Problem 81 - Chemistry An Atoms First Approach

Question

Write Lewis structures that obey the octet rule for each of the following molecules. a. \(\mathrm{CCl}_{4}\) b. \(\mathrm{NCl}_{3}\) c. \(\operatorname{Sec} 1_{2}\) d. ICl In each case, the atom listed first is the central atom.

Step-by-Step Solution

Answer

a. The Lewis structure of \(\mathrm{CCl}_{4}\): Cl | Cl - C - Cl | Cl b. The Lewis structure of \(\mathrm{NCl}_{3}\): Cl | Cl - N - Cl : c. The Lewis structure of \(SeO_{2}\): O : O - Se - O : d. The Lewis structure of ICl: I - Cl : : :

Step 1: a. Drawing the Lewis Structure of \(\mathrm{CCl}_{4}\)

1. Determine the total number of valence electrons: Carbon has 4 valence electrons and each chlorine atom has 7. Since there are 4 chlorine atoms, the total number of valence electrons is 4 + (4 × 7) = 32. 2. Identify the central atom: Carbon is the central atom. 3. Connect the chlorine atoms to the central carbon atom with single bonds: C-Cl. We have used 8 valence electrons so far. 4. Complete the octets of chlorine atoms by adding lone pair electrons to each chlorine atom: Each Cl atom needs 6 more electrons (3 lone pairs) to complete the octet. Thus, the Lewis structure of \(\mathrm{CCl}_{4}\) becomes: Cl | Cl - C - Cl | Cl

Step 2: b. Drawing the Lewis Structure of \(\mathrm{NCl}_{3}\)

1. Determine the total number of valence electrons: Nitrogen has 5 valence electrons and each chlorine atom has 7. Since there are 3 chlorine atoms, the total number of valence electrons is 5 + (3 × 7) = 26. 2. Identify the central atom: Nitrogen is the central atom. 3. Connect the chlorine atoms to the central nitrogen atom with single bonds: N-Cl. We have used 6 valence electrons so far. 4. Complete the octets of chlorine atoms and nitrogen atom by adding lone pair electrons: Each Cl atom needs 6 more electrons (3 lone pairs) to complete the octet. Nitrogen needs 2 more electrons (1 lone pair) to complete the octet. The Lewis structure of \(\mathrm{NCl}_{3}\) becomes: Cl | Cl - N - Cl :

Step 3: c. Drawing the Lewis Structure of \(SeO_{2}\)

1. Determine the total number of valence electrons: Selenium has 6 valence electrons and each oxygen atom has 6. Since there are 2 oxygen atoms, the total number of valence electrons is 6 + (2 × 6) = 18. 2. Identify the central atom: Selenium is the central atom. 3. Connect the oxygen atoms to the central selenium atom with single bonds: Se-O. We have used 4 valence electrons so far. 4. Complete the octets of oxygen atoms and selenium atom by adding lone pair electrons: Each O atom needs 6 more electrons (3 lone pairs) to complete the octet. Selenium needs 2 more electrons (1 lone pair) to complete the octet. The Lewis structure of \(SeO_{2}\) becomes: O : O - Se - O :

Step 4: d. Drawing the Lewis Structure of ICl

1. Determine the total number of valence electrons: Iodine has 7 valence electrons and chlorine has 7 valence electrons, for a total number of valence electrons of 7 + 7 = 14. 2. Identify the central atom: Iodine is the central atom. 3. Connect the chlorine atom to the central iodine atom with a single bond: I-Cl. We have used 2 valence electrons so far. 4. Complete the octets of iodine and chlorine atoms by adding lone pair electrons: Each atom needs 6 more electrons (3 lone pairs) to complete the octet. The Lewis structure of ICl becomes: I - Cl : : :

Key Concepts

Octet Rule

In chemistry, the Octet Rule is a fundamental guideline that helps us understand the behavior of atoms in a molecule. According to this rule, atoms strive to have eight electrons in their outermost shell, or valence shell. This configuration is similar to the stable arrangement of noble gases.

The Octet Rule is crucial when drawing Lewis structures as it ensures each atom achieves a stable configuration. By forming bonds, atoms share electrons to complete their octet:

• Atoms like carbon, nitrogen, and oxygen commonly follow this rule.
• Exceptions occur with elements such as hydrogen (which aims for two electrons) and larger elements that can expand their octet.

Understanding this rule is key to predicting the formation of covalent bonds and the structure of molecules, aiding in the visualization of molecular geometry.

Valence Electrons

Valence electrons play a vital role in determining how atoms interact with each other. These are the electrons present in the outermost shell of an atom, which participate in bonding with other atoms.

To draw Lewis structures, you first need to tally the total number of valence electrons for the atoms involved:

• Carbon, for example, has four valence electrons.
• Chlorine has seven, while nitrogen has five.

Knowing the number of valence electrons helps in predicting how many bonds an atom can form.

For instance, when writing the Lewis structure of \(\text{CCl}_4\), you would calculate the electrons from one carbon and four chlorine atoms, summing to 32. These electrons are represented as dots in Lewis structures, placed around the atoms to showcase any shared or lone pair electrons.

Covalent Bonds

A covalent bond is a chemical bond formed when two atoms share one or more pairs of electrons, enabling them to satisfy the Octet Rule. Covalent bonds are essential for constructing molecules and determining their properties.

These bonds involve the mutual sharing of electrons to achieve stable electron configurations:

• Single bonds share one pair of electrons, like in \(\text{H}_2\) or \(\text{Cl}_2\).
• Double or triple bonds, sharing two or three pairs, add more strength and stability to molecules.

In Lewis structures, covalent bonds are represented by lines between atoms.

For example, in \(\text{NCl}_3\), the nitrogen atom forms three covalent bonds with chlorine atoms, each represented as a line, helping to fulfill the electron requirements of each atom efficiently.

Central Atom Identification

Identifying the central atom in a molecule is usually the first step when drawing Lewis structures. The central atom is typically the least electronegative, excluding hydrogen, and offers clarity on how other atoms are bonded around it.

Here are a few pointers:

• The atom listed first in the formula is often the central atom, as seen with \(\text{CCl}_4\) where carbon is central.
• Elements like carbon, nitrogen, and sulfur frequently serve as central atoms in their compounds.

Choosing the correct central atom ensures the structural accuracy of the molecule.

This process is depicted in molecules like \(\text{SeO}_2\), where selenium sits centrally, allowing the oxygen atoms to form bonds around it, thereby creating a balanced structure that obeys the Octet Rule.