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. \(\mathrm{H}_{2} \mathrm{S}\) b. \(\operatorname{SiF}_{4}\) c. \(C_{2} H_{4}\) d. \(\mathrm{C}_{3} \mathrm{H}_{8}\)

Short answer

a. \(\mathrm{H}_{2} \mathrm{S}\): ``` H H \ . . / S ``` b. \(\operatorname{SiF}_{4}\): ``` F F \ / .-Si-. / \ F F ``` c. \(C_{2} H_{4}\): ``` H H \\ // C=C / \ H H ``` d. \(\mathrm{C}_{3} \mathrm{H}_{8}\): ``` H H H | | | H--C--C--C--H | | | H H H ```

Step-by-step solution

Determine the total number of valence electrons

Determine the total number of valence electrons for each element in the molecule. This can be done by finding the group number, which corresponds to the number of valence electrons for elements in that group. a. \(\mathrm{H}_{2} \mathrm{S}\): Hydrogen has 1 valence electron, and Sulfur has 6 valence electrons. Therefore, the molecule has a total of \((2\times1)+6=8\) valence electrons. b. \(\operatorname{SiF}_{4}\): Silicon has 4 valence electrons and Fluorine has 7 valence electrons. Therefore, the molecule has a total of \(4+(4\times7)=32\) valence electrons. c. \(C_{2} H_{4}\): Carbon has 4 valence electrons, and Hydrogen has 1 valence electron. Therefore, the molecule has a total of \((2\times4)+(4\times1)=12\) valence electrons. d. \(\mathrm{C}_{3} \mathrm{H}_{8}\): Carbon has 4 valence electrons, and Hydrogen has 1 valence electron. Therefore, the molecule has a total of \((3\times4)+(8\times1)=20\) valence electrons.

Arrange the atoms

Arrange the atoms in the molecule in a way that helps minimize repulsion forces between the electron pairs and satisfy the octet rule for the non-Hydrogen atoms. Hydrogen atoms can only have 2 electrons at most, so it will always be bonded only to one other atom. a. \(\mathrm{H}_{2} \mathrm{S}\): In this molecule, Sulfur is the central atom, with two Hydrogen atoms bonded to it. b. \(\operatorname{SiF}_{4}\): Here, Silicon is the central atom. Each of the four Fluorine atoms is bonded to it. c. \(C_{2} H_{4}\): In this molecule, there are two Carbon atoms bonded to each other. Each Carbon atom is also bonded to two Hydrogen atoms. d. \(\mathrm{C}_{3} \mathrm{H}_{8}\): In this molecule, three Carbon atoms form a chain with Hydrogen atoms bonded to the Carbon atoms. The central Carbon atom has 2 Hydrogen atoms, while the other two have 3 Hydrogen atoms each.

Draw the Lewis structures

Finally, draw the Lewis structures for each molecule by using lines to represent bonding electron pairs and dots for nonbonding electron pairs. a. \(\mathrm{H}_{2} \mathrm{S}\): ``` H H \ . . / S ``` b. \(\operatorname{SiF}_{4}\): ``` F F \ / .-Si-. / \ F F ``` c. \(C_{2} H_{4}\): ``` H H \\ // C=C / \ H H ``` d. \(\mathrm{C}_{3} \mathrm{H}_{8}\): ``` H H H | | | H--C--C--C--H | | | H H H ```

Key concepts

Valence Electrons

Valence electrons play a pivotal role in chemical bonding as they are the electrons located in the outermost shell of an atom. Understanding how to determine the number of valence electrons is essential for depicting Lewis structures. For example, hydrogen (H), being in group 1, has one valence electron, while sulfur (S) in group 16 possesses six.

The total number of valence electrons for a molecule is the sum of valence electrons of all atoms present. This aids in drawing the correct Lewis structure and predicting molecule geometry. In the case of SiF₄, we multiply Silicon's four valence electrons with the seven from each of the four Fluorine atoms to get 32 valence electrons, which forms the basis for further steps in creating the molecule's Lewis structure.

Chemical Bonding

Chemical bonding involves the attraction between atoms that enables the formation of chemical substances containing two or more atoms. The Lewis structure visually represents these bonds. Bonding valence electrons are portrayed by lines between atoms, suggesting a stable electron configuration through sharing.

For instance, in ethene (C₂H₄), the double bond between carbon atoms is represented by two lines, indicating that each carbon shares two electrons. This visualises how atoms fulfill their need for electrons by either sharing, gaining, or losing them to achieve a full outer shell, known as the octet rule, which is a stable configuration.

Molecule Geometry

The geometry of a molecule is determined by the spatial distribution of atoms around a central atom and is influenced by the repulsion between electron pairs. This is perfectly illustrated in the Lewis structure. For example, the molecule of propane (C₃H₈) shows three carbon atoms forming a chain, with hydrogen atoms bonded to the carbon atoms.

Applying the VSEPR (Valence Shell Electron Pair Repulsion) theory allows us to predict that the molecule of H₂S will have a bent shape due to the two pairs of nonbonding electrons on sulfur, which pushes down the hydrogen atoms. In contrast, SiF₄ with four bonding pairs will form a tetrahedral shape. Understanding molecule geometry is crucial for predicting how molecules will interact with each other, influencing the physical and chemical properties of substances.