Question

The following electrostatic potential diagrams represent \(\mathrm{CH}_{4}\) , \(\mathrm{NH}_{3},\) or \(\mathrm{H}_{2} \mathrm{O}\) . Label each and explain your choices.

Short answer

Based on the molecular geometries and electrostatic potential diagrams, the tetrahedral diagram with equal charge distribution should be labeled as CH4, due to the nonpolar nature of C-H bonds and symmetric distribution of charge in the molecule. The trigonal pyramidal diagram with partial charges should be labeled as NH3, as it has a lone pair of electrons on the nitrogen atom contributing to a partial negative charge, and three hydrogen atoms contributing to partial positive charges. Lastly, the bent or V-shaped diagram with partial charges should be labeled as H2O, with the oxygen atom having a partial negative charge due to two lone pairs of electrons, and two hydrogen atoms having partial positive charges.

Step-by-step solution

Molecular Geometries of CH4, NH3, and H2O

Each molecule has a distinct molecular geometry due to the arrangement of atoms in the molecule and their bond angles. 1. CH4 (Methane): CH4 has a tetrahedral geometry with bond angles of 109.5 degrees. 2. NH3 (Ammonia): NH3 has a trigonal pyramidal geometry with bond angles of 107 degrees. 3. H2O (Water): H2O has a bent or V-shaped geometry with bond angles of 104.5 degrees. Step 2: Examine electrostatic potential diagrams

Electrostatic Potential Diagram Analysis

Electrostatic potential diagrams show the distribution of charge in a molecule. Blue areas indicate positive charge (electrophilic regions), and red areas indicate negative charge (nucleophilic regions). With this information, we can analyze and label the given diagrams: 1. The diagram with equal distribution of charge around the central atom, with a tetrahedral arrangement, corresponds to CH4. This is due to the nonpolar nature of C-H bonds and symmetric distribution of charge in the molecule. 2. The diagram with three surrounding atoms and a lone pair on the central atom, having a trigonal pyramidal arrangement, corresponds to NH3. The molecule has a lone pair of electrons on the nitrogen atom which contributes to a partial negative charge, while the three hydrogen atoms contribute to partial positive charges. 3. Finally, the diagram showing two surrounding atoms and two lone pairs on the central atom, with a bent arrangement, corresponds to H2O. The oxygen atom has a partial negative charge due to the two lone pairs of electrons, and the two hydrogen atoms have partial positive charges. Step 3: Label the diagrams

Final Labeling

Based on our analysis of the molecular geometries and electrostatic potential diagrams: 1. Label the tetrahedral diagram with equal charge distribution as CH4. 2. Label the trigonal pyramidal diagram with partial charges as NH3. 3. Label the bent or V-shaped diagram with partial charges as H2O. Make sure to mention the geometry and distribution of charges as reasons for the labeled choices.

Key concepts

Electrostatic Potential Diagrams

Electrostatic potential diagrams are a visual representation of the charge distribution within a molecule. These diagrams help us understand how electrons are distributed across a molecule.
Red areas on these diagrams indicate regions of high electron density, or nucleophilic areas, and typically correspond to atoms with higher electronegativity. In contrast, blue areas suggest regions of low electron density, known as electrophilic regions, and typically correlate with less electronegative atoms.

By interpreting these diagrams, we can predict how molecules will interact with each other—specifically, how they might attract or repel each other based on charge. For instance, a molecule with a predominantly red area will have a tendency to attract electrophilic atoms. Understanding this electrostatic potential is key in fields like chemistry and biochemistry, as it helps in predicting molecular behavior and reactivity.

Tetrahedral Geometry

Tetrahedral geometry is a basic molecular shape that occurs when a central atom is bonded to four surrounding atoms, with no lone pairs affecting the shape. This geometry features bond angles of approximately 109.5 degrees, creating an equilateral shape around the central atom. Methane (\(\mathrm{CH}_{4}\)) is a classic example of a molecule possessing tetrahedral geometry.

The characteristics of a tetrahedral molecule result in a symmetrical distribution of charge, often leading to nonpolar molecules if all surrounding atoms are identical, like in methane. This symmetry means there are no distinct regions of positive or negative charge, making tetrahedral molecules like methane quite inert and nonpolar, as all the dipoles cancel each other out.

Trigonal Pyramidal Geometry

Trigonal pyramidal geometry is a molecular shape that arises in molecules with a central atom that has three bonds and one lone pair, resulting in unsymmetrical geometry. Ammonia (\(\mathrm{NH}_{3}\)) is a well-known example.
Unlike tetrahedral geometry, this shape has bond angles slightly less than 109.5 degrees—usually around 107 degrees—due to the repulsion caused by the lone electron pair.

The lone pair on the nitrogen atom in ammonia leads to a distortion from perfectly tetrahedral to trigonal pyramidal, creating a partial negative charge on the nitrogen due to the added electron concentration. The geometric arrangement pulls the hydrogen atoms closer together, and due to the uneven distribution of charges, ammonia is a polar molecule with a permanent dipole moment.

Bent Molecular Geometry

Bent, or V-shaped, molecular geometry occurs when a central atom is bonded to two other atoms and possesses two lone pairs, resulting in a significant deviation from linear geometry. Water (\(\mathrm{H}_{2} \mathrm{O}\)) exemplifies this shape.
Due to the lone pairs, the bond angles in water are around 104.5 degrees, smaller than those in a perfect tetrahedral arrangement.

The presence of two lone pairs on the oxygen atom causes a considerable repulsion, bending the molecule and creating a strong dipole moment. This bend results in distinct positive charges near the hydrogen atoms and a negative charge on the oxygen atom, making water highly polar. Understanding the bent geometry of water helps explain many of its unique properties, such as its high boiling point and distinct solvent capabilities.