Question

Determine the electron geometry, molecular geometry, and idealized bond angles for each molecule. In which cases do you expect deviations from the idealized bond angle? \begin{equation}\begin{array}{lllll}{\text { a. } \mathrm{CF}_{4}} & {\text { b. } \mathrm{NF}_{3}} & {\text { c. } \mathrm{OF}_{2}} & {\text { d. } \mathrm{H}_{2} \mathrm{S}}\end{array}\end{equation}

Short answer

CF4: Electron geometry - tetrahedral, Molecular geometry - tetrahedral, Bond angle - 109.5 degrees; NF3: Electron geometry - tetrahedral, Molecular geometry - trigonal pyramidal, Bond angle - <109.5 degrees; OF2: Electron geometry - tetrahedral, Molecular geometry - bent, Bond angle - <<109.5 degrees; H2S: Electron geometry - tetrahedral, Molecular geometry - bent, Bond angle - ~92 degrees. Deviations in bond angles are expected in NF3, OF2, and H2S due to the presence of lone pairs.

Step-by-step solution

Identify the Electron Geometry for CF4

Count the number of bonds and lone pairs on the central atom, carbon (C) in CF4. Carbon forms four single bonds with fluorine atoms and has no lone pairs, making a total of four electron groups. The electron geometry for a molecule with four electron groups is tetrahedral.

Determine the Molecular Geometry for CF4

Since all the electron groups in CF4 are bonding pairs with no lone pairs on the central atom, the molecular geometry is the same as the electron geometry, which is tetrahedral.

Describe the Idealized Bond Angles in CF4

The ideal bond angles in a tetrahedral geometry are approximately 109.5 degrees.

Identify the Electron Geometry for NF3

Nitrogen (N) is the central atom in NF3 with three single bonds to fluorine atoms and one lone pair of electrons. This gives a total of four electron groups. The electron geometry for a molecule with four electron groups is tetrahedral.

Determine the Molecular Geometry for NF3

With three bonding pairs and one lone pair, the molecular geometry is trigonal pyramidal.

Describe the Idealized and Actual Bond Angles in NF3

The ideal bond angles in a trigonal pyramidal geometry are somewhat less than 109.5 degrees due to greater repulsion from the lone pair compared to bonding pairs.

Identify the Electron Geometry for OF2

Oxygen (O) is the central atom in OF2 with two single bonds to fluorine atoms and two lone pairs of electrons. This also gives four electron groups, resulting in a tetrahedral electron geometry.

Determine the Molecular Geometry for OF2

With two bonding pairs and two lone pairs, the molecular geometry is bent or angular.

Describe the Idealized and Actual Bond Angles in OF2

The ideal bond angles in a bent geometry are less than 109.5 degrees and will be even smaller than in NF3 due to the two lone pairs exerting more repulsion.

Identify the Electron Geometry for H2S

Hydrogen sulfide (H2S) has the sulfur (S) atom as the central atom with two bonds to hydrogen atoms and two lone pairs, making four electron groups and resulting in a tetrahedral electron geometry.

Determine the Molecular Geometry for H2S

With two bonding pairs and two lone pairs, similar to OF2, H2S has a bent molecular geometry.

Describe the Idealized and Actual Bond Angles in H2S

Like OF2, the ideal bond angles in a bent geometry for H2S are less than 109.5 degrees. However, due to the larger size of sulfur relative to oxygen and weaker bond pair-lone pair repulsion, the actual bond angle in H2S is typically closer to 92 degrees.

Expectation of Deviations from the Idealized Bond Angle

Deviations from idealized bond angles can be expected in NF3, OF2, and H2S due to the presence of lone pairs that exert greater electron repulsion than bonding pairs. CF4 has no lone pairs and therefore, ideal tetrahedral bond angles of 109.5 degrees are expected without any deviations.

Key concepts

Electron Geometry

Understanding electron geometry is essential for predicting the shape of a molecule and how the atoms within it are arranged. The electron geometry reflects the arrangement of electron groups around the central atom without distinguishing between bonding pairs of electrons and lone pairs. For instance, in the case of CF₄, the central carbon atom is surrounded by four bonding pairs of electrons, resulting in a tetrahedral electron geometry.

This concept is crucial because electron groups, which include both bonds and lone pairs, will repel each other as per the principles of VSEPR theory, and this repulsion will determine the most stable arrangement of electrons around the central atom. Each molecular geometry, whether it is linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, or others, is based on a specific number of electron groups. When identifying electron geometry, simply count the total number of electron groups and apply the corresponding geometry.

Bond Angles

In molecular geometry, the bond angles refer to the angle between two adjacent bonds originating from the same central atom. The bond angles are determined by the electron geometry of the molecule. For instance, in a tetrahedral electron geometry, the ideal bond angles are approximately 109.5 degrees as seen in molecules like CF₄.

Deviations from Ideal Bond Angles

However, deviations from the ideal bond angles can occur due to factors such as the presence of lone pairs which exert greater repulsion than bonding pairs, as seen in NF₃ and OF₂. Molecules with lone pairs on the central atom will usually have smaller bond angles than the idealized figures. The VSEPR theory helps to predict these deviations by considering the repulsive forces between electron pairs. Therefore, knowing the ideal bond angles is just a starting point, and understanding electron pair repulsion is essential for predicting the actual bond angles in a molecule.

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory provides a systematic way to predict the geometry of molecules based on the idea that electron groups around a central atom will repel each other and thus arrange themselves as far apart as possible to minimize this repulsion.

According to VSEPR theory, lone pairs of electrons occupy more space than bonding pairs because their electron density is exclusively located around the central atom. This increased repulsion from lone pairs dictates that they will try to be as far from each other as possible, affecting bond angles and hence the molecular geometry. For example, in NF₃, the lone pair on the nitrogen atom pushes the bonding pairs closer together, resulting in a smaller than ideal bond angle for a tetrahedral geometry.

VSEPR theory allows us to anticipate the shape and bond angles of a molecule, which is essential for understanding chemical reactions and properties. It's a fundamental concept that forms the basis for more complex molecular modeling and is a valuable tool for students and chemists alike.