Problem 62 - Chemistry Principles and Practice

Question

The connectivity of HNO could be either HNO or HON. Draw a Lewis structure for each and predict which connectivity is the more favorable arrangement.

Step-by-Step Solution

Answer

HNO is the more favorable arrangement due to lower formal charges.

Step 1: Draw Lewis Structure for HNO

Start by counting the total valence electrons. Hydrogen (H) has 1, nitrogen (N) has 5, and oxygen (O) has 6, making a total of 12 electrons. Place H, N, and O in sequence as H-N-O. Distribute electrons to satisfy the octet for N and O. You will end up with N sharing three pairs with O to form a triple bond and sharing one pair with H.

Step 2: Calculate Formal Charges for HNO

Formal charges can help determine stability. To calculate the formal charge use: \( \text{Formal charge} = \text{Valence electrons} - \text{Non-bonding electrons} - \frac{1}{2} \text{Bonding electrons} \). For HNO: N has 5 valence, used all 5 perfectly (Formal Charge = 0), O has 6 valence but effectively uses 7 (-1), H has 1 (0). Total charge: 0.

Step 3: Draw Lewis Structure for HON

With HON, again start with 12 total valence electrons. Place hydrogen bonded to oxygen, and oxygen double bonded to nitrogen. Distribute electrons to satisfy the octet rule, while allowing N and O to share a double bond, and H to be bonded to O.

Step 4: Calculate Formal Charges for HON

In this arrangement, H and O both have no charge, but the formal charge calculation for N reveals it has a +1 charge: N(+) is less stable than O(-). Thus, each partially creates a structure: H = 0, O = 0, N = +1. Total: +1.

Step 5: Determine More Favorable Arrangement

Typically, the most stable arrangement is one with the smallest or zero total formal charges. Since HNO has a total formal charge of 0 while HON has a total formal charge of +1, the HNO connectivity is favored over HON based on the formal charge assessment.

Key Concepts

Valence Electrons

Valence electrons are the outer electrons of an atom that can participate in forming chemical bonds. Key to understanding Lewis structures, they help determine how atoms will bond with one another. Let's break down their role:

• Valence electrons are specific to each element, depending on its position in the periodic table.
• Elements in the same group have the same number of valence electrons.
• The number of valence electrons dictates the chemical reactivity and bonding preferences of the atom.

In the case of the molecule HNO, the total valence electrons are the sum of those from hydrogen (1 electron), nitrogen (5 electrons), and oxygen (6 electrons), providing us with 12 electrons to work with in drawing the Lewis structure. Keeping track of these electrons is essential for visualizing how the atoms connect and form bonds.

Formal Charge

The concept of formal charge provides insight into the stability of a molecule's Lewis structure. Think of it like a bookkeeping system for electrons. To calculate formal charge, use the formula:\[ \text{Formal charge} = \text{Valence electrons} - \text{Non-bonding electrons} - \frac{1}{2} \text{Bonding electrons} \]Here's why it's helpful:

• Structures where formal charges are minimized or zero are usually more stable.
• Total formal charge tells us about the overall charge of the molecule or ion.
• It can help determine the arrangement of atoms that best represents the actual molecule.

For HNO, calculating formal charges for each connectivity (H-N-O and H-O-N) revealed that H-N-O achieves a total formal charge of 0, making it more stable compared to the HON arrangement with a formal charge of +1.

Octet Rule

The octet rule is a guiding principle in chemistry that helps explain how atoms form bonds. It states that atoms are generally most stable when they have eight electrons in their valence shell, similar to noble gases. This rule is crucial in creating Lewis structures for molecules, particularly for:

• Ensuring that atoms like nitrogen and oxygen in HNO satisfy the octet rule by sharing electrons appropriately.
• Determining whether to draw single, double, or triple bonds between atoms to fulfill the octet requirement for non-hydrogen atoms.
• Understanding exceptions such as hydrogen (which only needs two electrons for stability in its "duet").

In the case of HNO, both nitrogen and oxygen share electrons to complete their octets, while hydrogen forms a simple bond with nitrogen. Observing how each atom attains electronic stability through the octet rule aids in predicting the most stable Lewis structure.