unit cells & the structure of solids

simple/primitive cubic

• cube with one atom at each corner

• 1/8 of each atom is an individual unit cell

  • 1 atom per unit cell

• edge length = 2r

• 6

body-centered cubic

• one atom at each corner and one in the center

• 2 atoms per unit cell

• edge length = 4r/sqrt(3)

• 8

face-centered cubic

• one atom at each corner and at the center of each face

• 4 atoms/unit cell

• edge length = 2sqrt(2r)

• 12

molecular solid

• lattice sites occupied by molecules

• molecules have strong covalent bonds, but weaker IMFs between them

  • low melting points

  • gases at room temp (CO2, methane)

ionic solid

• lattice sites occupied by ions

• incredibly strong electrostatic forces (ionic bonds) holding them together

  • very high melting points

  • hard and brittle

  • low conductivity as a solid; high conductivity when aqueous

lattice energy

increases as:

• ionic charge increases

• ionic size decreases

charge is more important than size

atomic solid

• the lattice sites are occupied by atoms

• metallic, covalent network, non-bonding solids

atomic solids - metallic solids

• electron sea model

• very good conductors of heat and electricity

• malleable and ductile

• variable hardness and melting points

atomic solids - covalent network solids

• graphite, diamond, SiO₂, SiC

• very high melting points

• harder than metallic solids

• do not effectively conduct energy

atomic solids - nonbonding solids

• generally solids of noble gases

• held together by intermolecular forces ONLY

  • noble gases - ONLY LDFs

• very, very low melting points