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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...
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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 .
- Spring '08