When substances are classified by bonding type, they can be organized into four types: ionic crystals, molecular crystals, covalent-network crystals, and metallic crystals. Let us review their differences.
Basic Knowledge
Comparison table of the four types of crystals (frequently tested in basic chemistry):
| Type | Bond | Constituent particles | Melting point | Electrical conductivity | Examples |
|---|---|---|---|---|---|
| Ionic crystal | Ionic bond | Cations and anions | High | Solid×, liquid◯ | NaCl, CaCO3 |
| Molecular crystal | Intermolecular forces | Molecules | Low | Does not conduct | Ice, I2, CO2 |
| Covalent-network crystal | Covalent bonds | Atoms | Extremely high | Does not conduct (exception: graphite) | Diamond, SiO2, graphite |
| Metallic crystal | Metallic bonds | Metal ions + free electrons | Various | Conducts well | Fe, Cu, Au, Al |
A covalent-network crystal is a giant lattice in which atoms are connected three-dimensionally by covalent bonds. Diamond and silicon dioxide (SiO2, the main component of quartz and rock crystal) are typical examples. They have extremely high melting points and are exceptionally hard. In graphite, atoms within each layer are joined by covalent bonds, while layers are held together by weak van der Waals forces. Electrons can move freely between layers, so graphite conducts electricity (an exception among covalent-network crystals).
Ionic crystal（A crystal formed by Coulomb forces, such as NaCl）
Molecular crystal（Ice, dry ice, and iodine. Molecules gather through intermolecular forces）
Covalent-network crystal（A giant covalent network. Diamond, SiO2, graphite）
Metallic crystal（Metal ions + free electrons. Fe, Cu, Al）
Polymer（A compound in which many monomers are connected by covalent bonds）
Chemical formula（A general term for formulas representing substances. Molecular, empirical, condensed, and structural formulas）
Further Study (Background and Significance)
Tips for identifying bond types:
① Metal + nonmetal → ionic bond (NaCl, MgO, CaCO3)
② Nonmetal + nonmetal → covalent bond (H2O, CO2, NH3, CH4, SiO2)
③ Metal + metal → metallic bond (alloys and pure metals)
④ Exceptions that contain only nonmetals but form giant covalent networks: diamond C, silicon Si, and silicon dioxide SiO2.
Bond types in familiar substances:
・Table salt NaCl = ionic bond; sugar (sucrose C12H22O11) = molecular crystal with covalent bonds.
・Ice = molecular crystal (with hydrogen bonds).
・Quartz (SiO2) and diamond = covalent-network crystals.
・Copper Cu, iron Fe, and aluminum Al = metallic crystals.
・Concrete = mainly calcium carbonate and silicates (an ionic and covalent hybrid).
・Plastic = a polymer (a long chain of covalent bonds).
Understanding bonding is the starting point for predicting physical properties and connects to organic synthesis, inorganic materials, and life science.
- Four crystal types: ionic, molecular, covalent-network, and metallic
- Metal + nonmetal → ionic bond
- Nonmetal + nonmetal → covalent bond (molecular or covalent-network crystal)
- Metal + metal → metallic bond
- Covalent-network crystals: diamond, SiO2, Si, graphite (conducts electricity)
- Molecular crystals have low melting points, are soft, and sublime
- Sugar is a molecular crystal; table salt is an ionic crystal
- Polymers are giant molecules formed by covalent bonds
Points to Note (Common Confusions)
① Covalent-network crystals (diamond, SiO2) and molecular crystals (ice, CO2) both involve covalent bonds, but their structures differ. In the former, the entire lattice is covalently bonded; in the latter, covalent bonds exist only within molecules. ② Graphite is a covalent-network crystal, but weak forces between layers allow it to conduct electricity. ③ Ionic crystals do not conduct electricity as solids but conduct as liquids and aqueous solutions; metallic crystals conduct even as solids. ④ Chemical formula is a general term; a molecular formula represents a molecule, an empirical formula represents an ionic crystal, a condensed formula shows functional groups (C2H5OH), and a structural formula uses lines to show bonds.
Practice
- What type of crystal is each of diamond, ice, sodium chloride, and copper?
- What is the only covalent-network crystal that conducts electricity?
- Explain why ionic and metallic crystals differ in electrical conductivity as solids.