Question

Write the Lewis structures for the following molecules and polyatomic ions. In each case, the first atom is the central atom. (a) \(\mathrm{CCl}_{4}\) (b) \(\mathrm{NCl}_{3}\) (c) \(\mathrm{COCl}_{2}\) (d) \(\mathrm{SO}_{3}^{2-}\)

Short answer

Question: Draw Lewis structures for the following molecules and polyatomic ions with the given central atom: (a) CCl4, (b) NCl3, (c) COCl2, (d) SO3^2- Answer: (a) The Lewis structure of CCl4 consists of a central Carbon atom bonded to 4 Chlorine atoms. (b) The Lewis structure of NCl3 consists of a central Nitrogen atom bonded to 3 Chlorine atoms. (c) The Lewis structure of COCl2 consists of a central Carbon atom bonded to an Oxygen atom and 2 Chlorine atoms. (d) The Lewis structure of SO3^2- consists of a central Sulfur atom bonded to 3 Oxygen atoms with a 2- charge.

Step-by-step solution

(a) CCl4

1. Determine the valence electrons: Carbon (C) has 4 valence electrons and Chlorine (Cl) has 7 valence electrons. Since there are 4 Cl atoms, the total valence electrons are \(4+4\times 7=32\). 2. Arrange the atoms: Place the Carbon atom in the center and surround it with the 4 Chlorine atoms. 3. Distribute the electrons: Assign 2 electrons to each of the C-Cl bonds (8 electrons). Assign the remaining 24 electrons to the Chlorine atoms, with each Cl atom getting 6 more electrons to complete its octet. 4. Stability check: Carbon has 8 electrons in its outer shell and each Chlorine atom has 8 electrons in its outer shell. The Lewis structure for \(\mathrm{CCl}_{4}\) is stable.

(b) NCl3

1. Determine the valence electrons: Nitrogen (N) has 5 valence electrons and Chlorine (Cl) has 7 valence electrons. Since there are 3 Cl atoms, the total valence electrons are \(5+3\times 7=26\). 2. Arrange the atoms: Place the Nitrogen atom in the center and surround it with the 3 Chlorine atoms. 3. Distribute the electrons: Assign 2 electrons to each of the N-Cl bonds (6 electrons). Assign the remaining 20 electrons to the Chlorine atoms, with each Cl atom getting 6 more electrons to complete its octet. 4. Stability check: Nitrogen has 8 electrons in its outer shell and each Chlorine atom has 8 electrons in its outer shell. The Lewis structure for \(\mathrm{NCl}_{3}\) is stable.

(c) COCl2

1. Determine the valence electrons: Carbon (C) has 4 valence electrons, Oxygen (O) has 6 valence electrons, and Chlorine (Cl) has 7 valence electrons. Since there are 2 Cl atoms, the total valence electrons are \(4+6+2\times 7=24\). 2. Arrange the atoms: Place the Carbon atom in the center, with Oxygen on one side and the two Chlorine atoms on the other side. 3. Distribute the electrons: Assign 2 electrons to each of the C-O and C-Cl bonds (8 electrons). Assign the remaining 16 electrons to the Oxygen and Chlorine atoms, with Oxygen getting 4 more electrons and each Cl atom getting 6 more electrons to complete their octet. 4. Stability check: Carbon has 8 electrons in its outer shell, Oxygen has 8 electrons, and each Chlorine atom has 8 electrons. The Lewis structure for \(\mathrm{COCl}_{2}\) is stable.

(d) SO3^2-

1. Determine the valence electrons: Sulfur (S) has 6 valence electrons and Oxygen (O) has 6 valence electrons. Since there are 3 O atoms and there is a 2- charge, the total valence electrons are \(6+3\times 6+2=26\). 2. Arrange the atoms: Place the Sulfur atom in the center and surround it with the 3 Oxygen atoms. 3. Distribute the electrons: Assign 2 electrons to each of the S-O bonds (6 electrons). Assign the remaining 20 electrons to the Oxygen atoms, with each O atom getting 6 more electrons to complete its octet. 4. Stability check: Sulfur has 8 electrons in its outer shell and each Oxygen atom has 8 electrons in its outer shell. The Lewis structure for \(\mathrm{SO}_{3}^{2-}\) is stable.

Key concepts

Valence Electrons

Valence electrons are crucial when drawing Lewis structures as they determine how atoms bond with each other. These are the electrons located in the outermost shell of an atom and are responsible for the chemical behavior of the element. Different elements have different numbers of valence electrons, and knowing this number helps to construct accurate Lewis structures.

For instance, Carbon has 4 valence electrons, which allows it to form four bonds, as seen in \(\mathrm{CCl}_4\). Chlorine, with 7 valence electrons, is eager to gain one more electron to complete its octet, making it highly reactive. By adding up the valence electrons from all present atoms, you can determine the total count needed to distribute when forming the molecule or polyatomic ion.

Using the correct number of valence electrons ensures that each atom achieves a stable electronic configuration through sharing, gaining, or losing electrons, leading to the formation of stable molecules.

Octet Rule

The octet rule is an essential guideline that helps predict the arrangement of electrons in an atom's outer shell within a molecule. In simple terms, most atoms strive to attain a stable configuration similar to the noble gases, possessing 8 electrons in their valence shell. This rule explains why atoms tend to engage in chemical bonding.

For example, in \(\mathrm{NCl}_3\), Nitrogen initially has 5 valence electrons and it forms bonds with three Chlorine atoms to reach the desired octet—a total of 8 valence electrons. Each Chlorine atom also fulfills the octet rule by sharing electrons with Nitrogen through covalent bonds. However, there are exceptions to the octet rule, especially for elements in the third period and beyond, like Sulfur in \(\mathrm{SO}_3^{2-}\), which sometimes utilizes d orbitals to accommodate more than 8 electrons.

While drawing Lewis structures, always count the electrons around each atom to ensure that, ideally, 8 electrons are achieved, providing maximum stability for the molecule.

Molecular Geometry

Understanding molecular geometry is vital in determining the shape and properties of the molecules. Molecular geometry refers to the three-dimensional arrangement of the atoms in a molecule. It influences many physical and chemical properties, such as boiling point, polarity, and reactivity.

The geometry is determined by several factors, including the number of bonds and lone pairs around the central atom. For example, in \(\mathrm{CCl}_4\), the carbon atom is surrounded by four chlorine atoms, which creates a symmetrical "tetrahedral" shape due to equal repulsion of the bond pairs.

In \(\mathrm{SO}_3^{2-}\), although you'll arrange atoms initially to visualize the Lewis structure, using VSEPR (Valence Shell Electron Pair Repulsion) theory can further help in predicting the trigonometry, resulting in a particular arrangement. This geometric perspective allows for a better understanding of why molecules have their specific shapes, significantly impacting interactions in the chemistry world.