Chemical Bonds and Properties of Matter

Covalent Molecular vs. Atomic Covalent Solids

Covalent molecular substances are linked together by intermolecular forces, which are generally weak. Consequently, molecular substances are usually gases or liquids at Standard Temperature and Pressure (STP). In contrast, in atomic covalent solids such as diamond or silica, the molecules are linked together by covalent bonds. These bonds are very strong, resulting in greater cohesion between the molecules; therefore, they are solid at STP.

Molecular substances have low boiling temperatures because small thermal agitation can break the intermolecular bonds. However, covalent solids are much stronger; they only melt or boil at very high temperatures because these links offer strong resistance to being scratched. Finally, regarding solubility, molecular substances are soluble in polar solvents like water (due to the attraction of charges) and insoluble in nonpolar solvents. Nonpolar molecular substances are soluble only in nonpolar solvents such as benzene (where particles interact as solute-solvent). However, covalent solids form a crystal lattice and are soluble in any solvent.

Scientific Explanations of Chemical Properties

Boiling Points of Hydrides and Water

Among the compounds H₂O, H₂S, H₂Se, and H₂Te, water is the one with the higher boiling temperature. This is because water molecules are joined by hydrogen bonds, which are very strong and therefore require a higher temperature to break.

Electrical Conductivity in Ionic Compounds

Ionic compounds are poor conductors in a solid state but good conductors if they are fused. Ionic compounds do not conduct electricity in the solid state because they lack free electrons that can move at a potential difference; the ionic bond is responsible for this. However, when molten or in an aqueous solution, the rigid ionic bond disappears, leaving free ions. These are free charges that can move at a potential difference and conduct electricity.

Phase Changes and Covalent Bonds in Methane

Although methane is a compound with covalent bonds, these bonds do not break when methane evaporates. When evaporating, methane (which is a gas at room temperature) does not break its covalent bonds; instead, the intermolecular bonds are simply weakened or broken. This allows the methane molecules to be dispersed while the covalent bonds that hold the Carbon (C) with Hydrogen (H) in the CH₄ molecules remain intact.

Mechanical Properties of Ionic and Metallic Solids

Unlike brittle ionic solids, metallic solids are ductile and malleable. This is because, in ionic solids, applying a blow distorts the crystal lattice, causing charges of the same sign to approach each other and repel. These repulsive forces cause the solid to break. However, in a metallic solid, the cation network is surrounded by an electron cloud that holds it together. This cohesion prevents the metal from breaking when hit, giving it good mechanical properties: it is ductile (can be drawn into wire) and malleable (can be formed into sheets).