Chemistry – Chapter 6.4 Metallic Bonding

Objectives Describe the electron-sea model of metallic bonding, and explain why metals are good electrical conductors. Explain why metal surfaces are shiny. Explain why metals are malleable and ductile but ionic-crystalline compounds are not.

Metallic Bonding Chemical bonding is different in metals than it is in ionic, molecular, or covalent-network compounds. Difference is reflected in the unique properties of metals. They are excellent electrical conductors in the solid state. Property is due to the highly mobile valence electrons of the atoms that make up a metal. In metals, the valence electrons are shared by all surrounding atoms.

Delocalization Within a metal, the vacant orbitals in the atoms’ outer energy levels overlap. The overlapping of orbitals allows the outer electrons of the atoms to roam freely through the entire metal. The electrons are delocalized, which means they do not belong to any one atom but move freely about the metal’s network of empty atomic orbitals.

Mobile Electrons These mobile electrons form a sea of electrons around the metal atoms, which are packed together in a crystal lattice. Metallic Bonding: The chemical bonding that results from the attraction between metal atoms and the surrounding sea of electrons. Mobile electrons in the metallic bond are responsible for luster, thermal conductivity, and electrical conductivity.

Metallic Properties The freedom of electrons to move in a network of metal atoms accounts for the high electrical and thermal conductivity characteristic of all metals. Because they contain many orbitals separated by extremely small energy differences, metals can absorb a wide range of light frequencies. Absorption results in the excitation of the metal atoms’ electrons to higher energy levels. The electrons immediately fall back down to lower levels, emitting energy in the form of light. De-excitation is responsible for their shiny appearance. The metals basically absorb then re-emit the light.

Malleability and Ductility Most metals are also easy to form into desired shapes. Malleability: the ability of a substance to be hammered or beaten into thin sheets. Ductility: The ability of a substance to be drawn, pulled, or extruded through a small opening to produce a wire, without breaking. The malleability and ductility of metals are possible because metallic bonding is the same in all directions throughout the solid. In other words, the fact that metals are malleable and ionic crystals are brittle is explained in terms of their chemical bonds.

Metallic Bond Strength Metallic bond strength varies with the nuclear charge of the metal atoms and the number of electrons in the metal’s electron sea. Both of these factors are reflected in a metal’s heat of vaporization. When a metal is vaporized, the bonded atoms in the normal (usually solid) state are converted to individual metal atoms in the gaseous state. The amount of heat needed to vaporize the metal is a measure of the vapor strength of the bonds that hold the metal together. In general, the strength of the metallic bond increases moving from left to right on any row of the periodic table.