Compare the properties of ionic compounds and covalent compounds.

Ionic bonds form a foundational concept in chemistry that helps explain the characteristics of ionic compounds. In essence, an ionic bond is an electrostatic attraction between positively charged metal cations and negatively charged non-metal anions. These opposite charges attract each other to form a bond that is strong and directional, resulting in the formation of a crystal lattice structure, which effectively organizes ions in a repeating three-dimensional pattern.

This structure contributes to many of the properties students learn about. For example, it’s the reason why such compounds tend to be hard and have high melting and boiling points. The energy needed to break these electrostatic forces is significant, which is why we don’t see ionic compounds melting under mild conditions. But when they do melt or dissolve in water, these ions become free to move, which makes them capable of conducting electricity – a notable feature of these compounds widely explored in applications such as electrolysis.

Contrastingly, covalent bonds occur when atoms—usually non-metals—share electron pairs. Unlike the transfer of electrons seen in ionic bonding, covalent bonding involves electrons spending time in the orbitals of both atoms, creating a stable balance. This is where terminology such as 'molecule' becomes common, describing the discrete groups of atoms held together by shared electrons.

Because the bonds are based on shared electrons rather than electrostatic attractions, covalent compounds do not exhibit the same rigidity as ionic structures. This difference results in generally lower melting and boiling points, which is why water — a covalent compound — is liquid at room temperature. Moreover, these compounds are not great conductors of electricity, as the electrons are more or less localized within the bonds, unlike the free ions in ionic compounds. Additionally, the varied nature of molecular structures means that covalent compounds can take on numerous physical states, including gases, as found in the air we breathe.

Electronic conductivity is an intriguing property that distinguishes between ionic and covalent compounds. A core factor in conductivity is the presence of charged particles that can move. In ionic compounds, when dissolved in water or melted, the ions act as charge carriers and facilitate the flow of electricity. It is fascinating that these same compounds, though made of charged particles, do not conduct electricity when solid, as the ions are locked in place within the crystal lattice.

On the other hand, covalent compounds are usually poor conductors because they lack free-moving charged particles. Even when melted or dissolved, the molecules remain neutral and do not provide a path for electrical current. However, it's worthy to note exceptions like acids and certain polymers that can conduct electricity when they ionize or when appropriate doping is achieved, showing that the world of covalent chemistry holds many surprises.

The solubility of compounds speaks volumes about their nature. Ionic compounds typically dissolve well in polar solvents like water, as the polar solvent molecules surround the ions, effectively 'pulling' them into solution. This phenomenon is commonly observed in table salt dissolving in water. Solubility in water is not only a matter of convenience but also has biological implications, as it allows for ionic compounds to be transported through the aqueous environment of the human body.

Conversely, covalent compounds are generally more soluble in nonpolar solvents. Nonpolar molecules are better suited to interacting with similar nonpolar substances due to a concept known as 'like dissolves like'. This is why oils, which are covalent, mix well with other nonpolar compounds but not with water. However, students should be aware that exceptions exist and that polarity, molecular structure, and hydrogen bonding play critical roles in the solubility of covalent substances. For instance, alcohol has a polar end, making it soluble in water to a degree, despite being predominantly covalent.