Problem 34 - Chemistry: Principles and Reactions

Question

Predict the geometry of the following species: (a) NNO (b) \(\mathrm{ONCl}\) (c) \(\mathrm{NH}_{4}{\underline{\phantom{xx}}}^{+}\) (d) \(\mathrm{O}_{3}\)

Step-by-Step Solution

Answer

Question: Based on the VSEPR theory, predict the geometries of the following species: (a) NNO, (b) ONCl, (c) NH4+, and (d) O3. Answer: (a) NNO has a linear geometry, (b) ONCl has a bent geometry, (c) NH4+ has a tetrahedral geometry, and (d) O3 has a bent geometry.

Step 1: Identify the central atoms

For each given species, determine the central atom, which usually has the lowest electronegativity. (a) NNO: The central atom is N. (b) ONCl: The central atom is O. (c) NH4+: The central atom is N. (d) O3: The central atom is O.

Step 2: Determine electron pairs and lone pairs around the central atoms

Using Lewis structures, count the number of bonding electron pairs (single, double, or triple bonds) and lone pairs of electrons around each central atom. (a) NNO: NN triple bond and NO double bond. Total of 2 bonding pairs and 0 lone pairs. (b) ONCl: O single bond with N and N single bond with Cl. Total of 2 bonding pairs and 1 lone pair. (c) NH4+: N has 4 single bonds with H. Total of 4 bonding pairs and 0 lone pairs. (d) O3: central O double bond with one O and single bond with another O. Total of 2 bonding pairs and 1 lone pair.

Step 3: Apply VSEPR theory to predict geometries

Use the VSEPR theory to predict the geometries of the given species by minimizing repulsion between electron pairs. The geometries are determined by the arrangement of electron pairs around the central atom. (a) NNO: linear geometry. (b) ONCl: bent geometry. (c) NH4+: tetrahedral geometry. (d) O3: bent geometry.

Key Concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) Theory is a fundamental concept in chemistry used to determine the shapes of molecules. The theory rests on the idea that electron pairs surrounding a central atom will arrange themselves as far apart as possible to minimize repulsion. This leads to specific geometric structures that are characteristic of different numbers of electron pairs. VSEPR theory considers both bonding pairs of electrons (those involved in chemical bonds) and lone pairs (non-bonding valence electrons).

• Bonding pairs contribute to the size and shape of the molecule.
• Lone pairs also affect the geometry as they occupy space and repel other electron pairs, potentially altering bond angles.

The theory is typically applied as a step in determining molecular geometry: first the number of electron pairs is counted, then the geometry is adjusted to ensure maximum spacing between all electron pairs.

Lewis Structures

Lewis structures provide a visual representation of the valence electrons in a molecule. By showing these electrons, Lewis structures help identify how atoms are bonded in a molecule and how electrons are distributed. To create a Lewis structure:

• Determine the total number of valence electrons in the molecule.
• Distribute these electrons to form bonds between atoms, aiming for full octets where possible.
• Any remaining electrons are placed as lone pairs, which can influence the shape predicted by the VSEPR theory.

Lewis structures are crucial for understanding molecular geometry as they facilitate the identification of bonding pairs and lone pairs, which are critical inputs for the VSEPR model.

Bonding Pairs

Bonding pairs of electrons are the shared electron pairs that form the chemical bonds between atoms in a molecule. Each bond, whether single, double, or triple, is formed by the sharing of one or more pairs of electrons.

• Single bonds involve one bonding pair, double bonds involve two, and triple bonds have three.
• These pairs are shared between atoms to achieve stability by fulfilling the octet rule.

The number of bonding pairs directly affects the molecular geometry, as indicated by VSEPR theory. For instance, in NNO, the N atom shares triple and double bonds with another N and O respectively, leading to a linear geometry.

Lone Pairs

Lone pairs are valence electrons not involved in bonding. These electrons remain on a single atom and have a significant effect on molecular shape due to their repulsion of other electron pairs.

• An atom with lone pairs will often have bond angles smaller than expected due to this repulsion.
• Lone pairs can change the idealized geometry to a position of minimal repulsion.

In the molecule O3 (ozone), there is a lone pair on the central oxygen atom. This lone pair influences the geometry, resulting in a bent shape rather than a triangular planar arrangement, closely following predictions by VSEPR theory.