Write Lewis structures that obey the octet rule (duet rule for H) for each of the following molecules. a. \(\mathrm{H}_{2} \mathrm{CO} \quad\) c. \(\mathrm{HCN}\) b. \(\mathrm{CO}_{2}\)

Short Answer

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The Lewis structures for the given molecules are as follows: a. H2CO: H-C=O-H b. CO2: O=C=O c. HCN: H-C≡N

Step by step solution

01

Counting valence electrons

For each molecule, we first count the total number of valence electrons available for bonding. We do this by adding the valence electrons of each individual atom in the molecule. a. H2CO: - Hydrogen: 1 valence electron (each hydrogen atom) - Carbon: 4 valence electrons - Oxygen: 6 valence electrons Total: 1(2) + 4 + 6 = 12 valence electrons b. CO2: - Carbon: 4 valence electrons - Oxygen: 6 valence electrons (each oxygen atom) Total: 4 + 6(2) = 16 valence electrons c. HCN: - Hydrogen: 1 valence electron - Carbon: 4 valence electrons - Nitrogen: 5 valence electrons Total: 1 + 4 + 5 = 10 valence electrons **Step 2: Determine the Central Atom**
02

Determining the central atom

In each molecule, the atom with the highest bonding capacity, which is usually the least electronegative element, will be the central atom. a. H2CO: Carbon is the central atom. b. CO2: Carbon is the central atom. c. HCN: Carbon is the central atom. **Step 3: Arrange Atoms and Distribute Electrons**
03

Arranging atoms and electron distribution

Next, we arrange the atoms around the central atom and distribute the electrons to form bonds and fulfill the octet rule for each atom (excluding hydrogen). a. H2CO: Place the carbon atom in the center, with the two hydrogen atoms and the oxygen atom around it. Two electrons form a bond between each hydrogen atom and the carbon atom. Then, we add a double bond between carbon and oxygen, since oxygen needs two more electrons to follow the octet rule, and we have 4 valence electrons left in our pool. This leaves us with the Lewis structure: H-C=O-H. b. CO2: Place the carbon atom in the center with both oxygen atoms on either side. Form two double bonds, one between each oxygen atom and the carbon atom. This uses the 16 valence electrons available, and the Lewis structure is: O=C=O. c. HCN: Place the carbon atom in the center with hydrogen atom and nitrogen atom on either side. One electron pair forms a bond between hydrogen and carbon. A triple bond between carbon and nitrogen uses the remaining six valence electrons, fulfilling the octet rule for both carbon and nitrogen, while hydrogen follows the duet rule. The Lewis structure is: H-C≡N. Now we have successfully drawn the Lewis structures for all three molecules that obey the octet and duet rules.

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