Bonding in Solids
Crystalline solids fall into one of four categories. All four categories involve packing discrete molecules or atoms into a lattice or repeating array, though network solids are a special case. The categories are distinguished by the nature of the interactions holding the discrete molecules or atoms together. Based on the nature of the forces that hold the component atoms, molecules, or ions together, solids may be formally classified as ionic, molecular, covalent (network), or metallic. The variation in the relative strengths of these four types of interactions correlates nicely with their wide variation in properties
table 1.1: Solids may be formally classified as ionic, molecular, covalent (network), or metallic
| Type of Solid |
Interaction |
Properties |
Examples |
| Ionic |
Ionic |
High Melting Point, Brittle, Hard |
NaCl, MgO |
| Molecular |
Hydrogen Bonding,
Dipole-Dipole,
London Dispersion |
Low Melting Point, Nonconducting |
H2, CO2 |
| Metallic |
Metallic Bonding |
Variable Hardness and Melting Point (depending upon strength of metallic bonding), Conducting |
Fe, Mg |
| Network |
Covalent Bonding |
High Melting Point, Hard, Nonconducting |
C (diamond),
SiO2 (quartz) |
In ionic and molecular solids, there are no chemical bonds between the molecules, atoms, or ions. The solid consists of discrete chemical species held together by intermolecular forces that are electrostatic or Coulombic in nature. This behavior is most obvious for an ionic solid such as NaCl
, where the positively charged Na+ ions are attracted to the negatively charged Cl− ions. Even in the absence of ions, however, electrostatic forces are operational. For polar molecules such as CH2Cl2, the positively charged region of one molecular is attracted to the negatively charged region of another molecule (dipole-dipole interactions). For a nonpolar molecule such as CO2
, which has no permanent dipole moment, the random motion of electrons gives rise to temporary polarity (a temporary dipole moment). Electrostatic attractions between two temporarily polarized molecules are called London Dispersion Forces.
Hydrogen bonding is a term describing an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation. (See the IUPAC Provisional Recommendation on the definition of a hydrogen bond.) Dots are employed to indicate the presence of a hydrogen bond: X–H•••Y. The attractive interaction in a hydrogen bond typically has a strong electrostatic contribution, but dispersion forces and weak covalent bonding are also present.
In metallic solids and network solids, however, chemical bonds hold the individual chemical subunits together. The crystal is essential a single, macroscopic molecule with continuous chemical bonding throughout the entire structure. In metallic solids, the valence electrons are no longer exclusively associated with a single atom. Instead these electrons exist in molecular orbitals that are delocalized over many atoms, producing an electronic band structure. The metallic crystal essentially consists of a set of metal cations in a sea of electrons. This type of chemical bonding is called metallic bonding.
