This is why we can make so many structural parts from

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: . Most metals have a face-centered cubic, hexagonal close-packed, or body-centered cubic structure, which provides the densest packing of atoms. The packing, along with the atomic weight, determines the density of a metal. 4.3 Covalent Bonding Covalent solids are mainly formed from non-metallic elements. In covalent materials, the bonded atoms share electrons between them. Most semiconductors are covalent. The atom must have a half-filled p-orbital. For example, silicon, with 14 electrons, is covalently bonded. Each silicon atom is bonded to 4 others in a tetrahedral bond, which leads to the diamond cubic crystal structure. The electronic structure of Si is 1s22s22p63s2 3p2. When the four Si atoms create tetrahedral covalent bonds, the 3s and 3p electrons form a new set of hybrid orbitals called 3sp. Thus the electronic configuration becomes 1s22s22p6(3sp)4. Germanium (Ge) is another covalent semiconductor, with the structure 1s22s22p63s23p6 3d10(4sp)4. Tetrahedral bonds are highly directional and there is little probability of an electron being outside the vicinity of this bond. High temperatures or other sources of energy are needed to remove an electron from the strong covalent bond. This is why semiconductors have relatively low electrical conductivities unless they have special impurities (dopants) added. Because of the directionality of the bond, atoms in a covalent solid cannot be easily displaced from their equilibrium positions, making covalent solids brittle. 4.4 Ionic Bonding Solids with more than one type of atom often possess ionic bonds. This includes ceramic materials, such as oxides and silicates, as well as salts. In an ionic bond an electron is transferred from the cation to the anion. This then creates an electrostatic attraction between them, creating a strong ionic bond. Electronegative atoms are those that have a few empty p-orbitals; they tend to acquire electrons and become negative anions. Electropositive atoms have only a few electrons in an outer shell, and tend to give up electrons, becoming cations. Thus none of the atoms in an ionic solid are neutral; all atoms in the crystal are ions with either a plus charge (cation) or a minus charge (anion). The electron swapping lowers the energy of the crystal by providing each ion with an electron configuration closer to a filled outer shell. For example, in NaCl, when the Na gives up one electron (and becomes Na+), it has a filled 3s shell and becomes more stable. When the Cl accepts the electron from the Na (becoming Cl-), it now has a filled 3p shell and is more stable. Not all combinations of elements can form ionic bonds; only pairs that complement each other can combine. Rev 4.1 1-6 Crystals 1 MatE 25 San Jose State University Lab Notes It is difficult to deform ionic solids because of the strong electrostatic force between the ions. Thus ceramic materials are brittle and cannot be deformed easily. The electrical conductivity in general is very low because there are no free electrons to conduct current. However, some ionic solids have ionic conductivity, in which small...
View Full Document

This test prep was uploaded on 02/19/2014 for the course EE 98 taught by Professor Raychen during the Spring '08 term at San Jose State University .

Ask a homework question - tutors are online