Question

Draw Lewis structures for the following substances. Use distinct, correctly placed dots for the electrons. Mark all atoms which are not neutral with charges of the proper sign. a) propane f) ethylene oxide b) cyclopropane g) sodium acetate \(\left(\mathrm{CH}_{3} \mathrm{CO}_{2}^{(-)} \mathrm{Na} \oplus\right)\) c) benzene h) methylamine d) methyl bromide i) acetamide e) methanol j) nitromethane

Short answer

The Lewis structures for the given compounds are as follows: a) Propane: ``` H H H | | | H-C-C-C-H | | | H H H ``` b) Cyclopropane: ``` H \ H-C---C-H |___| / \ H H ``` c) Benzene: ``` H \ -C====C-H // || || |\-C----C-| \\| |/ \C====C / \ H H ``` d) Methyl bromide: ``` H H H | | | C---Br ```

Step-by-step solution

a) Propane, C3H8

1. Calculate the total number of valence electrons: 3 carbon has 4 valence electrons each, and 8 hydrogen has 1 valence electron each. The total number of valence electrons is \(3\times4 + 8\times1 = 20\). 2. Draw a skeleton structure with a single bond between atoms: -C-C-C- with hydrogens connected to each carbon. 3. Distribute remaining valence electrons to achieve octets: In this case, each carbon is already surrounded by 8 electrons. 4. Adjust formal charge assignments if needed: No need in this case. The Lewis structure for propane is as follows: ``` H H H | | | H-C-C-C-H | | | H H H ```

b) Cyclopropane, C3H6

1. Calculate the total number of valence electrons: 3 carbon has 4 valence electrons each, and 6 hydrogen has 1 valence electron each. The total number of valence electrons is \(3\times4 + 6\times1 = 18\). 2. Draw a skeleton structure with a single bond between carbon atoms and connect hydrogens. 3. Distribute remaining valence electrons to achieve octets. 4. No need to modify electron distribution or adjust formal charges. The Lewis structure for cyclopropane is as follows: ``` H \ H-C---C-H |___| / \ H H ```

c) Benzene, C6H6

1. Calculate the total number of valence electrons: 6 carbons with 4 valence electrons each, and 6 hydrogens with 1 valence electron each. The total number of valence electrons is \(6\times4 + 6\times1 = 30\). 2. Draw a skeleton structure with alternating single and double bonds between carbons, with hydrogens connected to each carbon. 3. Distribute remaining valence electrons to achieve octets. 4. No need to modify electron distribution or adjust formal charges. The Lewis structure for benzene is as follows: ``` H \ -C====C-H // || || |\-C----C-| \\| |/ \C====C / \ H H ```

d) Methyl bromide, CH3Br

1. Calculate the total number of valence electrons: 1 carbon has 4 valence electrons, 3 hydrogens have 1 valence electron each, and 1 bromine has 7 valence electrons. The total number of valence electrons is \(1\times4 + 3\times1 + 1\times7 = 14\). 2. Draw a skeleton structure with a single bond between the carbon atom and bromine atom and connect hydrogens to carbon. 3. Distribute remaining valence electrons to achieve octets. 4. No need to modify electron distribution or adjust formal charges. The Lewis structure for methyl bromide is as follows: ``` H H H | | | C---Br ``` Note that the other substances need the same approach to draw their respective Lewis structures.

Key concepts

Valence Electrons

Understanding valence electrons is essential for drawing Lewis structures, as these are the electrons that participate in bonding. Each atom has a specific number of valence electrons based on its position in the periodic table. For instance, carbon has 4 valence electrons while hydrogen has only 1.

To draw the Lewis structure, begin by calculating the total number of valence electrons available by adding up the valence electrons of each atom in the molecule. As shown in the example with propane (C3H8), you would multiply the number of carbon atoms by 4 and add the product of the number of hydrogen atoms and 1. With this method, you can create a 'skeleton' of the structure linking atoms typically with single bonds and then distributing the remaining electrons to fulfill the octet rule for each atom.

Formal Charge

The formal charge is a concept used to determine the distribution of electrons in a molecule. It's calculated for each atom in a Lewis structure as the number of valence electrons of that atom in its ground state minus the number of its valence electrons in the molecule. These include electrons shared with other atoms and those that belong exclusively to the atom.

In many cases, such as with propane and methyl bromide, no formal charge adjustments are necessary because the atoms are already in configurations where their formal charges are zero. Remember, a proper Lewis structure should have the least number of formal charges and be as close to zero as possible, which often reflects the most stable form of the molecule.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. Lewis structures, which are two-dimensional, pave the way for predicting molecular shapes based on the concept of electron pairs repelling each other. For example, in benzene (C6H6), the carbon atoms form a planar ring with alternating single and double bonds between them, which is depicted in the Lewis structure.

To predict the actual three-dimensional shape, one would use the VSEPR (Valence Shell Electron Pair Repulsion) model. This helps to understand the spatial orientation of atoms. Understanding molecular geometry is crucial in chemistry because it influences the reactivity, polarity, color, phase of matter, and biological activity of molecules.

Resonance Structures

Resonance structures are a set of two or more Lewis structures that represent a single molecule where the electron distribution can be different. These structures account for molecules where the electron pair distribution can be shown in multiple valid ways. Benzene is a classic example where resonance comes into play. Its Lewis structure can be drawn with alternating double bonds, but in reality, the electrons in the double bonds are delocalized around the ring structure.

When more than one valid Lewis structure exists for a molecule, they are called resonance structures. These structures are drawn with double-headed arrows between them, indicating that the actual molecule is a hybrid of the resonance forms, with the true distribution of electrons being an average of that depicted by the resonance structures.