Problem 28 - Chemistry The Central Science

Question

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

Step-by-Step Solution

Answer

(a) AsF3: Lewis structure has As as the central atom with three single bonds to F atoms and two lone pairs on As. Electron-domain geometry is trigonal bipyramidal, and molecular geometry is T-shaped. (b) CH3+: Lewis structure has C as the central atom with three single bonds to H atoms. Electron-domain geometry and molecular geometry are both trigonal planar. (c) BrF3: Lewis structure has Br as the central atom with three single bonds to F atoms and two lone pairs on Br. Electron-domain geometry is trigonal bipyramidal, and molecular geometry is T-shaped. (d) ClO3-: Lewis structure has Cl as the central atom with three single bonds to O atoms and one lone pair on Cl. Electron-domain geometry is tetrahedral, and molecular geometry is trigonal pyramidal. (e) XeF2: Lewis structure has Xe as the central atom with two single bonds to F atoms and three lone pairs on Xe. Electron-domain geometry is trigonal bipyramidal, and molecular geometry is linear. (f) BrO2-: Lewis structure has Br as the central atom with two single bonds to O atoms and one lone pair on Br. Electron-domain geometry is tetrahedral, and molecular geometry is trigonal pyramidal.

Step 1: (a) AsF3 Lewis structure, electron-domain and molecular geometry

1. Count valence electrons: As has 5 valence electrons and F has 7. There are 3 F atoms, so the total number of valence electrons is 5 + (3 × 7) = 26. 2. As the central atom is As, we will connect each F atom to As with single bonds. 3. As the octet rule requires 8 electrons, there are 8 × 4 = 32 needed for complete octets. We have 26, so we will put the remaining 6 electrons on the As atom as lone pairs. 4. The electron-domain geometry is trigonal bipyramidal, with three bonding pairs and two lone pairs in the equatorial positions. This results in a molecular geometry of T-shaped.

Step 2: (b) CH3+ Lewis structure, electron-domain and molecular geometry

1. Count valence electrons: C has 4 valence electrons and H has 1. There are 3 H atoms and a positive charge, so the total number of valence electrons is 4 + (3 × 1) - 1 = 6. 2. C is the central atom. Connect each H atom to the central C atom with single bonds. 3. All atoms have their required number of electrons, no additional electrons are needed. 4. The electron-domain geometry is trigonal planar, as there are three bonding pairs and no lone pairs. This results in a molecular geometry of trigonal planar.

Step 3: (c) BrF3 Lewis structure, electron-domain and molecular geometry

1. Count valence electrons: Br has 7 valence electrons and F has 7. There are 3 F atoms, so the total number of valence electrons is 7 + (3 × 7) = 28. 2. Br is the central atom. Connect each F atom to the central Br atom with single bonds. 3. We need 8 × 4 = 32 electrons for complete octets, and we have 28. We will put the remaining 4 electrons on the Br atom as lone pairs. 4. The electron-domain geometry is trigonal bipyramidal, with three bonding pairs and two lone pairs in the equatorial positions. This results in a molecular geometry of T-shaped.

Step 4: (d) ClO3- Lewis structure, electron-domain and molecular geometry

1. Count valence electrons: Cl has 7 valence electrons and O has 6. There are 3 O atoms and a negative charge, so the total number of valence electrons is 7 + (3 × 6) + 1 = 26. 2. Cl is the central atom. Connect each O atom to the central Cl atom with single bonds. 3. We need 8 × 4 = 32 electrons for complete octets, and we have 26. We will distribute the remaining 6 electrons as lone pairs on O atoms. 4. The electron-domain geometry is tetrahedral, with three bonding pairs and one lone pair on the central atom. This results in a molecular geometry of trigonal pyramidal.

Step 5: (e) XeF2 Lewis structure, electron-domain and molecular geometry

1. Count valence electrons: Xe has 8 valence electrons and F has 7. There are 2 F atoms, so the total number of valence electrons is 8 + (2 × 7) = 22. 2. Xe is the central atom. Connect each F atom to the central Xe atom with single bonds. 3. We need 8 × 3 = 24 electrons for complete octets, and we have 22. We will put the remaining 2 electrons on the Xe atom as lone pairs. 4. The electron-domain geometry is trigonal bipyramidal, with two bonding pairs and three lone pairs in the axial positions. This results in a molecular geometry of linear.

Step 6: (f) BrO2- Lewis structure, electron-domain and molecular geometry

1. Count valence electrons: Br has 7 valence electrons and O has 6. There are 2 O atoms and a negative charge, so the total number of valence electrons is 7 + (2 × 6) + 1 = 20. 2. Br is the central atom. Connect each O atom to the central Br atom with single bonds. 3. We need 8 × 3 = 24 electrons for complete octets, and we have 20. We will distribute the remaining 4 electrons as lone pairs on O atoms. 4. The electron-domain geometry is tetrahedral, with three bonding pairs and one lone pair on the central atom. This results in a molecular geometry of trigonal pyramidal.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a vital role in chemical bonding. They are responsible for the formation of bonds between atoms in a compound. When drawing Lewis structures, it's crucial to correctly count the number of valence electrons to predict how the atoms will bond and arrange themselves in a molecule.

• To determine the number of valence electrons, look at the group number of an element in the periodic table. For instance, nitrogen in Group 15 has 5 valence electrons.
• Elements gain, lose, or share valence electrons to fulfill the octet rule, which leads to more stable electron configurations.
• When dealing with compounds or ions, consider any charges that alter the total count of valence electrons. Add electrons for negative charges and subtract electrons for positive charges.

Understanding valence electrons helps predict the potential molecular geometry and the types of bonds formed during the chemical reactions.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. It determines many physical and chemical properties, such as polarity and reactivity. By knowing the molecular geometry, students can predict how molecules interact with each other and their environment.

• Geometry depends on the number of bonding pairs and lone pairs of electrons around the central atom.
• Common geometries include linear, trigonal planar, tetrahedral, and more complex shapes like trigonal bipyramidal and octahedral.
• For example, CH₃⁺ has a trigonal planar geometry with no lone pairs on the carbon, resulting in a flat, triangular shape.

Visualizing molecular structures as geometric shapes helps in understanding the spatial arrangement and potential interactions between molecules.

Electron-Domain Geometry

Electron-domain geometry considers the spatial arrangement of all electron domains (bonding and non-bonding) around a central atom. This concept is essential for predicting and explaining the molecule's shape before assessing molecular geometry.

• Electron domains include solitary bonds and lone pairs of electrons.
• The

  VSEPR Model

  Valence Shell Electron Pair Repulsion (VSEPR) theory helps determine electron-domain geometry by predicting that electron pairs will orient themselves to minimize repulsion.
• In BrF₃, the electron-domain geometry is trigonal bipyramidal due to three bonding pairs and two lone pairs around bromine, leading to a T-shaped molecular geometry.

The correct determination of electron-domain geometry provides a foundation for further understanding complex molecules.

Octet Rule

The octet rule is a guiding principle in chemistry, stating that atoms tend to combine in such a way that they each have eight electrons in their valence shell, similar to the noble gases. This principle helps predict the structure and stability of most molecules.

• Atoms bond by sharing, gaining, or losing electrons to achieve a stable octet configuration.
• Exceptions to this rule include hydrogen, which can be stable with just two electrons, and elements like phosphorus or sulfur, which can have expanded octets.
• In AsF₃, arsenic shares its electrons with three fluorine atoms to achieve an octet configuration, forming stable covalent bonds guided by the octet rule.

A good grasp of the octet rule allows for the accurate prediction of molecular structures and the types of bonds an atom can form.