Lesson 4.4 Bonding in Metals

Lesson 4.4 Bonding In Metals

Structure of Metal Crystal

The properties of solid metals can be explained by the structure of metal atoms and the bonding among those atoms. When metal atoms combine chemically with atoms of other elements, they usually lose valence electrons. They then become positively charged metal ions. Metal atoms lose electrons easily because they do not hold onto their valence electrons very strongly. The loosely held valence electrons in metal atoms result in a type of bonding that happens in metals. Most metals are crystalline solids. A metal crystal is composed of closely packed, positively charged metal ions. The valence electrons drift among the ions. Each metal ion is held in the crystal by a metallic bond – an attraction between a positive metal ion and the electrons surrounding it.

How aluminum foil is made

What are the properties of Metals?Almost all metals are solids at room temperature. They can be hammered into sheets or drawn out (pulled) into thin wires. Metallic bonding explains many of the common physical properties of metals. Properties of metals include shiny luster, and high levels of malleability, ductility, electrical conductivity, and thermal conductivity. Each of these properties is related to the behavior of valence electrons in metal atoms.

Luster – Polished metals have a shiny and reflective luster, called metallic luster. The luster of metal is due to its valence electrons. When light strikes these electrons, they absorb the light and then re-emit the light. Example: chromium on a motorcycle/cars.

Malleability and Ductility – metals are ductile and can be bent easily and pulled into thin strands or wires. Metals are also malleable and can be rolled into thin sheets like aluminum foil. Metals act this way because the positive metal ions are attracted to the loose electrons all around them rather than to other metal ions. These ions can be made to change position. However, the metallic bonds prevent the metal ions from breaking apart from one another.

Thermal Conductivity – Remember that thermal energy is the total energy of motion of all the particles in an object. Thermal energy flows from warmer matter to cooler matter. This transfer of thermal energy is known as heat. Metals conduct heat easily because the valence electrons within a metal are free to move. Electrons in the warmer part of the metal can transfer energy to electrons in the cooler part of the metal.

Electrical conductivity – Charged particles that are free to move can conduct an electric current. Metals conduct electric current easily because the valence electrons in a metal can move freely among the atoms.

Alloys – very few of the “metals” you use every day are made up of just one element. Most of the metallic objects you see and use are made of alloys. An alloy is a mixture made of two or more elements, at least one of which is a metal. Alloys are generally stronger and less reactive than pure metals. Pure gold is shiny, but is soft and easily bent, so gold jewelry is made of an alloy of gold mixed with a harder element, such as copper or silver. Gold alloys are much harder than pure gold, but still look like gold.

Iron is a strong metal, but iron rusts when exposed to air and water. So iron is often alloyed with one or more other elements to make steel. Iron alloys are much stronger than iron and resist rust. Forks and spoons made of stainless steel can be washed over and over without rusting. Stainless steel is an alloy of iron, carbon, nickel, and chromium.