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Covalent-network solids are a class of materials where atoms are connected by a continuous network of covalent bonds. These solids are exceptionally stable and have high melting and boiling points due to the strong bonds that hold their atoms together. Imagine a vast 3D grid where each node is an atom bonded to several other atoms; this is typical of covalent-network solids.

A classic example of a covalent-network solid is diamond, where each carbon atom is tetrahedrally bonded to four other carbon atoms. Another example, from our exercise, is InAs (indium arsenide), where indium and arsenic atoms are bonded covalently to form a solid, rigid lattice structure that extends throughout the material.

Ionic solids are those composed of positively and negatively charged ions. These ions are held together by the electric force of attraction, known as ionic bonds. The ions form a repeating pattern, or lattice, which contributes to the characteristic high melting and boiling points of ionic solids. They are often brittle and can conduct electricity when melted or dissolved in water.

For example, table salt (NaCl) is a well-known ionic solid where sodium ions and chloride ions alternate in a cubic lattice. Following our textbook exercise, MgO (magnesium oxide) and HgS (mercury sulfide) are also ionic solids. The Mg and Hg atoms concede electrons to become positively charged cations, while the O and S atoms accept electrons becoming negatively charged anions.

Metallic solids are comprised purely of metal atoms. They feature metallic bonding, which is a type of chemical bonding where valence electrons are not tied to any particular atom and can move freely throughout the material. This creates a ‘sea’ of delocalized electrons surrounding a lattice of positive metal ions.

These freely moving electrons allow for unique properties such as electrical conductivity, malleability, ductility, and luster. A prime example of a metallic solid from our exercise is indium (In), a soft and malleable metal commonly used in electronics. Due to the nature of metallic bonding, indium can conduct electricity and is ductile, making it suitable for being drawn into wires.

Molecular solids consist of atoms or molecules held together by intermolecular forces, which are weaker than covalent bonds. These forces can include London dispersion forces, dipole-dipole interactions, and hydrogen bonds. Molecular solids typically have lower melting and boiling points compared to other types of solids, and many are soft and insulating.

Water ice is a familiar example of a molecular solid, where molecules of H2O are held together by hydrogen bonds. In our problem set, HBr (hydrogen bromide) is classified as a molecular solid. When HBr is in the solid phase, the molecules are close together but not bonded as tightly as in a covalent-network or ionic solid, attributing to its relatively low melting point.