Question

Draw the Lewis structure for each of the following molecules or ions, and predict their electron-domain and molecular geometries: (a) \(\operatorname{AsF}_{3},(\mathbf{b}) \mathrm{CH}_{3}^{+},(\mathbf{c}) \operatorname{Br} \mathrm{F}_{3},(\mathbf{d}) \mathrm{ClO}_{3},(\mathbf{e}) \mathrm{XeF}_{2}\) \((\mathbf{f}) \mathrm{BrO}_{2}^{-}\)

Short answer

The short answer is as follows: (a) AsF3: Lewis structure shows As with single bonds to 3 F atoms and one lone pair. The electron-domain geometry is tetrahedral, and the molecular geometry is trigonal pyramidal. (b) CH3+: Lewis structure shows C with single bonds to 3 H atoms. The electron-domain geometry and molecular geometry are both trigonal planar. (c) BrF3: Lewis structure shows Br with single bonds to 3 F atoms and 2 lone pairs. The electron-domain geometry is trigonal bipyramidal, and the molecular geometry is T-shaped. (d) ClO3-: Lewis structure shows Cl with single bonds to 3 O atoms and one lone pair. The electron-domain geometry is tetrahedral, and the molecular geometry is trigonal pyramidal. (e) XeF2: Lewis structure shows Xe with single bonds to 2 F atoms and 3 lone pairs. The electron-domain geometry is trigonal bipyramidal, and the molecular geometry is linear. (f) BrO2-: Lewis structure shows Br with single bonds to 2 O atoms and one lone pair. The electron-domain geometry is trigonal planar, and the molecular geometry is bent.

Step-by-step solution

(a) AsF3 Lewis structure and geometry prediction

1. Total valence electrons: As (5) + 3F (3x7) = 5+21 = 26 2. Lewis structure: As is the center atom, with single bonds to each of the 3 F atoms. As has one lone pair on it, and each of the F atoms has 3 lone pairs. 3. Electron-domain geometry: With 4 electron domains (3 bonded pairs and 1 lone pair), we have a tetrahedral geometry. 4. Molecular geometry: Due to the lone pair-bond pair repulsion, the molecular geometry is trigonal pyramidal.

(b) CH3+ Lewis structure and geometry prediction

1. Total valence electrons: C (4) + 3H (3x1) - 1 (due to positive charge) = 4+3-1 = 6 2. Lewis structure: C is the center atom, with single bonds to each of the 3 H atoms. There is no lone pair on C or H atoms. 3. Electron-domain geometry: With 3 electron domains (all bonded pairs), we have a trigonal planar geometry. 4. Molecular geometry: All electron domains are bonded pairs, so the molecular geometry is also trigonal planar.

(c) BrF3 Lewis structure and geometry prediction

1. Total valence electrons: Br (7) + 3F (3x7) = 7+21 = 28 2. Lewis structure: Br is the center atom, with single bonds to each of the 3 F atoms. Br has 2 lone pairs on it, and each of the F atoms has 3 lone pairs. 3. Electron-domain geometry: With 5 electron domains (3 bonded pairs and 2 lone pairs), we have a trigonal bipyramidal geometry. 4. Molecular geometry: As there are 2 lone pairs on the equatorial positions, the molecular geometry is T-shaped.

(d) ClO3- Lewis structure and geometry prediction

1. Total valence electrons: Cl (7) + 3O (3x6) + 1 (due to negative charge) = 7+18+1 = 26 2. Lewis structure: Cl is the center atom, with single bonds to each of the 3 O atoms. Each O atom has 2 lone pairs, and Cl has 1 lone pair. 3. Electron-domain geometry: With 4 electron domains (3 bonded pairs and 1 lone pair), we have a tetrahedral geometry. 4. Molecular geometry: Due to the lone pair-bond pair repulsion, the molecular geometry is trigonal pyramidal.

(e) XeF2 Lewis structure and geometry prediction

1. Total valence electrons: Xe (8) + 2F (2x7) = 8+14 = 22 2. Lewis structure: Xe is the center atom, with single bonds to the 2 F atoms. Xe has 3 lone pairs on it, and each of the F atoms has 3 lone pairs. 3. Electron-domain geometry: With 5 electron domains (2 bonded pairs and 3 lone pairs), we have a trigonal bipyramidal geometry. 4. Molecular geometry: All 3 lone pairs occupy the equatorial positions, so the molecular geometry is linear.

(f) BrO2- Lewis structure and geometry prediction

1. Total valence electrons: Br (7) + 2O (2x6) + 1 (due to negative charge) = 7+12+1 = 20 2. Lewis structure: Br is the center atom, with single bonds to both O atoms. Br has one lone pair, and each O atom has 3 lone pairs. 3. Electron-domain geometry: With 3 electron domains (2 bonded pairs and 1 lone pair), we have a trigonal planar geometry. 4. Molecular geometry: Due to the lone pair-bond pair repulsion, the molecular geometry is bent.

Key concepts

Electron-Domain Geometry

Understanding the electron-domain geometry of molecules is fundamental in predicting their structure and behavior. The electron-domain geometry considers both the bonding and nonbonding electron pairs, or 'lone pairs', surrounding a central atom. These electron pairs repel each other and arrange themselves as far apart as possible to minimize repulsion. This concept follows the VSEPR theory (Valence Shell Electron Pair Repulsion theory), which states that the shape of the molecule is largely determined by these repulsions.

For example, a molecule with four electron domains, such as ammonia (NH₃), has a tetrahedral electron-domain geometry. This is because it has three hydrogen atoms and one lone pair of electrons around the nitrogen atom, constituting four areas of electron density. These areas are arranged in such a way that they point towards the corners of a tetrahedron. Similarly, a molecule like water (H₂O), with two bonded pairs and two lone pairs, also has a tetrahedral electron-domain geometry, although its molecular geometry is different.

Molecular Geometry

While electron-domain geometry considers both bonded atoms and lone pairs, molecular geometry focuses exclusively on the arrangement of the atoms themselves. This arrangement gives us a visual description of a molecule's shape. For instance, while the electron-domain geometry of water is tetrahedral due to the four electron pairs around the oxygen atom, its molecular geometry is described as 'bent' or 'angular' since we only consider the position of the hydrogen atoms.

Let's consider methane (CH₄) as another example. Methane has a tetrahedral electron-domain geometry, similar to the previous example of ammonia, but here, all four domains are occupied by hydrogen atoms, which means its molecular geometry is also tetrahedral. These geometries predict how molecules will interact with each other and can affect their physical and chemical properties, like boiling and melting points, reactivity, and polarity.

Valence Electrons

Valence electrons play the key role in bond formation and molecular structure. They are the electrons located in the outermost shell of an atom and are responsible for the chemical properties of the element. Knowing the number of valence electrons is crucial when determining the Lewis structure of a molecule.

For instance, in carbon dioxide (CO₂), carbon has four valence electrons and each oxygen atom has six. These valence electrons are shared or transferred between atoms to reach a more stable electronic configuration, often leading to the complete filling of the valence shell, known as the octet rule. In CO₂, carbon forms double bonds with each oxygen atom, sharing two pairs of electrons with each, and consequently, every atom achieves an octet. The count of valence electrons guides us in drawing the Lewis structure, which is a simple representation that shows how the electrons are arranged among the atoms in a molecule.