Question

What kinds of intermolecular forces are present in each substance? (a) \(\mathrm{NCl}_{3}\) (trigonal pyramidal) (b) \(\mathrm{NH}_{3}\) (trigonal pyramidal) (c) \(\mathrm{SiH}_{4}\) (tetrahedral) (d) \(\mathrm{CCl}_{4}\) (tetrahedral)

Short answer

NCl3 displays dipole-dipole interactions and London dispersion forces. NH3 shows hydrogen bonding, dipole-dipole interactions, and London dispersion forces. SiH4 and CCl4 only exhibit London dispersion forces due to their nonpolar nature.

Step-by-step solution

Analyze Molecular Geometry and Polarity for NCl3

The molecule trichloride (NCl3) has a trigonal pyramidal shape, which typically suggests that it is not symmetrical. Since nitrogen and chlorine have different electronegativities, this asymmetry leads to a polar molecule with a net dipole moment.

Identify Intermolecular Forces for NCl3

Since NCl3 is polar, it will exhibit dipole-dipole interactions. Additionally, all molecules exhibit London dispersion forces (induced dipole-induced dipole interactions), including NCl3. Therefore, the intermolecular forces in NCl3 include dipole-dipole forces and London dispersion forces.

Analyze Molecular Geometry and Polarity for NH3

Ammonia (NH3) has a trigonal pyramidal shape and is also polar due to the difference in electronegativity between nitrogen and hydrogen and the lack of symmetry in its shape. This results in a net dipole moment.

Identify Intermolecular Forces for NH3

Besides dipole-dipole interactions and London dispersion forces, NH3 also engages in hydrogen bonding due to the presence of a highly electronegative nitrogen atom bonded to hydrogen.

Analyze Molecular Geometry and Polarity for SiH4

Silane (SiH4) has a tetrahedral shape and is nonpolar because the electronegativity difference between silicon and hydrogen is not significant, and its symmetrical shape cancels any dipole moments.

Identify Intermolecular Forces for SiH4

Because SiH4 is nonpolar, the only intermolecular force present is the London dispersion force.

Analyze Molecular Geometry and Polarity for CCl4

Carbon tetrachloride (CCl4) has a tetrahedral geometry and is nonpolar. Despite chlorine being more electronegative than carbon, the symmetry of the molecule results in the cancellation of the dipole moments.

Identify Intermolecular Forces for CCl4

Similar to SiH4, CCl4 only experiences London dispersion forces because it is a nonpolar molecule.

Key concepts

Molecular Geometry

Understanding the shape of molecules, known as molecular geometry, is crucial because it can affect a molecule's polarity and, therefore, its intermolecular forces. For example, NCl₃ and NH₃, both with trigonal pyramidal geometry, exhibit a lack of symmetry that leads to a net dipole moment. This asymmetry is less evident in molecules with tetrahedral geometry like SiH₄ and CCl₄, which are symmetrical enough that any individual dipole moments cancel out, resulting in nonpolar molecules.

The three-dimensional arrangement of atoms dictates how molecule-to-molecule interactions occur. Even slight differences in geometry can lead to significant changes in how molecules behave and interact with each other, especially when it comes to solubility in different solvents, reaction mechanisms, and biological interactions.

Electronegativity

Electronegativity is a measure of how strongly an atom can attract or hold onto electrons when it forms a chemical bond. Atoms like nitrogen, oxygen, and chlorine have high electronegativities compared to hydrogen, or to silicon and carbon in the context of the given exercise.

When atoms with different electronegativities bond together, unequal sharing of electrons occurs, resulting in partial charges on the atoms. This can lead to the formation of polar bonds within the molecules, such as those seen in NCl₃ and NH₃, where the difference in electronegativity between nitrogen and the bonding atoms results in polar bonds and a net dipole in the molecule.

Polar and Nonpolar Molecules

Polarity in molecules arises from the asymmetrical distribution of electrons within the structure, leading to a molecule having an uneven charge distribution. In molecules like NCl₃ and NH₃, the shapes are asymmetrical, and the atoms involved have different electronegativities, resulting in polar molecules with a net dipole moment. Conversely, molecules such as SiH₄ and CCl₄ are nonpolar; despite differences in electronegativity, their symmetrical shapes allow for an even charge distribution, canceling out any dipoles.

This distinction is important as it not only determines the types of intermolecular forces that can occur but also influences properties like boiling and melting points, solubility, and reactivity.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that have permanent dipole moments. These forces are the electrostatic attractions between the positive end of one polar molecule and the negative end of another. In the case of NCl₃, for instance, the uneven charge distribution within the molecule creates a scenario where the nitrogen end is partially negative while the chlorine ends are partially positive, allowing it to align with and attract other polar molecules.

Think of dipole-dipole interactions as small magnets within a material, where the opposite poles attract. This is one reason why polar substances tend to have higher boiling points than their nonpolar counterparts—they require more energy to overcome these intermolecular forces during phase changes.

London Dispersion Forces

London dispersion forces are the weakest type of intermolecular force, yet they are also the most universal, as they are present in both polar and nonpolar molecules. They originate from temporary dipoles that occur when the electron cloud around an atom or molecule is momentarily unevenly distributed, inducing a dipole in a neighboring atom or molecule.

Even nonpolar molecules like SiH₄ and CCl₄ experience these forces because electrons are constantly moving and can form instantaneous dipoles. For molecules without permanent dipoles, these forces are the primary source of attraction between the particles. London dispersion forces are also more pronounced in larger atoms or molecules because there is a greater probability of temporary dipoles forming.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction, but it only occurs when hydrogen is bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine, as is the case in NH₃. This interaction is responsible for many unique properties of substances—in water's high boiling point, for example, and in the structure of DNA.

The electronegative atom attracts the electron from the hydrogen atom, resulting in a partially positive hydrogen that can interact strongly with a lone pair on another electronegative atom. These bonds are essential in biochemistry, as they play a key role in maintaining the structure and function of biomolecules such as proteins and nucleic acids.