Properties of metals Knowledge-Based Study

Study notes

1. The Nature of Metallic Bonding

   Metals generally possess low ionisation energies and few valence electrons. In the solid state, metal atoms do not share electron pairs locally (as in covalent bonds) nor do they permanently transfer electrons to form distinct anions (as in ionic bonds). Instead, metal atoms lose their valence electrons to form an ordered, three-dimensional lattice of positively charged metal ions (cations).

   The surrendered valence electrons become delocalised and move freely throughout the entire crystalline structure. This arrangement is commonly described as the electron sea model:

   • Lattice cations: Fixed positive atomic cores occupying regular positions in a crystalline lattice.
   • Delocalised electrons: An electron cloud permeating the spaces between the cations.
   • Metallic bond: The strong, non-directional electrostatic attraction between the positive metal ions and the surrounding mobile sea of delocalised electrons.

   Virtually all physical and mechanical properties characteristic of metallic elements arise directly from this unique bonding architecture.

   Transport Properties of Metals

   Metals are outstanding conductors of both electricity and heat in both solid and molten states.

   1. Electrical Conductivity

   Electric current in a conductor is defined as the net flow of charge carriers. In non-metallic covalent substances, electrons are localised in specific bonds or lone pairs, and in solid ionic compounds, ions are held rigidly in place. In metals:

   • In the absence of an external electric field, delocalised electrons move randomly at high speeds.
   • When an electric potential difference (voltage) is applied across a metal, the mobile electrons experience an electrostatic force and acquire a net directional drift toward the positive terminal.
   • Because the electrons remain delocalised even after melting, liquid metals (such as molten iron or liquid mercury) remain excellent electrical conductors.

   2. Thermal Conductivity

   When one end of a metal is heated, kinetic energy is transferred rapidly throughout the material via two mechanisms:

   • Electronic conduction: Mobile electrons gain thermal kinetic energy in hotter regions and rapidly collide with other particles across cooler regions, dispersing energy very quickly.
   • Lattice vibrations (phonons): Cations vibrate more intensely and pass energy to neighbouring cations. In metals, the electronic mechanism predominates, making metals far better thermal conductors than non-metals.

   Why Metals Bend Without Breaking

   Unlike ionic solids, which are brittle and shatter under mechanical stress, metals exhibit remarkable plastic deformability:

   • Malleability: The ability to be hammered, rolled, or pressed into thin sheets without fracturing.
   • Ductility: The ability to be drawn or stretched into thin wires under tensile stress.

   Comparison with Ionic Crystals

   The mechanical difference between metals and ionic lattices illustrates the significance of bonding geometry:

   • Ionic solids: Bonding forces are highly directional and alternate between cations and anions. When a mechanical shear stress is applied, planes of ions slide relative to one another. This displacement brings ions of identical charge into direct alignment. Strong electrostatic repulsion between like charges immediately forces the crystal planes apart, causing cleavage and brittle fracture.
   • Metallic solids: Metallic bonds are non-directional. When a mechanical force shifts one layer of cations over another, the delocalised electron sea instantly adjusts and flows around the newly positioned cations. The uniform electrostatic attraction between the mobile electron cloud and the cations is fully maintained, preventing repulsive cleavage and allowing permanent deformation without fracture.

   Thermal Stability and Optical Characteristics

   1. Melting and Boiling Points

   The strength of a metallic bond determines the thermal energy required to disrupt the crystal lattice:

   • Charge density and valence electrons: Greater positive charge on the metal cations and a higher number of delocalised valence electrons yield stronger electrostatic attractions. For instance, magnesium (Mg2+ with two delocalised electrons per atom) possesses stronger metallic bonding and a higher melting point (650 °C) than sodium (Na+ with one delocalised electron per atom, melting at 98 °C).
   • Cation radius: Smaller ionic radii allow the delocalised electron cloud to approach the positively charged nucleus more closely, increasing bond strength.

   2. Metallic Lustre

   Freshly cut metal surfaces display a characteristic shiny appearance known as lustre. Because delocalised electrons are not confined to discrete energy levels as in isolated atoms or localised bonds, they form closely spaced energy bands:

   • Incident light of virtually all visible frequencies excites these freely moving surface electrons.
   • The excited electrons immediately oscillate and re-emit (reflect) light of the same frequencies, giving metals their characteristic reflective and opaque sheen.