F4 Solids and gases Flashcards
structures of solids
molecular structures
covalent network structures
metals
ionic compounds
ex. of covalent network structures
diamond, graphite, buckministerfullerene
Allotropy, allotropes
Different forms of the same element
Polymorphs
Different crystal structures of the same compound
Ceramics
Compounds formed between metallic and non-metallic elements. Ex.: porcelain, cement, bricks
(Al 2Si2O5(OH)4)
• Covalent network structures
• Often ions enclosed in the network
• Crystalline materials with hardness, wear-resistance,
brittleness, electrical & thermal insulators
glasses
Sand = quartz (SiO2) => Made amorphous when melted and
cooled rapidly -> Glass. Amorphous solid = no long-range order
=> liquid-like structure
• Addition of a second oxide can improve some
properties of the glass
– NaCO3 tougher (bottles, windows)
– H3BO3 low thermal expansion
(-> laboratory equipment, e.g. Pyrex)
close packing of spheres
cubic close packing (ABCABC…)
hexagonal close packing (ABAB…)
Coordination number:
Number of closest neighbors
packing efficiency
packing efficiency = Volume atoms / Volume unit cell x 100 (in %)
Cell projection diagrams
A 2D representation of the unit cell, projection down the z-axis
plane indices: z=0 bottom, z=1 top
What happens with the empty space in the close packed structures, ≈26%?
→ Octahedral and tetrahedral sites (interstitial sites)
Occupied by ions in many ionic compounds
Metals loose their … relatively easy
-> Metals are good …
valence electrons
electrical conductors
metal bonding - band theory
Filled valence band: Insulator or semiconductor
A partially filled valence band gives electrical conductivity
conductor - very small band gap
semiconductor - small band gap
insulator - large band gap
Ionic compounds
Cations and anions held together by …
electrostatic force
predicting bond type
large difference in electronegativity -> …
electronegativities are both high -> …
electronegativities are both low -> …
ionic bond
covalent bond
alloy (legering)
