A fundamental difference between compounds containing ionic bonds and those containing covalent bonds is the existence of molecules. Fiplain why molecules exist in solid covalent compounds but not in solid ionic compounds.

When we delve into the fascinating world of chemistry, one key aspect we encounter is the molecular structure. This term describes the three-dimensional arrangement of atoms within a molecule. In the case of covalent compounds, atoms come together by sharing electrons, establishing a bond that is both strong and directional. This sharing leads to the creation of distinct, stable units called molecules, each with a precise number of atoms and a defined shape that dictates the compound's properties and behaviors.

For instance, water (H₂O) has a molecular structure wherein each hydrogen atom shares electrons with the central oxygen atom, resulting in a bent shape. This structure contributes to the unique properties of water, such as its ability to dissolve many substances, a characteristic indispensable to life on Earth. Molecular structures are foundational in understanding how substances interact, react, and exist in various states of matter.

Stepping further into the atomic realm, we encounter the intricacies of covalent bonding. Atoms bond covalently in a quest to achieve stability by filling their outer electron shells. This is achieved by sharing pairs of electrons, with each shared pair constituting a single covalent bond.

Take a simple molecule like methane (CH₄) as an example. The carbon atom forms covalent bonds with four hydrogen atoms, sharing electrons such that the outer shells are completed, leading to a stable tetrahedral structure. This sharing is highly specific, as it occurs in predictable patterns, allowing us to forecast the molecular structure of a vast array of covalent substances. Covalent bonding is fundamentally a 'give and take' relationship at the atomic level, leading to the immense variety of organic and inorganic molecules we know.

In stark contrast to the mutual sharing seen in covalent bonds, ionic bonding hinges on the transfer of electrons. When an atom loses electrons, it becomes positively charged (a cation), and when an atom gains electrons, it becomes negatively charged (an anion). Ionic compounds form through the electrostatic attraction between these oppositely charged ions.

Iterations of this process can be observed in compounds like sodium chloride (NaCl). Sodium (Na) donates an electron to chlorine (Cl), resulting in Na⁺ and Cl⁻ ions. These ions are then bound in a lattice by electrostatic forces, a starkly different structure from the individual molecules found in covalent compounds. Ionic bonding leads to properties such as high melting points and the ability to conduct electricity when dissolved or molten, which are key identifiers for substances that contain ionic bonds.

Crystal lattice structure is the architectural marvel of the ionic compound world. Ions congregate in a geometrically ordered three-dimensional array, creating a solid material that extends in all directions. This is the hallmark of ionic compounds; a continuous pattern, void of the distinct molecules that characterize covalent compounds.

Every ion in this impressive network is surrounded by ions of the opposite charge, a configuration that significantly contributes to the overall stability of the substance. Salt crystals, for example, showcase ions arrayed in repeating patterns to form a solid with a high degree of symmetry. This structure imparts ionic substances with their signature high melting points and brittleness, and also features in industrial applications ranging from the manufacture of jewelry to the production of electrical components. Understanding the crystal lattice is crucial for envisioning how ionic substances are constructed and how they differ fundamentally from the discreetly bonded world of covalent compounds.