Question

Use VSEPR theory to predict the geometric shapes of the following molecules and ions: (a) \(\mathrm{N}_{2}\); (b) HCN; (c) \(\mathrm{NH}_{4}^{+} ;\) (d) \(\mathrm{NO}_{3}^{-} ;\) (e) NSF.

Short answer

The shapes of the molecules and ions are as follows: (a) \(\mathrm{N}_{2}\) - Linear; (b) HCN - Linear; (c) \(\mathrm{NH}_{4}^{+} ;\) - Tetrahedral; (d)\(\mathrm{NO}_{3}^{-} ;\) - Trigonal Planar; (e) NSF - Bent or V-Shaped.

Step-by-step solution

Calculate Steric Number for N2

As nitrogen (N) has five valence electrons, N2 means that each nitrogen shares three electrons with each other. Thus, the steric number becomes 3 and the shape is Linear.

Predict Shape for HCN

Carbon (C), the central atom here, has 4 valence electrons. In HCN, it forms a double bond with Nitrogen (N) and a single bond with Hydrogen (H). Total bonded electrons = 4, so the steric number is 4 (3 from N and 1 from H). Hence, the shape is Linear.

Predict Shape for NH4+

Nitrogen (N) has 5 valence electrons. In Ammonium ion (NH4+), it forms 4 bonds with Hydrogens so total bonded electrons = 8 (4x2 from each H). The steric number is 4, which leads to a Tetrahedral shape.

Predict Shape for NO3-

In Nitrate ion (NO3-), Nitrogen (N) is the central atom with 5 valence electrons. It forms three bonds (2 with one Oxygen and 1 with two others). In total, bonded electrons = 6 (2x3 from each O). The steric number is 3, hence the shape is Trigonal Planar.

Predict Shape for NSF

In NSF, Sulphur (S) is the central atom with 6 valence electrons. It forms 2 single bonds with Nitrogen and Fluorine, so total bonded electrons = 4 (2x2 from each N, F). The steric number is 4, therefore the shape is Bent or V-Shaped.

Key concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This geometry determines many chemical properties, including reactivity and polarity. Using the Valence Shell Electron Pair Repulsion (VSEPR) theory, we can predict these shapes by examining the repulsion between electron pairs. These pairs repel each other to be as far apart as possible, leading to specific geometric structures.
For example, molecules like \( ext{N}_2\) and HCN have linear molecular geometry because the atoms align in a straight line due to minimal repulsion in a simple bonding scenario. The NH\(^4\)^+ ion, with its four bond pairs, adopts a tetrahedral shape to maximize distance between them. In contrast, \( ext{NO}_3^-\) is trigonal planar because the nitrogen forms three bonds that spread evenly in the plane.

Steric Number

The steric number is essential for predicting the shape of a molecule using VSEPR theory. It is calculated by summing the number of atoms bonded to the central atom and the number of lone pairs on the central atom. A higher steric number indicates more regions of electron density around the central atom.
For example:

• In \( ext{N}_2\), with a steric number of 3 (from sharing electrons), the structure is linear.
• HCN has a steric number of 2 from its bonded atoms, leading to a linear shape.
• NH\(^4\)^+, with a steric number of 4, forms a tetrahedral configuration.
• The trigonal planar shape of \( ext{NO}_3^-\) is due to a steric number of 3.
• NSF has a steric number of 4, resulting in a bent shape because of electron pair arrangement.

Understanding the steric number helps us anticipate molecular geometry more accurately.

Electron Pair Repulsion

Electron pair repulsion is the driving force behind a molecule's geometry. The VSEPR model is based on the idea that electron pairs around a central atom will repel each other and position themselves to be as far apart as possible. These pairs may include bonded electron pairs as well as non-bonded lone pairs.
For instance, in the NH\(^4\)^+ ion, four hydrogen atoms share electrons with nitrogen, avoiding any lone pair repulsion, hence a regular tetrahedral shape. However, molecules like NSF demonstrate bent geometry due to lone pair-bond pair repulsions altering the ideal angles. \( ext{NO}_3^-\) lacks lone pairs on the central atom, emphasizing bonds in a planar spread.

Chemical Bonding

Chemical bonding is the foundation of molecular formation and geometry prediction. The type and number of bonds influence the molecular shape and stability.
Consider the following bonding scenarios:

• In \( ext{N}_2\), triple bonds dominate, creating a linear shape due to lack of additional bonds or lone pairs.
• HCN features a combination of a triple bond and a single bond, which results in a linear structure due to repulsion minimization.
• NH\(^4\)^+ experiences single bonds with hydrogen, allowing for a symmetric tetrahedral arrangement.
• With double and single bonds, \( ext{NO}_3^-\) attains a planar triangular form.
• Lastly, NSF includes single bonds and exhibits a bent shape due to lone pair repulsion effects.

The variety of chemical bonds—single, double, triple—contributes to diverse and interesting molecular geometries within chemical compounds.