elements (electronegativity of 3.98), while boron has a lower electronegativity value (2.04). Despite this difference in electronegativity, the BF3 molecule has equal distribution of electron density due to its symmetric trigonal planar structure. Therefore, any individual bond dipoles will cancel each other out. This makes the molecule nonpolar. (a) The molecule BF3 is nonpolar.

3. Determine the geometry of BF3^2- ion

The ion BF3^2- indicates that the BF3 molecule now has an additional two electrons. Adding two extra electrons to the existing 3 pairs of bonding electron domains will provide us with 4 pairs of electron domains. With four bonded electron pairs, the molecule will assume a tetrahedral electron domain geometry. However, the question asks whether the ion is planar, not whether it is tetrahedral. (b) The ion BF3^2- is not planar, as it has a tetrahedral electron domain geometry.

4. Determine the geometry of BF2Cl molecule

For BF2Cl, boron remains the central atom with one fluorine atom replaced by a chlorine atom. The molecule has three pairs of bonding electron domains similar to BF3. Therefore, its molecular geometry continues to be trigonal planar.

5. Identify electronegativity and dipole moment in BF2Cl

In the BF2Cl molecule, fluorine has the highest electronegativity (3.98), followed by chlorine (3.16) and boron (2.04). The bond dipoles of boron-fluorine and boron-chlorine will not cancel out each other due to the difference in electronegativity between fluorine and chlorine. This will give rise to a net dipole moment in the molecule. (c) The molecule BF2Cl has a dipole moment.

Key concepts

Polarity

Polarity in molecules refers to the distribution of electrical charge over the atoms joined by the bond. In each bond, two atoms share electrons. However, these shared electrons may not be equally divided between the two atoms. This creates a molecular dipole or vector pointing from the less electronegative atom (lesser pull on electrons) to the more electronegative atom (greater pull on electrons).
For a molecule to be considered polar, it must have a net dipole moment, meaning that the dipole vectors do not cancel out. Some key factors influencing polarity include:

• Electronegativity Differences: Greater differences lead to more polarized bonds.
• Molecular Geometry: Symmetrical structures often result in nonpolar molecules as dipoles cancel out.

Taking \( ext{BF}_{3}\) as an example, it's nonpolar despite having highly polar B-F bonds due to its symmetrical trigonal planar shape which results in the cancellation of the dipole moments. Polarity significantly affects a molecule's physical properties, such as boiling and melting points, and its interactions with other molecules.

VSEPR Theory

The VSEPR (Valence Shell Electron Pair Repulsion) Theory is a model used to predict the geometry of individual molecules based on the repulsion between the electrons in a molecule's valence shell. The basic idea is that electron pairs around a central atom will position themselves as far apart as possible to minimize repulsion.
This model plays a crucial role in determining molecular shape:

• Linear: Two bonded pairs (180° apart)
• Trigonal Planar: Three bonded pairs (120° apart)
• Tetrahedral: Four bonded pairs (109.5° apart)

By applying VSEPR theory, we find that \( ext{BF}_{3}\) has a trigonal planar shape due to three electron pairs surrounding the central boron atom, leading to three \( ext{F}- ext{B}- ext{F}\) bond angles of 120°. In contrast, the \( ext{BF}_{3}^{2-}\) ion, with two additional electrons, would adopt a tetrahedral electron geometry, as the four electron pairs will spread out to minimize repulsion. VSEPR theory helps us to visualize whether the shape of the molecule allows for dipole moments to cancel or reinforce one another, directly influencing molecular polarity.

Dipole Moment

The dipole moment is a vector quantity representing the separation of positive and negative charges within a molecule. It is measured in Debye units, where a larger dipole moment indicates a higher polarity.
Factors contributing to the dipole moment include:

• Bond Polarity: The difference in electronegativity between bonded atoms creates a bond dipole.
• Molecular Shape: The 3D layout affects how these bond dipoles interact and cancel out.

In the case of \( ext{BF}_{2} ext{Cl}\), the bond dipoles do not cancel out due to the uneven electronegativities and the asymmetric shape, resulting in a net dipole moment. Chlorine, being less electronegative than fluorine, results in unequal sharing of electrons and a non-cancelling arrangement of bond dipoles. This gives \( ext{BF}_{2} ext{Cl}\) a definitive dipole moment, making it a polar molecule. Molecules with dipole moments tend to interact better with polar solvents like water, impacting their solubility and reactivity.