Problem 126 - Chemistry The Science in Context

Question

The products of the reaction between boron and NO can be trapped in solid argon matrices. Among the products is BNO. Draw the Lewis structure for BNO, including any resonance forms. Assign formal charges and predict which structure provides the best description of the bonding in this molecule. Do any of your structures contain atoms without complete octets? Predict the molecular geometry of BNO.

Step-by-Step Solution

Answer

Answer: The molecular geometry of BNO is trigonal planar, with bond angles approximately 120 degrees.

Step 1: Draw the Lewis Structure with Resonance Forms

We begin by drawing the Lewis structure of BNO. First, we count the total number of valence electrons. Boron has 3, nitrogen has 5, and oxygen has 6, giving us a total of 14 valence electrons. We then connect the atoms and spread the electrons to complete the octets. The Lewis structure of BNO is: `B - N = O`

Step 2: Calculate Formal Charges

Now, we calculate the formal charges for each atom in the structure. We can use the formula: Formal Charge = Valence Electrons - Non-bonding Electrons - 1/2 (Bonding Electrons). We find: - Formal charge on B: 3 - 0 - 1/2(4) = 0 - Formal charge on N: 5 - 2 - 1/2(6) = 0 - Formal charge on O: 6 - 4 - 1/2(4) = 0 All formal charges are 0, which is desirable for a Lewis structure.

Step 3: Identify the Best Structure Based on Formal Charges

Since there are no formal charges for the atoms in the BNO structure, it does not have any resonance. The structure with zero formal charges is considered the most stable and therefore the best description of the bonding in BNO.

Step 4: Check for Complete Octets

Now, we will check for complete octets in the molecule. Nitrogen and oxygen already have complete octets. However, boron has only 6 electrons surrounding it. Therefore, boron does not have a complete octet, which means it is electron-deficient.

Step 5: Predict Molecular Geometry

Finally, to predict the molecular geometry of BNO, we can use the VSEPR (Valence Shell Electron Pair Repulsion) theory. The central atom, nitrogen, has three electron groups around it (one single bond to boron, a double bond to oxygen, and a lone pair). This configuration results in a trigonal planar arrangement in which all the bond angles should be approximately 120 degrees.

Key Concepts

Resonance Forms

When working with Lewis structures, resonance forms are alternative ways of drawing the same molecule that show different placements of electrons. In the case of BNO, resonance forms can help us understand possible variations of how electrons are distributed among the atoms in the molecule. However, in the BNO molecule, all formal charges are zero. This suggests that a single structure effectively describes the electron distribution without needing multiple resonance forms. In general, resonance forms are important as they represent molecules that can't be accurately depicted by a single Lewis structure, due to delocalized electrons.

Formal Charges

Formal charges are used to determine the most accurate Lewis structure by calculating the charge of an atom in the molecule. This is done using the formula:

• Formal Charge = Valence Electrons - Non-bonding Electrons - 1/2 (Bonding Electrons)

For BNO:

• Boron has a formal charge of 0: 3 valence electrons - 0 non-bonding electrons - 1/2 of 4 bonding electrons.
• Nitrogen also has a formal charge of 0: 5 valence electrons - 2 non-bonding electrons - 1/2 of 6 bonding electrons.
• Oxygen has a formal charge of 0: 6 valence electrons - 4 non-bonding electrons - 1/2 of 4 bonding electrons.

A formal charge of zero on all atoms suggests a stable configuration for the molecule.

Octet Rule

The octet rule is a fundamental concept in chemistry that states atoms tend to bond in such a way that each atom has eight electrons in its valence shell, mimicking the electron configuration of a noble gas. For BNO, nitrogen and oxygen fulfill the octet rule, having complete sets of valence electrons. However, boron is an exception; it is electron-deficient with only six electrons. Boron often forms stable compounds even when it does not satisfy the octet rule, which is common for some of the elements in Group 13 of the periodic table.

Molecular Geometry

Molecular geometry refers to the 3D arrangement of atoms within a molecule. To predict this for BNO, we examine the central nitrogen atom, which is bound to boron and oxygen. It has three groups around it: a single bond, a double bond, and a lone pair. This forms a trigonal planar geometry according to the electron pairs' repulsion, with bond angles of approximately 120 degrees. Understanding the molecular geometry gives insight into the molecule's reactivity, polarity, and physical properties.

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory explains how molecular shapes are determined based on electron pairs' mutual repulsion. In BNO, the nitrogen's electron pairs arrange themselves to minimize repulsion. This leads to a trigonal planar shape for the molecule as described earlier. According to VSEPR, lone pairs repel more strongly than bonded pairs, slightly adjusting bond angles if lone pairs are present. This theory aids in visualizing and predicting molecular shapes crucial for advanced chemical understanding and predicting molecular behavior in reactions.