Types of Crystalline Solids Which solid in each pair has the higher melting point and why? \(\begin{array}{ll}{\text { a. }} & {\mathrm{TiO}_{2}(s) \text { or } \mathrm{HOOH}(s)} & {\text { b. } \mathrm{CCl}_{4}(s) \text { or } \mathrm{SiCl}_{4}(s)} \\ {\text { c. }} & {\mathrm{Kr}(s) \text { or } \mathrm{Xe}(s)} & {\text { d. } \mathrm{NaCl}(s) \text { or } \mathrm{CaO}(s)}\end{array}\)

Chemical bonding is the force of attraction between atoms or ions in a compound. There are several types of chemical bonds, each affecting the physical properties of materials differently. For instance, the melting points of crystalline solids are directly related to the strength of their chemical bonds.

• Ionic bonds involve the transfer of electrons from one atom to another, leading to the formation of charged ions that are held together by electrostatic forces.
• Covalent bonds, on the other hand, involve sharing electrons between atoms and can vary greatly in strength. At one end are covalent network solids, extremely strong due to their 3D network of bonds, like in diamond or quartz.
• Metallic bonds are found in metals where electrons are delocalized over a lattice of positive ions, providing unique properties like conductivity and malleability.
• Finally, molecules held together by weaker van der Waals forces or hydrogen bonds tend to have lower melting points since less energy is required to disrupt these interactions.

Understanding the type of bonding is crucial when predicting the melting point of a solid.

Intermolecular forces are attractions between molecules, influencing the behavior of substances, especially regarding their phase changes, like melting.

The significance of these forces lies in their varying strengths:

• Hydrogen bonds are especially strong intermolecular forces that occur between hydrogen and electronegative atoms like oxygen or nitrogen.
• Dipole-dipole interactions are found between polar molecules, where positive and negative charges attract.
• London dispersion forces, also known as van der Waals forces, are the weakest and result from temporary dipoles induced in atoms or molecules.

Substances with strong intermolecular forces, like hydrogen bonds, usually have higher melting points compared to those with just van der Waals forces, as more thermal energy is required to overcome the attraction between molecules.

Ionic bonds are one of the strongest types of chemical bonds and are a major factor in the high melting points of ionic compounds. These bonds form when electrons are transferred from a metal to a non-metal, creating positively charged cations and negatively charged anions.

These ions are then held together in a lattice structure by the strong electrostatic forces between them. The greater the charge and the smaller the ionic radii, the stronger the ionic bond will be. For example, in the case of CaO, the presence of Ca²⁺ and O^2- ions, which have double charges compared to Na⁺ and Cl^- in NaCl, results in a significantly higher melting point for CaO.

Van der Waals forces are the weakest type of intermolecular interaction and consist of London dispersion forces and dipole-dipole interactions. These forces originate from the temporary fluctuations in electron density within atoms and molecules, which create temporary dipoles that induce further dipoles in neighboring particles, leading to a chain of attractions.

Despite their weak nature, van der Waals forces can be significant in larger atoms or molecules where there are more electrons to contribute to the dispersion forces. Thus, substances like xenon (Xe) with more electrons and a larger atomic radius will have stronger van der Waals forces and a higher melting point compared to smaller atoms like krypton (Kr).