Ionic Crystals

• Crystalline solids are characterized by a highly ordered, repeating arrangement of atoms, ions, or molecules, forming a crystal lattice

Ionic Crystals

• Composed of positively and negatively charged ions held together by electrostatic attraction.
• Ions arrange themselves in a lattice structure that maximizes attraction between ions of opposite charge and minimizes repulsion between ions of the same charge.
• Ionic bonds are strong, which lead to high melting and boiling points, as well as hardness and brittleness.
• Generally poor conductors of electricity in the solid state because the ions are fixed in place
• When dissolved in water or melted, ionic compounds become good conductors, as the ions are free to move and carry charge
• Examples: sodium chloride (NaCl), magnesium oxide (MgO), and calcium fluoride (CaF2).
• The arrangement of ions in an ionic crystal depends on the relative sizes and charges of the ions involved.
• Crystal structures include the rock salt structure (e.g., NaCl), the cesium chloride structure (e.g., CsCl), and the zinc blende structure (e.g., ZnS).
• Lattice energy measures the strength of ionic bonds and is the energy required to separate one mole of an ionic compound into its gaseous ions; high lattice energy results in a stable crystal structure.

Covalent Crystals

• Consist of atoms held together by covalent bonds in a continuous network extending throughout the material.
• Each atom is covalently bonded to its neighbors, resulting in a strong, rigid structure.
• Covalent crystals are typically very hard and have high melting points because of the strong covalent bonds that must be broken to disrupt the structure.
• They are generally poor conductors of electricity because the electrons are localized in the covalent bonds and are not free to move.
• Diamond is a classic example: each carbon atom is tetrahedrally bonded to four other carbon atoms, forming a giant, three-dimensional network.
• Silicon and germanium, which have the same crystal structure as diamond, are important semiconductors.
• Quartz (SiO2) is another example of a covalent network solid, where silicon and oxygen atoms are covalently bonded in a complex three-dimensional network.
• Graphite is a layered structure, with strong covalent bonds within each layer but weak van der Waals forces between layers, making it soft and useful as a lubricant.
• The properties of covalent crystals depend on the strength and directionality of the covalent bonds, as well as the arrangement of atoms in the crystal lattice.

Metallic Crystals

• Consist of metal atoms arranged in a regular pattern, with valence electrons delocalized throughout the entire crystal.
• Metal atoms are packed closely together, and the valence electrons are free to move among the atoms, forming a "sea" of electrons.
• This electron sea is responsible for the high electrical and thermal conductivity of metals, as the electrons can easily transport charge and energy.
• Metallic bonds are generally strong, but they are non-directional, allowing metal atoms to slide past each other without breaking bonds, which makes metals ductile and malleable.
• Metals typically have high melting points, though there is a wide range depending on the specific metal and the strength of the metallic bonds.
• Examples include copper (Cu), aluminum (Al), iron (Fe), and gold (Au).
• Common crystal structures for metals include face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP).
• In FCC structures, atoms are located at the corners and the center of each face of the cube; examples include copper, aluminum, and gold.
• In BCC structures, atoms are located at the corners and the center of the cube; examples include iron, tungsten, and chromium.
• In HCP structures, atoms are arranged in a hexagonal pattern in each layer, with alternating layers stacked in an ABAB pattern; examples include zinc, titanium, and magnesium.
• Alloys are mixtures of two or more metals, and their properties can be tailored by varying the composition and processing methods.