Question

Predict the shapes and give approximate bond angles for the following molecules. (a) OCS (c) FCN (e) \(\mathrm{PF}_{3}\) (g) \(\mathrm{NO}_{2}\) (b) FNO (d) \(\mathrm{HN}_{3}\) (f) \(\mathrm{SF}_{2}\)

Short answer

Shapes and approximate bond angles for the molecules are: OCS - Linear, 180 degrees; FCN - Linear, 180 degrees; PF3 - Trigonal Pyramidal, <109.5 degrees; NO2 - Bent, <120 degrees; FNO - Bent, <120 degrees; HN3 - Linear, 180 degrees; SF2 - Bent, <109.5 degrees.

Step-by-step solution

Understand VSEPR Theory

VSEPR theory or the Valence Shell Electron Pair Repulsion theory is used to predict the shape of a molecule based on the electron domains (which include bonded atoms and lone electron pairs) around the central atom. The theory states that the electron pairs (both bonding and lone pairs) in the outermost shell or valence shell of a central atom will repel each other, forcing the molecule to take on a shape that minimizes their repulsion.

Apply VSEPR Theory to OCS, FCN, PF3, and NO2

For molecule OCS, there are two electron domains so it's shape is linear with bond angle of 180 degrees. For molecule FCN, there is also two electron domains so it's linear too with bond angle of 180 degrees. For molecule PF3, there are four electron domains (three being from bonded atoms and one from lone pair), it adopts a trigonal pyramidal shape with bond angle of less than 109.5 degrees (because lone pair-bond pair repulsion is more than bond pair-bond pair repulsion). NO2 has three electron domains (two from bonded atoms and one from lone pair), so it has a bent shape, with a bond angle of less than 120 degrees.

Apply VSEPR Theory to FNO, HN3, and SF2

For molecule FNO, there are three electron domains (two from bonded atoms and one from lone pair), so it's shape is bent with bond angle of less than 120 degrees. For molecule HN3, there are two electron domains so it's shape is linear with bond angle of 180 degrees. For molecule SF2, there are four electron domains (two being from bonded atoms and two from lone pairs), so it has a bent shape, with a bond angle of less than 109.5 degrees.

Key concepts

Molecular Geometry

Molecular geometry is all about the three-dimensional arrangement of atoms within a molecule. This concept helps define not only the shape but also the way a molecule interacts with others. The VSEPR theory plays a crucial role in determining molecular geometry. By focusing on electron domains, which include bonded atoms and lone pairs, we predict the geometry that minimizes repulsion among these regions. For instance, a molecule like OCS with two electron domains forms a linear shape, whereas PF3 with four domains adopts a trigonal pyramidal shape.

Bond Angles

Bond angles are pivotal in understanding the spatial configuration of molecules. They are the angles between adjacent bonds at an atom. VSEPR theory provides predictions for these angles by considering electron pair repulsions. For linear molecules like OCS and HN3, the bond angles are around 180 degrees. However, molecules like NO2 and SF2, which contain lone pairs, have smaller angles. In NO2, the bond angle is less than 120 degrees, while in SF2, it's less than 109.5 degrees due to lone-pair repulsion.

Electron Domains

Electron domains are regions where electrons are most likely found. This includes bonded atoms and lone pairs surrounding a central atom. Each domain influences the overall shape of the molecule. More electron domains equate to more interaction and repulsion, influencing molecular geometry. For example, PF3 has four electron domains but one is a lone pair, leading to a trigonal pyramidal shape. Similarly, SF2's two lone pairs and two bonding pairs push the molecule into a bent shape, exemplifying the effect of electron domains on molecular configuration.