Chapter 12 – Solids and Modern Materials

Types of Solids Metallic solids are held together by a sea of electrons. Ionic solids are sets of cations and anions that are attracted to each other. They have very high melting and boiling points. Ex: NaCl Covalent solids are joined by covalent bonds. Ex: Diamond Molecular solids are separate molecules that are weakly attracted by each other. Ex: Graphite

Solids Crystalline solids are made of regular repeating patterns in nature. They possess rigid and long range order where atoms, molecules, or ions occupy specific positions. Amorphous solids lack the order in the arrangement of atoms. A unit cell is the basic repeating structural unit of a crystalline solid. The structure of a crystalline solid is defined by the size and shape of the unit cell & the location of atoms within the unit cell.

Three Types of Cubic Cells The structures of many metals may have one of the following cubic unit cells: simple cubic, body-centered cubic, or face-centered cubic. 1 atom/unit cell a = 2r 2 atoms/unit cell a = 4r/√3 4 atoms/unit cell a = √8r

Practice Problems Ex: Gold crystallizes in a cubic close packed structure (FCC) and has a density of 19.3 g/cm3. Calculate the atomic radius of gold in picometers (pm). Ex: The edge of NaCl unit cell (FCC) is 564 pm. What is the density of NaCl in g/cm3?

Alloys Alloys are materials that contain more than one element and have the characteristic properties of metals. Three types of alloys include: Substitutional alloys: A second element takes the place of a metal atom. Interstitial alloys: A second element fills a space in the lattice of metal atoms. Heterogeneous alloys: components not dispersed uniformly

Molecular Orbital Theory As the number of atoms in a chain increases, the energy gap between the bonding orbitals and between the antibonding orbitals disappears, resulting in a continuous band of energy. The approach seen here only takes into account s-orbital population.

MO Approach with More Orbitals Most metals have d and p orbitals to consider. Their MO diagrams lead to more bands that better explain conductivity and other properties of metals.

Semiconductors Among elements, only Group IVA, all of which have 4 valence electrons, are semiconductors. Inorganic semiconductors (like GaAs) tend to have an average of 4 valence electrons (3 for Ga, 5 for As). Conductivity of a semiconductor can be changed by adding an element with more or fewer electrons. N-type semiconductors have more electrons, so the negative charge travels in the conductance band. P-type semiconductors have fewer electrons, so the “hole” travels in the valence band.

What are Polymers? Polymers are molecules of high molecular weight made by joining smaller molecules, called monomers. There are two types of polymers: Addition polymers are formed when a bond breaks, and the electrons in that bond make two new bonds. Condensation polymers are formed when a small molecule is removed between two large molecules.