Question

Write Lewis structures and predict whether each of the following is polar or nonpolar. a. HOCN (exists as \(\mathrm{HO}-\mathrm{CN}\) ) b. \(\operatorname{COS}\) c. \(\mathrm{XeF}_{2}\) d. \(\mathrm{CF}_{2} \mathrm{Cl}_{2}\) e. \(\mathrm{SeF}_{6}\) f. \(\mathrm{H}_{2} \mathrm{CO}(\mathrm{C}\) is the central atom. \()\)

Short answer

a. \(\mathrm{HO}-\mathrm{CN}\): Linear, polar b. \(\operatorname{COS}\): Linear, polar c. \(\mathrm{XeF}_{2}\): Linear, nonpolar d. \(\mathrm{CF}_{2} \mathrm{Cl}_{2}\): Tetrahedral, polar e. \(\mathrm{SeF}_{6}\): Octahedral, nonpolar f. \(\mathrm{H}_{2} \mathrm{CO}\): Trigonal planar, polar

Step-by-step solution

Draw Lewis Structures for Each Compound

For each given compound, follow these steps: 1) Count the total number of valence electrons. 2) Connect the central atom with single bonds to each of the surrounding atoms. 3) Distribute the remaining electrons on each atom to complete the octet rule (except for hydrogen which needs two electrons). For the compounds given: a. \(\mathrm{HO}-\mathrm{CN}\): Oxygen central atom b. \(\operatorname{COS}\): Carbon as the central atom c. \(\mathrm{XeF}_{2}\): Xenon central atom d. \(\mathrm{CF}_{2} \mathrm{Cl}_{2}\): Carbon central atom e. \(\mathrm{SeF}_{6}\): Selenium central atom f. \(\mathrm{H}_{2} \mathrm{CO}\): Carbon central atom After completing this step, you will have the Lewis structures for each compound.

Determine the Molecular Geometry

For each Lewis structure, determine the molecular geometry of the compound using VSEPR (Valence Shell Electron Pair Repulsion) theory. Count the number of bonding pairs and lone pairs surrounding the central atom, and use the VSEPR model to predict the molecular geometry.

Determine the Polarity of Each Molecule

To determine the polarity of the molecules, consider their molecular geometries and the presence of polar bonds. A molecule is polar if it has an uneven distribution of electron density due to presence of polar bonds and asymmetrical molecular geometry. For each compound: a. \(\mathrm{HO}-\mathrm{CN}\) b. \(\operatorname{COS}\) c. \(\mathrm{XeF}_{2}\) d. \(\mathrm{CF}_{2} \mathrm{Cl}_{2}\) e. \(\mathrm{SeF}_{6}\) f. \(\mathrm{H}_{2} \mathrm{CO}\) Identify the presence of polar bonds and examine the molecular geometry, then determine the polarity of the molecule as either polar or nonpolar.

Key concepts

Valence Electrons

Valence electrons are the electrons that reside in the outermost electron shell of an atom. These are the electrons involved in chemical bonding and reactions. Understanding the total number of valence electrons in a molecule is crucial for drawing its Lewis structure—a diagram that represents the distribution of electrons in a molecule.

For instance, in the compound HO-CN, the total count of valence electrons is determined by adding the valence electrons of Hydrogen (1), Oxygen (6), Carbon (4), and Nitrogen (5), resulting in a total of 16 valence electrons. The Lewis structure can be drawn by firstly creating single bonds between the atoms and then distributing the remaining electrons to fulfill the octet rule, where each element (except Hydrogen) aims to be surrounded by eight electrons.

VSEPR Theory

VSEPR theory, short for Valence Shell Electron Pair Repulsion theory, is a model used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. According to this theory, electron pairs in the valence shell of an atom will adopt a spatial arrangement that minimizes repulsion, thus determining the molecule's geometry.

For the molecule COS, the VSEPR theory helps us understand the molecular geometry. By counting the bonding pairs (electron pairs found in shared bonds between atoms) and the lone pairs (non-bonding electron pairs) on the central carbon atom, we can use VSEPR to predict that COS has a linear molecular geometry due to the presence of multiple bonding pairs and no lone pairs on the carbon atom.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. Each distinct geometric shape is defined by both the number of bond pairs and the number of lone pairs surrounding the central atom. For instance, the molecular geometry could be linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral, among others.

Using molecular geometry, we can predict the shape of the molecule XeF₂. Xenon (Xe) is the central atom, with two bonded fluorine atoms and three lone pairs of electrons. The VSEPR theory predicts a linear arrangement of the bonded atoms because the lone pairs occupy equatorial positions in a trigonal bipyramidal electron-pair geometry, resulting in a molecule with a linear shape.

Electron Distribution

Electron distribution in a molecule refers to the way electrons are dispersed or spread around different atoms. The electron distribution influences molecular polarity, which is the uneven sharing of electron density across a molecule. Electron distribution is significant when determining whether a molecule is polar or nonpolar.

In the molecule CF₂Cl₂, electron distribution must be assessed by looking at both the geometry and the electronegativity of the atoms. Carbon is the central atom bonded to two fluorine atoms and two chlorine atoms. Despite having four polar bonds, the molecule has a symmetrical tetrahedral shape leading to an overall uniform distribution of electron density, making CF₂Cl₂ a nonpolar molecule.