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The Nature of Solids
By Allen Tang, Dan Jacobson, and Ted Dennis.
A Model for Solids
• Properties of solids reflect the arrangement of their particles
• Melting point-the temperature at which a solid changes into a liquid– Ionic compounds have high melting points– Molecular solids have relatively low melting points
Crystal Structure
• Crystal-the particles are arranged in an orderly, repeating, three-dimensional pattern called a crystal lattice
• Shape of a crystal reflect the arrangement of the particles
OBJECTIVE
17. What makes solids different from liquids and gases according to KMT?
• In solids, the particles are arranged in fixed locations.
OBJECTIVE18. Compare and contrast the terms crystal system and unit cell.
Crystal Systems
• A crystal has sides, or faces• Crystals are classified into seven distinct
crystal groups, or crystal systems• The angles at which the faces of a crystal
intersect are always the same for a given substance. – Thus, the crystal system of a sample of galena (a
cubic system) will be identical in shape to the crystal system of another similarly sized sample.
Unit Cell
• A unit cell is the smallest group of particles within a crystal that retains the geometric shape of the crystal.
• A crystal lattice forms from a repeating array of any one of the fourteen kinds of unit cells.
• A unit cell is similar to a crystal system in that it defines the shape of said system by the bonding of its atoms, yet it is different in that a crystal system can consist of any amount of compounds while still retaining the structure of its original system, while a unit cell exists only as the smallest group of particles that create a crystal system
Allotropes
• Two or more different molecular forms of the same element in the same physical state.
• They have different properties because their structures are different, even though they are all made up of the same element.
• Example: Diamond vs. Graphite (carbon)
Objective
19. Explain how three allotropes of carbon produce different functions.
• Diamond, graphite and fullerenes are all composed of carbon, but have different molecular structures. Because of this the physical properties are different.
• Diamond has a high density and is very hard.• Graphite has a low density and is soft.• Fullerenes are strong and rigid.
Non-Crystalline Solid
• Amorphous solid- lacks an ordered internal structure; the atoms are randomly arranged (Ex: rubber, plastic and asphalt).
• Glass-The transparent fusion product of inorganic substances that have cooled to a rigid state without crystallizing