10.5 The Solid State of Matter

Crystalline Solids

Solids can be either crystalline (atoms arranged in a repeating, orderly pattern) or amorphous (atoms arranged without long-range order). Most of this section focuses on crystalline solids because their regular structure directly determines their physical properties.

Types of Crystalline Solids

There are four main types, and the key difference between them is what particles make up the solid and what forces hold them together.

• Ionic solids are made of positively and negatively charged ions held together by strong electrostatic attractions. They have high melting points because of these strong ionic bonds. They don't conduct electricity as solids (the ions are locked in place), but they do conduct when melted or dissolved in water because the ions can move freely. Examples: $NaCl$, $MgO$, $CaF_2$.
• Molecular solids are made of discrete molecules held together by weak intermolecular forces (van der Waals forces, hydrogen bonds). Because these forces are much weaker than ionic or covalent bonds, molecular solids generally have low melting points and are poor electrical conductors. Examples: ice ($H_2O$), sugar, solid iodine ($I_2$).
• Metallic solids consist of metal cations surrounded by a "sea" of delocalized electrons. This electron sea is what makes metals excellent electrical and thermal conductors. Metals are also malleable and ductile because the cations can slide past one another without breaking the metallic bonding. Melting points vary but are often high. Examples: copper, aluminum, gold.
• Covalent network solids are made of atoms covalently bonded in a continuous three-dimensional network. Every bond you'd need to break is a strong covalent bond, so these solids have very high melting points and are extremely hard but brittle. They're generally poor electrical conductors, with the notable exception of graphite (where delocalized electrons within its layered sheets can carry current). Examples: diamond, silicon, quartz ($SiO_2$).

Structure of Crystalline Solids

The structure of a crystal is described by its unit cell, the smallest repeating unit that, when stacked in all directions, builds the entire crystal. The shape and contents of the unit cell determine the solid's symmetry, density, and many physical properties.

Packing efficiency is the percentage of space in a unit cell actually occupied by atoms or ions. Higher packing efficiency generally means higher density and greater stability.

There are several common unit cell types:

1. Simple cubic (SC): Atoms sit at each corner of a cube. Packing efficiency is low (~52%).
2. Body-centered cubic (BCC): Atoms at each corner plus one atom in the center of the cube (~68% packing efficiency).
3. Face-centered cubic (FCC): Atoms at each corner plus one atom at the center of each face (~74% packing efficiency).
4. Hexagonal close-packed (HCP): Atoms arranged in a hexagonal pattern with alternating layers (~74% packing efficiency, same as FCC).

The coordination number tells you how many nearest neighbors surround each atom in the structure:

• SC: 6
• BCC: 8
• FCC: 12
• HCP: 12

Crystal systems categorize unit cells by their symmetry. There are seven total: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal. For an intro course, the cubic system (which includes SC, BCC, and FCC) is the one you'll work with most.

Crystal Analysis and Properties

X-ray crystallography is the main technique used to determine the atomic arrangement inside a crystal. X-rays are directed at a crystal, and the pattern in which they scatter (diffract) reveals the positions of atoms within the unit cell.

Lattice energy is the energy required to completely separate all the ions in an ionic solid into individual gaseous ions. Higher lattice energy means stronger ionic interactions, which translates to higher melting points and generally lower solubility in water.

Some substances can adopt more than one crystal structure depending on conditions like temperature and pressure. This is called polymorphism. For example, carbon can form diamond or graphite, and calcium carbonate can exist as calcite or aragonite.

Crystal Defects

Real crystals are never perfectly ordered. They contain defects, and these imperfections actually have a big impact on a material's properties.

• Point defects are localized to a single site. These include vacancies (missing atoms), interstitials (extra atoms squeezed into gaps between normal positions), and substitutional defects (impurity atoms replacing regular atoms). Point defects can alter electrical conductivity, mechanical strength, and optical properties.
• Line defects (dislocations) are misalignments that extend along a line through the crystal. Edge dislocations involve an extra half-plane of atoms inserted into the structure, and screw dislocations create a spiral arrangement around a line. Dislocations actually make metals more malleable because they allow planes of atoms to slip past each other more easily.
• Planar defects occur along a surface within the crystal. Grain boundaries are interfaces where regions of different crystal orientation meet, and stacking faults are local disruptions in the layering sequence. These affect mechanical strength, conductivity, and corrosion resistance.
• Bulk defects are three-dimensional flaws like pores (small voids), cracks (larger voids that can grow under stress), and inclusions (trapped impurities or foreign phases). These tend to weaken the material and can serve as starting points for corrosion or fracture.

Amorphous Solids

Not all solids are crystalline. Amorphous solids lack the long-range repeating order found in crystals. Their atoms or molecules are arranged more randomly, similar to a liquid that has been "frozen" in place.

Because of this disordered structure, amorphous solids don't have sharp melting points. Instead, they soften gradually over a range of temperatures. Their mechanical behavior also differs from crystalline solids: glass, for instance, shatters rather than cleaving along defined planes. Common examples include glass, plastics, and some ceramics.