Which Property Is Typical Of A Covalent Compound

Which Properties are Typical of a Covalent Compound?

Covalent compounds, formed by the sharing of electrons between atoms, exhibit a distinct set of properties that differentiate them from ionic compounds. Understanding these properties is crucial in chemistry, material science, and various other fields. This comprehensive guide will delve into the typical characteristics of covalent compounds, exploring the underlying reasons behind these properties and providing illustrative examples.

Low Melting and Boiling Points

One of the most prominent features of covalent compounds is their generally low melting and boiling points. This is in stark contrast to ionic compounds, which often have high melting and boiling points. The reason lies in the nature of the bonding.

Understanding the Intermolecular Forces

In covalent compounds, the atoms are held together by relatively weak intermolecular forces. These forces, such as London dispersion forces, dipole-dipole interactions, and hydrogen bonds, are responsible for the attraction between molecules. These forces are significantly weaker than the strong electrostatic attractions present in ionic compounds.

• London Dispersion Forces: These are the weakest intermolecular forces and are present in all molecules, regardless of polarity. They arise from temporary fluctuations in electron distribution, creating temporary dipoles. The larger the molecule, the stronger the London dispersion forces.

• Dipole-Dipole Interactions: These forces occur between polar molecules, which have a permanent dipole moment due to differences in electronegativity between atoms. The positive end of one molecule attracts the negative end of another.

• Hydrogen Bonds: A special type of dipole-dipole interaction, hydrogen bonds occur when a hydrogen atom is bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) and is attracted to another electronegative atom in a nearby molecule. Hydrogen bonds are relatively strong compared to other intermolecular forces.

The Impact on Melting and Boiling Points

Because the intermolecular forces in covalent compounds are weak, relatively little energy is required to overcome these forces and change the state of matter. This results in low melting and boiling points. For example, methane (CH₄), a covalent compound, boils at -161.5 °C, while table salt (NaCl), an ionic compound, melts at 801 °C.

Poor Electrical Conductivity

Covalent compounds generally exhibit poor electrical conductivity in both solid and liquid states. This is because, unlike ionic compounds, they do not have freely moving charged particles (ions) to carry electric current.

The Role of Electron Sharing

In covalent compounds, electrons are shared between atoms to form covalent bonds. These electrons are localized within the bonds and are not free to move throughout the compound. Consequently, there are no mobile charge carriers available to conduct electricity.

Exceptions: Conductivity in Aqueous Solutions

While covalent compounds generally do not conduct electricity in their solid or liquid states, some exceptions exist. Certain covalent compounds can conduct electricity when dissolved in water. This is because they can ionize in water, forming ions that can carry an electric current. For example, hydrogen chloride (HCl) dissolves in water to form hydrochloric acid, which is a strong electrolyte and a good conductor of electricity. This ionization, however, is a result of the interaction with the solvent (water), not an inherent property of the covalent compound itself.

Solubility in Water and Other Solvents

The solubility of covalent compounds varies greatly depending on the nature of the compound and the solvent. However, many covalent compounds are soluble in nonpolar solvents such as benzene or hexane, while they are often insoluble or only slightly soluble in polar solvents like water.

"Like Dissolves Like"

The principle of "like dissolves like" governs solubility. Polar solvents tend to dissolve polar substances, while nonpolar solvents tend to dissolve nonpolar substances. Many covalent compounds are nonpolar or have only weak polarity, making them more soluble in nonpolar solvents.

Exceptions: Polar Covalent Compounds

Some covalent compounds exhibit significant polarity due to the presence of highly electronegative atoms, such as oxygen or nitrogen. These polar covalent compounds can be soluble in polar solvents like water. For example, ethanol (C₂H₅OH), a polar covalent compound, is miscible with water due to the presence of the polar hydroxyl (-OH) group.

Low Hardness

Covalent compounds typically exhibit low hardness compared to ionic compounds. This is because the intermolecular forces holding the molecules together are relatively weak.

The Impact of Weak Forces on Hardness

The weak intermolecular forces are easily overcome by external forces, resulting in the material being easily scratched or deformed. This contrasts sharply with ionic compounds, where the strong electrostatic attractions between ions lead to high hardness. Diamond, however, is a notable exception, as its covalent network structure results in exceptional hardness.

Formation of Different Crystal Structures

Covalent compounds can form a variety of crystal structures depending on the arrangement of their molecules. These structures can be quite complex, ranging from simple molecular crystals to intricate network solids.

Molecular Crystals: Discrete Molecules

In many covalent compounds, the molecules exist as discrete units held together by relatively weak intermolecular forces. These compounds form molecular crystals, where the molecules are arranged in a regular, repeating pattern.

Network Solids: Extensive Covalent Bonding

Some covalent compounds, such as diamond and silicon dioxide (SiO₂), form network solids. In these materials, the atoms are covalently bonded to each other in an extensive three-dimensional network, resulting in a very strong structure. This explains the exceptional hardness of diamond.

Flammability and Reactivity

The flammability and reactivity of covalent compounds vary greatly depending on the specific compound. Some covalent compounds are highly flammable, while others are relatively unreactive.

Factors Affecting Flammability and Reactivity

Several factors influence the flammability and reactivity of covalent compounds, including the types of atoms present, the presence of multiple bonds, and the overall molecular structure. Compounds containing carbon and hydrogen, for instance, are often flammable.

Examples of Covalent Compounds and Their Properties

Let's examine some specific examples to illustrate these properties:

• Water (H₂O): A polar covalent compound with a relatively high boiling point (100 °C) due to strong hydrogen bonding. It's a good solvent for polar substances.

• Methane (CH₄): A nonpolar covalent compound with an extremely low boiling point (-161.5 °C) due to weak London dispersion forces. It's insoluble in water but soluble in nonpolar solvents.

• Diamond (C): A network covalent solid with exceptionally high hardness and melting point. It's insoluble in all common solvents.

• Carbon Dioxide (CO₂): A nonpolar covalent compound that exists as a gas at room temperature due to weak intermolecular forces. It's slightly soluble in water.

• Ethanol (C₂H₅OH): A polar covalent compound, it displays hydrogen bonding, making it miscible with water.

Conclusion

Covalent compounds showcase a unique set of properties arising from their electron-sharing nature. Their low melting and boiling points, poor electrical conductivity (except for some aqueous solutions), variable solubility, low hardness, diverse crystal structures, and varying flammability and reactivity all stem from the relatively weak intermolecular forces and localized nature of the electrons within the covalent bonds. Understanding these properties is fundamental to comprehending the behavior and applications of a vast range of materials in various scientific disciplines.