What is meant by (a) an ionic compound and (b) a molecular compound? What are the typical properties of the rwo classes of compounds?

An ionic compound is a substance formed when atoms transfer electrons to or from each other, resulting in the creation of ions. These ions have opposite charges; with the loss of electrons, an atom becomes a positively charged ion, known as a cation, and with the gain of electrons, an atom turns into a negatively charged ion, known as an anion. The ionic bond is the electrostatic attraction that holds cations and anions together, and this force is what gives ionic compounds some of their distinctive properties.

For instance, ionic compounds tend to have high melting and boiling points because the electrostatic forces between ions are strong, requiring more energy to break them apart. These compounds are often solid and crystalline at room temperature. When dissolved in water or melted, ionic compounds become excellent conductors of electricity, thanks to the free movement of ions. It's their rigid lattice structure that explains why ionic compounds are typically hard and brittle.

In contrast, a molecular compound, also known as a covalent compound, usually forms when atoms share electrons with each other. This electron sharing allows each atom to attain a complete valence shell, leading to the formation of a covalent bond. Such bonds are prevalent between nonmetal atoms, where neither atom has enough strength to outright steal electrons from the other.

Substances with molecular structures tend to have lower melting and boiling points compared to ionic compounds, as the forces between individual molecules – known as van der Waals forces – are much weaker than ionic bonds. Molecular compounds encompass various states of matter at room temperature, such as gases, liquids, or low-melting-point solids, and do not typically conduct electricity as they lack free ions. Their solubility in water varies, generally correlating with the polarity of the molecules.

The term electrostatic forces refers to the interactions between electrically charged particles. These forces are governed by Coulomb's Law, which states that like charges repel each other while opposite charges attract. In the context of ionic compounds, the electrostatic forces at play are what hold the oppositely charged ions together, creating a strong and stable lattice structure.

The strength and directional nature of these electrostatic forces help to explain the high melting and boiling points of ionic compounds, as well as their general trend towards solubility in polar solvents like water. The electrostatic interactions are crucial in determining the physical and chemical properties of ionic substances and play a significant role in various fields of chemistry and physics.

A covalent bond is the chemical bond formed when two atoms share a pair of electrons. This kind of bond is a key characteristic of molecular compounds and contributes to their unique properties. The shared electrons occupy an overlapping area known as the molecular orbital, and this sharing allows each atom to achieve a more stable electronic configuration.

Unlike the electrostatic forces seen in ionic compounds, covalent bonds rely on electron sharing rather than the full transfer of electrons. This difference results in lower melting and boiling points because the intermolecular forces in molecular substances are weaker than the ionic bonds in a salt crystal. The ability to share electrons between atoms is the foundation of molecule formation and defines much of the chemistry of life, including organic molecules and biochemical interactions.