Solid State Structures

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In theory, all substances are able to exist in the solid state. Some substances may only exist as solids, however, under exotic conditions. For instance, some substances must be cooled to near Absolute Zero before they will become solids. Other substances may have different exotic requirements. This presentation discusses the solid state as it applies to all systems, from a theoretical point of view. All solids as classified as one of five types.

 To a large extent, the primary difference between the types of solids is the mechanisms that hold them together as solids. These mechanisms will be responsible for many physical characteristics, such as melting points, boiling points, hardness and water solubility.

There are three basic mechanism used to hold solid state systems together. In all three cases, they will be electrostatic forces, and must follow the requirements established by Coulomb's Law.

Generally, Bonding is recognized as the strongest mechanism of attachment. Intermolecular forces are the weakest mechanism of attachment. Metallic bonding covers a widely varying set of strengths. Bonding can be broken down into two types and Intermolecular Forces can be classified as three types. The strength of the electrostatic forces in a system will depend on Coulomb's Law. In other words, the variables to be considered are the magnitude of charge on the objects and the distance between the centers of the charged objects.

 Bonds

1. Ionic : The strength of an ionic bond depends on the size of the charge on each ion and on the radius of each ion.

• The larger the charges on the ions the greater the force of attraction, or the stronger the bond. This factor places larger values for the variables q₁ and q₂ in the numerator of Coulomb's Law.
• The larger the radii of the bonded ions, the weaker the force of attraction, or the weaker the ionic bond. This factor places a larger distance between the centers of the charged objects, and reduces the force of attraction according to Coulomb's Law.

2. Covalent : The strength of a covalent bond depends on the size of the charge from the shared electrons, the nuclear charges of the bonded atoms and the radii of the bonded atoms.

• The greater the number of electrons being shared, the greater the force of attraction, or the stronger the bond.
• The greater the nuclear charge on each bonded atom, the greater the force of attraction, or the stronger the bond.
• The greater the radii of the bonded atoms, the greater the distance between the bonded nuclei and the shared electrons. According to Coulomb's Law, this increased distance will reduce the force of attraction, or decrease the strength of the covalent bond.

 Intermolecular Forces

1. Hydrogen Bonding : Hydrogen bonds are strong intermolecular forces because of the very large partial charges on the hydrogen bonding sites and the very small sizes of the atoms that occur at the sites. The partial charges are large because of the large difference in the electronegativity values for the two bonded atoms. The N, O and F atoms when combined with H will create these large partial charges and be associated with atoms that have small radii--H, N, O, F.

2. Dipole-dipole : The strength of dipole-dipole interactions is associated with the size of the partial charges on the atoms and the radii of the atoms that are carrying the partial charges.

• The larger the partial charges on the bonded atoms, the stronger the intermolecular forces will be. The partial charges represent the q₁ and q₂ values found in Coulomb's Law.
• Unlike most other systems, the larger radii involved in dipole-dipole intermolecular forces generally lead to stronger forces of attraction. This is primarily a result of the larger radii being more readily able to sustain the partial charges that are associated with dipoles. As a result, the intermolecular forces are strengthened by the sustained partial charges.

3. London Dispersion Forces (or van der Waal's Forces) : The strength of London Dispersion Forces is determined through the same variables associated with regular dipole-dipole intermolecular forces. Since these systems lack the permanent dipoles of polar systems, the ability to sustain a temporary dipole on a nonpolar system becomes a much bigger issue. As a result, the radii of nonpolar systems will have significant control over the strength of this type of intermolecular force.

 Crystal Structures

There is a big difference between systems that use bonding without intermolecular forces and systems that use bonding with intermolecular forces. The intermolecular forces are a weak link in any structure that uses them. The result being that the systems will be softer, less rigid and melt or boil at lower temperatures than any system that uses bonding exclusively.

When bonding is used exclusively in a structure, then the system will lack molecules. The formula of such a system will represent the ratio of the different elements in the overall solid state. When a system has intermolecular forces, then they will serve as well-defined weak spots in the structure. These weak spots will be located between molecules. As a result, systems with intermolecular forces will exist as molecules and the formulas of these systems will correspond to the composition of a molecule. In other words, H₂O, a known Polar Molecular Solid actually exists as molecules composed of two hydrogens and one oxygen. A covalent structure such as SiO₂, which does not use interactions will not have a molecule. Rather, the SiO₂ says that a crystal of this substance will have twice as many oxygen atoms as it does silicon atoms.

Ionic Solids : Formula represents of ratio of cations to anions in the crystal.

Covalent Solids : Formula represents a ratio of the various atoms in the crystal.

Polar Molecular Solids : Formula represents a molecule.

Nonpolar Molecular Solids : Formula represents a molecule.

Metal Solids : Formula represents the symbol for the metallic element.

Solid State Crystal Structures

Electrical

Conductivity

Water

Solubility