Bonding and Structure in Solids SCH4U

Molecular Solids Intermolecular forces: dipole-dipole, London dispersion and H-bonds. Weak intermolecular forces give rise to low melting points & soft solids Room temperature gases and liquids usually form molecular solids at low temperatures. Here is the structure of ice. Notice how the hydrogen bonds become rigid forming a lattice linking each molecule of water together.

Structure in solids Crystalline Solids: the atoms, ions, or molecules are ordered in well-defined three-dimensional arrangements. Have faces and make definite angles. Amorphous solids: a solid in which particles have no orderly structure. There is no well-defined shape or face. Solids do not stack well together. Eg. Rubber or Glass

Crystalline vs. Amorphous Structures

Structures of Crystalline Solids (AP) Each repeating unit is reffered to as a Unit Cell. Unit cells create a crystal lattice. Unit Cell  Crystal Lattice 

Types of Unit Cells There are seven basic types of unit cells. Cubic Unit Cell Primitive Cubic Body Centered Cubic Face-Centered Cubic Each has a different arrangement of lattice points.

Types of Unit Cells

Sample Problem Determine the net number of Na+ and Cl- ions in the NaCl unit cell shown in the Figure.

Close Packing of Spheres Arrangement of particles in closest contact to maximize attractive forces. Hexagonal Close Packing ABAB pattern Cubic Close Packing  ABCA Pattern

Bonding in Crystalline Solids

Covalent Network Solids Consist of atoms held together, through the entire sample of material in large networks or chains by covalent bonds. Harder and have higher melting points than most molecular solids. Diamond, graphite, silicon, germanium, quartz, SiO2, silicon dicarbide, SiC, BN In diamond each C is bondend tetrahedrally to four other carbon atoms. In graphite, C are bonded in trigonal planar geometries to three other C to form interconnected hexognal rings

Ionic Solids Ions are at the lattice points. Strength of bonds depends on charges Electrostatic attraction keeps them frozen into this position.

Metallic Solids Also called metals, consist entirely of metal atoms. Bonding is very strong and is due to delocalized valence electrons throughout the entire solid. In general, the melting point increases as the number of electrons available for bonding increases.