Metallic Bonding & Properties of Materials
What Is Metallic Bonding?
Metals have a distinctive type of bonding that is different from both ionic and covalent bonding. In a metal, the atoms are arranged in a regular pattern called a lattice. The outer-shell electrons of each metal atom are delocalised — they are no longer attached to any particular atom and are free to move throughout the whole structure.
This creates a structure of positive metal ions (cations) surrounded by a “sea” of delocalised electrons. The strong electrostatic attraction between the positive metal ions and the negatively charged delocalised electrons holds the structure together. This attraction is what we call a metallic bond.
Metallic bonding is the strong electrostatic attraction between positive metal ions and a sea of delocalised electrons. The metal ions are arranged in a regular lattice structure.
Properties of Metals
The metallic bonding model explains several key properties of metals. Most metals are shiny solids at room temperature with high melting and boiling points. This is because a large amount of energy is needed to overcome the strong electrostatic attraction between the metal ions and the delocalised electrons.
Metals are good conductors of electricity because their delocalised electrons can move freely through the lattice, carrying electrical charge. They are also good conductors of heat because the delocalised electrons transfer kinetic energy rapidly through the structure.
Metals are malleable (can be hammered into shape) and ductile (can be drawn into wires). When a force is applied to a metal, the layers of ions can slide over each other without breaking the metallic bonds, because the delocalised electrons can shift position to maintain the attraction. This is different from ionic compounds, which shatter when layers shift because ions of the same charge are forced next to each other.
Metals have high melting points (strong bonds), conduct electricity (delocalised electrons carry charge), and are malleable (layers slide without breaking bonds). All three properties are explained by the sea of delocalised electrons.
Most non-metals have low boiling points and are poor conductors of electricity. This is because non-metals form covalent bonds rather than metallic bonds. In simple molecular substances, the molecules are held together by weak intermolecular forces, which require little energy to overcome. There are no delocalised electrons, so they cannot conduct electricity.
Comparing Types of Substance
In the exam, you may need to compare the properties of ionic, simple molecular, giant covalent, and metallic substances. Each type of structure has distinctive physical properties that link directly to its bonding and structure.
Ionic compounds (e.g. sodium chloride) have high melting points because of strong electrostatic forces between ions. They conduct electricity when molten or dissolved in water because the ions are free to move, but not when solid because ions are held in fixed positions in the lattice.
Simple molecular substances (e.g. water, methane) have low melting and boiling points because the intermolecular forces between molecules are weak. They do not conduct electricity because they have no free charged particles.
Giant covalent substances (e.g. diamond, graphite) have very high melting points because many strong covalent bonds must be broken. Diamond does not conduct electricity because all electrons are involved in bonding. Graphite is unusual — it conducts electricity because each carbon atom forms only three covalent bonds, leaving one delocalised electron per atom that can carry charge.
Diamond has each carbon atom bonded to four others in a rigid tetrahedral structure. This makes diamond extremely hard, which is why it is used in cutting tools. Graphite has carbon atoms arranged in layers of hexagons. The weak forces between the layers allow them to slide over each other, making graphite useful as a lubricant and in pencils. Fullerenes such as \(C_{60}\) (buckminsterfullerene) are hollow carbon molecules and carbon nanotubes are tiny cylinders of carbon atoms, both with potential uses in electronics, drug delivery, and as catalysts.
Polymers
Polymers such as poly(ethene) are large molecules consisting of long chains of carbon atoms. Because polymer molecules are large, the intermolecular forces between them are relatively strong, giving polymers higher melting points than simple molecular substances. However, they do not conduct electricity because there are no free electrons or ions.
The four types of substance — ionic, simple molecular, giant covalent, and metallic — can be distinguished by their melting points, solubility in water, and ability to conduct electricity. These properties are explained by the type of bonding and structure present.