Chemistry B: 1.3 The Nature of Solids

Key terms:

***The general properties of solids reflect the orderly arrangement of their particles and the fixed locations of their particles.*** ***The shape of a crystal reflects the arrangement of the particles within the solid.***

Cubic:

All sides of a cubic crystal are of equal length and meet at a 90° angle. Some cubic crystals are shaped like a cube. However, cubic crystals may also have other shapes, such as an octahedron or a dodecahedron. An octahedron is a polygon that has eight faces. A dodecahedron has 12. No matter how many faces a cubic crystal has, however, the sides of each face have the same length and they join at 90° angles.

Hexagonal:

If you look at a hexagonal crystal in cross section, it looks like a hexagon, or a polygon with six sides. Hexagonal crystals have two sides of equal length and a third that differs. They also have two 90° angles and one that is 120°.

Rhombohedral:

A rhombohedral crystal looks like a cubic crystal that has been distorted such that its base is a parallelogram instead of a square. Like a cubic crystal, all the sides of a rhombohedral crystal are of equal length. However, while two sides meet at 90°angles, one does not.

Crystal systems:

A crystal has sides, or faces. The angles at which the faces of a crystal intersect are always the same for a given substance and are characteristic of that substance. Crystals are classified into seven groups, or crystal systems. The edges are labeled a, b, and c. The angles are labeled α, β, and γ. The seven crystal systems differ in terms of the angles between the faces and in the number of edges of equal length on each face.

Monoclinic:

A monoclinic crystal looks like an orthorhombic crystal that has been distorted such that the rectangular prism has a parallelogram as its base instead of a square. Like an orthorhombic crystal, the length, width, and height of the sides of a monoclinic crystal are all different. However, while all sides of an orthorhombic crystal meet at a 90° angle, two sides of a monoclinic crystal meet at 90° angles, but one does not.

Tetragonal:

If you stretch a cubic crystal along one of its sides, it becomes a rectangular prism with a square base. This is a tetragonal crystal. All sides of a tetragonal crystal meet at a 90° angle but, unlike a cubic crystal, one side is longer than the other two. For example, the length and width may be the same, but the height is not. Tetragonal crystals may appear to be rectangular prisms or they may resemble double pyramids with one pyramid sitting atop the other.

Orthographic:

If you stretch a cubic crystal such that it becomes a rectangular prism with a rectangular base, it becomes an orthorhombic crystal. All sides of an orthorhombic crystal meet at a 90° angle, but the length, width, and height of the sides are all different.

Triclinic:

Triclinic crystals are the most unsymmetrical. None of the sides of a triclinic crystal are of equal length and none of the angles equals 90°.

*Glass*:

transparent fusion product of inorganic materials that have cooled to a rigid state without crystallizing. Glasses are sometimes called supercooled liquids. The irregular internal structures of glasses are intermediate between those of a crystalline solid and those of a free-flowing liquid. Glasses do not melt at a definite temperature. Instead, they gradually soften when heated. This softening with temperature is critical to the glassblower's art. When a crystalline solid is shattered, the fragments tend to have the same surface angles as the original solid. By contrast, when an amorphous solid, such as glass, is shattered, the fragments have irregular angles and jagged edges.

Melting solids:

When you heat a solid, its particles vibrate more rapidly as their kinetic energy increases. The organization of particles within the solid breaks down, and eventually the solid melts. At the mp the disruptive vibrations of the particles are strong enough to overcome the attractions that hold them in fixed positions. The melting and freezing points of a substance are at the same temperature. At that temperature, the liquid and solid phases are in equilibrium. * -------------- freezing --------------> Solid Liquid <------------ melting ---------------- * The type of bonding that exists between particles in crystals determines their melting points. *In general, ionic solids have high melting points because relatively strong forces hold them together. By contrast, molecular solids have relatively low melting points. Not all solids melt, however. Wood and cane sugar, for example, decompose when heated.*

*Crystal*:

a solid in which the atoms, ions, or molecules are arranged in an orderly, repeating, three-dimensional pattern called a crystal lattice.

*Allotropes*:

are two or more different molecular forms of the same element in the same physical state. Although allotropes are composed of atoms of the same element, they have different properties because their structures are different. Only a few elements have allotropes. In addition to carbon, these include the nonmetals phosphorus, sulfur, and oxygen (O2 and O3), and the metalloids boron and antimony.

*Amorphous solid*:

describes a solid that lacks an ordered internal structure; denotes a random arrangement of atoms. Rubber, plastic, glass, and asphalt are amorphous solids.

*Unit cell*:

the smallest group of particles within a crystal that retains the geometric shape of the crystal. A crystal lattice is a repeating array of any one of fourteen kinds of unit cells. There are from one to four types of unit cells that can be associated with each crystal system.

*Melting point (mp)*:

the temperature at which a substance changes from a solid to a liquid; the melting point of water is 0°C.