Unit IV metals

Part I

I. Discuss the following questions in class.

1. What are metals? Try to define the term in your own words.

2. What are the characteristic features of them?

3. Where and what do we use the metals for?

II. Read the text and decide if the following questions correspond to the information in the text or not.

1. Are the metals sometimes described as a lattice of positive ions surrounded by a cloud of delocalized electrons?

2. What is the traditional definition of metals?

3. Does this text give the detailed information about boron?

4. What is corrosion?

In chemistry, metal is an element that readily loses electrons to form positive ions (cations) and has metallic bonds between metal atoms. Metals form ionic bonds with non-metals. They are sometimes described as a lattice of positive ions surrounded by a cloud of delocalized electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals.

On the periodic table, a diagonal line drawn from boron to polonium separates the metals from the nonmetals. Most elements on this line are metalloids, sometimes called semi-metals: elements to the lower left are metals: elements to the upper right are nonmetals.

An alternative definition of metals is that they have overlapping conduction bonds and valence bonds in their electronic structure. This definition opens up the category for metallic polymers and other organic metals, which have been made by researchers and employed in high-tech devices. These synthetic materials often have the characteristic silvery-grey reflectiveness (luster) of elemental metals.

The traditional definition focuses on the bulk properties of metals. They tend to be lustrous, ductile, malleable, and good conductors of electricity, while nonmetals are generally brittle (if solid), lack of luster, and are insulators. Most metals are chemically reactive, reacting with oxygen in the air to form oxides over changing timescales (for example iron rusts over years and potassium burns in seconds). The transition metals take much longer to oxidize (such as iron, copper, zinc, nickel). Others, like palladium, platinum, and gold, do not react with the atmosphere at all. Some metals form a barrier layer of oxide on their surface which cannot be penetrated by further oxygen molecules and thus retain their shiny appearance and good conductivity for many decades (like aluminum, some steels, and titanium). Painting, anodizing or planting metals are good ways to prevent their corrosion. However, a more reactive metal in the electrochemical series must be chosen for coating, especially when chipping of the coating is expected. Water and the two metals form an electrochemical cell, and if the coating is less reactive than the coatee, the coating actually promotes corrosion.

Traditionally metals have certain characteristic physical properties. They are usually shiny (they have metallic luster), have a high density, are ductile and malleable, have a high melting point, are hard, are usually a solid at room temperature and conduct electricity, heat and sound well. While there are several metals that are low density, soft and have low melting points, these (the alkali and alkaline earth metals) are extremely reactive, and are rarely encountered in their elemental, metallic form. The electrical and thermal conductivity of metals originate from the fact that in the metallic bond the outer electrons of the metal atoms form a gas of nearly free electrons, moving as an electron gas in a background of positive charge formed by the ion cores.