Ch.9 - Molecular Geometry and Bonding Theories

Brown - Chemistry: The Central Science 14th Edition

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Brown 14th Edition

Ch.9 - Molecular Geometry and Bonding Theories

Problem 21b

Chapter 9, Problem 21b

How many nonbonding electron pairs are there in each of the following molecules: (b) HCN

Verified step by step guidance

Step 1: Draw the Lewis structure for the molecule. For HCN, the Lewis structure would be H-C≡N. This shows that hydrogen (H) is single-bonded to carbon (C), and carbon is triple-bonded to nitrogen (N).

Step 2: Determine the number of valence electrons for each atom. Hydrogen has 1, carbon has 4, and nitrogen has 5.

Step 3: Calculate the total number of electrons used in bonding. Each single bond uses 2 electrons, and each triple bond uses 6 electrons. So, for HCN, there are 2 electrons in the H-C bond and 6 electrons in the C≡N bond, totaling 8 electrons.

Step 4: Subtract the total number of bonding electrons from the total number of valence electrons. For HCN, this would be (1+4+5) - 8 = 2. These are the nonbonding electrons.

Step 5: Since each pair of electrons counts as one, divide the number of nonbonding electrons by 2 to get the number of nonbonding electron pairs. For HCN, this would be 2/2 = 1 nonbonding electron pair.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms in a molecule and the lone pairs of electrons that may exist. They help visualize how many valence electrons are present and how they are distributed among the atoms. Understanding Lewis structures is essential for determining the number of nonbonding electron pairs in a molecule.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial for forming bonds with other atoms. The number of valence electrons determines how many bonds an atom can form and how many nonbonding pairs may exist. In the case of HCN, knowing the valence electrons of hydrogen, carbon, and nitrogen is key to identifying nonbonding pairs.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule, which is influenced by the number of bonding and nonbonding electron pairs. The presence of nonbonding electron pairs can affect the shape and angles between bonds, impacting the overall structure of the molecule. Understanding molecular geometry is important for predicting the behavior and reactivity of molecules like HCN.

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