5.4 Bonding in Metals Key Concepts: How do the properties of metals and alloys compare? How are metal atoms bonded in solid metal? How does metallic bonding result in useful properties of metals? Key terms: alloy, metallic bond

Metals and alloys An alloy is a mixture made of two or more elements that has the properties of a metal. At least one element must be a metal. They are generally stronger and less likely to react with air or water than are the pure metals from which they are made

Properties The properties of solid metals and their alloys can be explained by the structure of metal atoms and the bonding between the atoms

Physical properties of alloys Alloys can retain many of the physical properties of metals. Example: Gold jewelry is often a gold alloy to make the gold stronger

Chemical properties Iron is often alloyed with one or more other elements to make steel because they resist rust much better.

Metallic bonding Recall that most metals have 1, 2, or 3 valence electrons. When metal atoms combine chemically with atoms of other elements, they usually lose valence electrons, becoming positively charged metal ions.

Metallic bonding, continued Each metal ion is held by a metallic bond – an attraction between the positive metal ion and the electrons surrounding it. “A metal or metal alloy consists of positively charged metal ions embedded in a “sea” of valence electrons. The more valence electrons an atom can add to the “sea”, the stronger the bond.

Metallic properties The “sea of electrons” model of solid metals explains the ease with which they can change shape, their ability to conduct electric current, their luster, and their ability to conduct heat.

Changes in shape Because positive ions are attracted to the loose electrons all around them, metals can be stretched, pushed or compressed without breaking

Conductivity Metals conduct current easily because the electrons in a metal can move freely.

Luster Polished metals exhibit luster because when light strikes the valence electrons, they absorb it and then give it off again

Heat conductivity In a metal, the freely moving valence electrons transfer energy to nearby atoms and other electrons.