Question

Write a Lewis structure for each of the following simple molecules. Show all bonding valence electron pairs as lines and all nonbonding valence electron pairs as dots. a. \(\mathrm{PH}_{3}\) b. \(\mathrm{SF}_{2}\) C. HBr d. \(\mathrm{CCl}_{4}\)

Short answer

The Lewis structures for the given molecules are as follows: a. $\mathrm{PH}_{3}$: H \ \(P: - H\) / H b. $\mathrm{SF}_{2}$: F | \(S:: - F\) c. HBr: \(H - Br\) d. $\mathrm{CCl}_{4}$: Cl | Cl - C - Cl | Cl

Step-by-step solution

1. Drawing the Lewis structure for \(\mathrm{PH}_{3}\)

: To draw the Lewis structure for \(\mathrm{PH}_{3}\), follow the steps mentioned above. Step 1: P (Group 15) has 5 valence electrons and each H (Group 1) has 1; thus, the total number of valence electrons is 5 + (3 × 1) = 8. Step 2: Connect each H with P using a single electron bond (pair of electrons). Step 3: Distribute the remaining electrons as lone pairs. P has 3 bonds with 3 H atoms and 2 lone pair electrons. Step 4: All valence electrons are distributed. H \ \(P: - H\) / H

2. Drawing the Lewis structure for \(\mathrm{SF}_{2}\)

: Step 1: S (Group 16) has 6 valence electrons, and each F (Group 17) has 7; thus, the total number of valence electrons is 6 + (2 × 7) = 20. Step 2: Connect each F with S by using a single electron bond (pair of electrons). Step 3: Distribute the remaining electrons as lone pairs. Each F atom takes 3 lone pairs to satisfy the octet rule. S has 2 bonds with 2 F atoms and will have 2 lone pairs. Step 4: All valence electrons are distributed. F | \(S:: - F\)

3. Drawing the Lewis structure for HBr

: Step 1: H (Group 1) has 1 valence electron, and Br (Group 17) has 7; thus, the total number of valence electrons is 1 + 7 = 8. Step 2: Connect H with Br by using a single electron bond (pair of electrons). Step 3: Distribute the remaining electrons as lone pair electrons around the Br atom so that it satisfies the octet rule. Br will have 3 lone pairs. Step 4: All valence electrons are distributed. \(H - Br\)

4. Drawing the Lewis structure for \(\mathrm{CCl}_{4}\)

: Step 1: C (Group 14) has 4 valence electrons and each Cl (Group 17) has 7; thus, the total number of valence electrons is 4 + 4 × 7 = 32. Step 2: Connect each Cl with C using a single electron bond (pair of electrons). Step 3: Distribute the remaining electrons as lone pairs. Each Cl atom will have 3 lone pairs to satisfy the octet rule. The central C atom's octet is already satisfied. Step 4: All valence electrons are distributed. Cl | Cl - C - Cl | Cl

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom, located in the highest energy level. They play a crucial role in chemical bonding since they are the electrons involved when atoms form bonds with each other. Knowing the number of valence electrons helps us predict how atoms will interact and bond.

For instance, phosphorus (\(\text{P}\)) in the molecule \(\text{PH}_3\) is in Group 15 of the periodic table, indicating it has 5 valence electrons. Hydrogen (\(\text{H}\)) atoms each have 1 because they are in Group 1. Together, they give us a total of 8 valence electrons for \(\text{PH}_3\).

In a similar manner, sulfur in \(\text{SF}_2\) is in Group 16, contributing 6 valence electrons, while each fluorine (\(\text{F}\)) atom brings in 7. The total then becomes 20 valence electrons for the molecule. Being aware of these electrons helps us draw correct Lewis structures which represent the bonding and non-bonding pairs in a simple and visual manner.

Octet Rule

The octet rule is a key concept in understanding molecular structures. It is based on the tendency of atoms to prefer having eight electrons in their valence shell, similar to the electron configuration of a noble gas.

When constructing a Lewis structure, ensuring that atoms satisfy the octet rule helps us predict stable molecular arrangements. For example, in \(\text{SF}_2\), each fluorine atom ends up with 8 electrons around it after forming a single bond with sulfur. After bonding and the distribution of remaining electrons as lone pairs, sulfur also fulfills the octet rule.

However, the rule isn’t always met by all atoms. Hydrogen is a notable exception as it only requires 2 electrons in its outer shell. Therefore, in \(\text{PH}_3\), hydrogen atoms pair up their lone electron with one of phosphorus' electrons, helping to form a stable molecule even without following the octet rule.

Electron Pair

Electron pairs are fundamental elements represented in Lewis structures. They can be categorized into two types: bonding pairs and lone pairs. Bonding pairs are shared between atoms within a molecule, forming covalent bonds, while lone pairs are not shared and reside on a single atom.

For example, in the molecule \(\text{CCl}_4\), every chlorine atom connects with carbon using a pair of electrons, forming a bonding pair. Carbon, with 4 valence electrons, uses each electron to bond with one chlorine, resulting in four \(\text{C-Cl}\) single bonds.

Lone pairs, on the other hand, are depicted as dots surrounding an atom. In \(\text{SF}_2\), three lone pairs of electrons are found on each fluorine atom, while sulfur is left with two lone pairs after bonding. These electron pairs, both bonding and non-bonding, significantly influence the molecular structure and its chemical properties.