Question

Draw Lewis structures and give the other information requested for the following molecules: (a) \(\mathrm{BF}_{3}\). Shape: planar or nonplanar? (b) \(\mathrm{ClO}_{3}^{-}\). Shape: planar or nonplanar? (c) HCN. Polar or nonpolar? (d) \(\mathrm{OF}_{2}\). Polar or nonpolar? (e) \(\mathrm{NO}_{2}\). Estimate the ONO bond angle.

Short answer

(a) BF3: Planar. (b) ClO3-: Nonplanar. (c) HCN: Polar. (d) OF2: Polar. (e) NO2: ~115° bond angle.

Step-by-step solution

Draw Lewis Structure for \( \mathrm{BF}_3 \)

Boron is the central atom with three fluorine atoms surrounding it. Each fluorine is connected to boron through a single bond. Boron has a complete octet with six electrons, three from its bonds with fluorine atoms. There are no lone pairs on boron, resulting in a trigonal planar shape.

Determine Shape of \( \mathrm{BF}_3 \)

The shape becomes evident based on the fact that there are no lone pairs on the central boron atom. With three bonding pairs and no lone pairs, the molecule is trigonal planar and hence planar.

Draw Lewis Structure for \( \mathrm{ClO}_3^- \)

Chlorine is the central atom attached to three oxygen atoms. Two of these bonds are single bonds (one oxygen carries a formal negative charge), and one is a double bond. There should be lone pairs on the central chlorine to satisfy its octet, thus creating a total of 26 electrons including the extra electron from the negative charge.

Determine Shape of \( \mathrm{ClO}_3^- \)

The presence of lone pairs on chlorine create a trigonal pyramidal shape for the molecule, which results in a nonplanar configuration.

Draw Lewis Structure for \( \mathrm{HCN} \)

Carbon is the central atom between hydrogen and nitrogen, with a single bond to hydrogen and a triple bond to nitrogen. The Lewis structure is H-C≡N.

Determine Polarity of \( \mathrm{HCN} \)

The molecule is linear, and the electronegativity difference between nitrogen and hydrogen creates a dipole moment. Thus, HCN is polar.

Draw Lewis Structure for \( \mathrm{OF}_2 \)

Oxygen is the central atom connected to two fluorine atoms through single bonds. Oxygen will have two lone pairs, resulting in a bent shape.

Determine Polarity of \( \mathrm{OF}_2 \)

Due to the bent shape and difference in electronegativity between oxygen and fluorine, the molecule has an overall dipole moment. Hence, \( \mathrm{OF}_2 \) is polar.

Draw Lewis Structure for \( \mathrm{NO}_2 \)

Nitrogen is the central atom with one single bond and one double bond to oxygen atoms, and a lone electron (free radical) on nitrogen. This results in a resonance structure.

Estimate \( \mathrm{ONO} \) Bond Angle in \( \mathrm{NO}_2 \)

The shape is bent due to resonance and lone electron on the nitrogen atom. The present resonance structures and lone electron abjure from 120° and lead to an estimated bond angle around 115°.

Key concepts

Molecular Shape

Understanding the molecular shape is crucial because it influences a molecule's properties and reactions. The shape is determined by the arrangement of atoms and electron pairs around a central atom. A useful theory for predicting molecular shape is the VSEPR (Valence Shell Electron Pair Repulsion) theory. According to VSEPR, electron pairs around a central atom will arrange themselves to minimize repulsion, leading to a specific molecular geometry.

• For example, in the molecule of \( \mathrm{BF}_3 \), the boron atom forms three single bonds with fluorine atoms and has no lone pairs. This results in a trigonal planar shape, which is flat and planar.
• In contrast, \( \mathrm{ClO}_3^- \) has chlorine as the central atom with lone pairs, creating a trigonal pyramidal shape, which is non-planar.

Recognizing these shapes helps predict how the molecule interacts with other molecules, such as in bonding and reactions.

Polarity

Molecular polarity is a key feature that influences properties like solubility, boiling point, and intermolecular interactions. A molecule's polarity depends on both its shape and the electronegativity of its atoms. Polarity arises when there is an uneven distribution of electron density across a molecule, leading to areas of partial positive and negative charges.

• HCN is a polar molecule because it has a linear shape and a significant electronegativity difference between hydrogen and the more electronegative nitrogen, causing a dipole moment.
• Similarly, \( \mathrm{OF}_2 \) is polar due to its bent shape and the high electronegativity of fluorine compared to oxygen, resulting in a net dipole moment.

Understanding polarity is crucial for predicting how molecules will interact with each other, such as in solution mixing or chemical reactions.

Bond Angles

Bond angles are another important aspect of molecular geometry, determining the 3D shape of a molecule. They are influenced by the presence of lone pairs and the arrangement of electron domains around the central atom. Bond angles can alter due to the repulsion between bonding and non-bonding electron pairs.

• For instance, in \( \mathrm{BF}_3 \), the bond angles are 120°, typical for a trigonal planar shape.
• In \( \mathrm{NO}_2 \), although it has a form that could ordinarily have a bond angle near 120°, the presence of a lone electron (forming a free radical) reduces the bond angle to around 115°.

Understanding bond angles is essential when predicting molecular behavior or when modeling the structure of more complex molecules.

Electron Configuration

Electron configuration is a foundational concept that gives insight into how atoms bond to form molecules. It describes the distribution of electrons in an atom’s orbitals and is fundamental in predicting how a molecule will form bonds. The configuration not only influences the atomic structure but also affects the chemical properties and reactivity of the molecule.

• For instance, boron in \( \mathrm{BF}_3 \) has three valence electrons, which bond with the fluorine atoms, filling its electron shell to satisfy the octet rule.
• For \( \mathrm{ClO}_3^- \), the presence of lone pairs on the central chlorine atom affects its electron configuration, contributing to its shape and reactivity.

Electron configuration is a critical concept for understanding and predicting the behavior of atoms in molecules, including how molecules interact in reactions.