Chapter 11 Sections 7 and 8

Sherry Matthew

Mary Same

Rachel Wolver

Amorphous Solids

• Solids can be either crystalline or amorphous.

• Solids whose particles have no orderly structure, lack well-defined faces and shapes.

• Usually are mixtures of molecules that don’t fit together well or are composed of large, complicated molecules. Ex: rubber, glass

• Solids whose atoms, ions, or molecules are ordered in well-defined arrangements, usually have flat surfaces with definite angles. Ex: diamond, quartz– melts at a specific temperature

• If a crystalline solid is melted and then rapidly cooled, an amorphous solid is sometimes formed if the atoms are unable to return to an orderly arrangement. Ex: When quartz melts, it forms an amorphous solid

Simple CUBIC

• lattice points are at corners only

Body CENTERED CUBIC

• lattice point occurs at center and edges of unit cell

FACE-CENTERED CUBIC

• lattice points at center of each face and at each corner

Crystalline Structure and Ratio

• The total cation-to-anion ratio of a unit cell must be the same as that of an entire crystal (NaCl-One Na for every Cl, CaCl2, one Ca for every 2 Cl)

• Crystalline solids adopt the structures they do because they bring particles closest together and maximize the attractive forces between them. In many cases, and in metallic solids, the particles that make us the solids are spherical or close to spherical.

Packing

• The particles that make up crystalline solids are mostly spherical

• the most efficient way to pack them is to surround one sphere by 6 others.

• However, there are different ways to stack them on top of each other, and cubic close packing. hexagonal close packing (pg 419 OB / 469 NB)

• Coordination number: number of particles immediately surrounding a particle in a crystal structure.

Molecular solids

• atoms/molecules held together by intermolecular forces.

• Since these forces are weak, these substances are soft and have low melting points.– Properties also depend on the abilities of the

molecules to pack efficiently in 3 dimensions

Covalent-Network Solids

• atoms are held together in large networks or chains by covalent bonds, covalent bonds are much stronger than intermolecular forces, so they are harder and have higher melting points (ex: diamond, graphite)

Ionic Solids

• ions held together by ionic bonds, structure depends on the charges of the molecules

Metallic Solids

• consist entirely of metal atoms, have hexagonal close-packed, cubic close-packed (face-centered cubic), or body-centered cubic structures.

• Bonding is due to valence electrons that are delocalized throughout the entire solid (sea of positive ions immersed in a sea of valence electrons)– strength of the bonding increases as the number of

electrons available for bonding increases.

Sample Problem:

• Which metal will have the higher melting point, sodium of chromium?

• Chromium, 6 valence electrons, bond is stronger.

• Which ionic solid will have the higher melting point? MgO or NaCl

• MgO has higher charges and therefore stronger ionic bonds