Properties of Metals
An element is a substance made up of all one kind of atom: for example, the element helium (the same stuff that fills balloons) is made up exclusively of helium atoms. Elements are generally classified as metals or nonmetals(although some elements have characteristics of both; these are called metalloids). The majority of elements listed in the periodic table are metals. These elements usually share three main properties:
- Luster: Metals are shiny when cut, scratched, or polished.
- Malleability: Metals are strong but malleable, which means that they can be easily bent or shaped. For centuries, smiths have been able to shape metal objects by heating metal and pounding it with a hammer. If they tried this with nonmetals, the material would shatter! Most metals are also ductile, which means they can be drawn out to make wire.
- Conductivity: Metals are excellent conductors of electricity and heat. Because they are also ductile, they are ideal for electrical wiring. (You can test this using some household items. See our metals science projects article for an activity.)
These are some of the most important characteristics of metals, but there are other ones as well. Most metals have high melting points and all except mercury are solid at room temperature. They are also sonorous - they often make a ringing sound when hit.
Metals share many characteristics, but they don't all have the same reactivity. Some metals don't react at all with other metals, and because of this they can be found in a pure form (examples are gold and platinum). Because copper is relatively inexpensive and has a low reactivity, it's useful for making pipes and wiring. Other metals are highly reactive and combine easily with other elements, such as oxygen. These metals are never found in a pure form, and are difficult to separate from the minerals they are found in. Potassium and sodium are the most reactive metals. They react violently with air and water; potassium will ignite on contact with water!
Metals are categorized in five different groups:
Noble Metals are found as pure metals because they are unreactive and don't combine with other elements to form compounds. Because they are so unreactive, they don't corrode easily. This makes them ideal for jewelry and coins. Noble metals include copper, palladium, silver, platinum, and gold.
Alkali Metals are very reactive. They have low melting points and are soft enough to be cut with a knife. Potassium and sodium are two alkali metals.
Alkaline Earth Metals are found in compounds with many different minerals. They are less reactive than alkali metals, as well as harder, and have higher melting points. This group includes calcium, magnesium, and barium.
Transition Metals are what we usually think of when we think of metals. They are hard and shiny, strong, and easy to shape. They are used for many industrial purposes. This group includes iron, gold, silver, chromium, nickel, and copper, some of which are also noble metals.
Poor Metals are fairly soft, and most are not used very much by themselves. They become very useful when added to other substances, though. Poor metals include aluminum, gallium, tin, thallium, antimony, and bismuth.
Alloys: Strong Combinations
The properties of these different metals can be combined by mixing two or more of them together. The resulting substance is called an alloy. Some of our most useful building materials are actually alloys. Steel, for example, is a mixture of iron and small amounts of carbon and other elements; a combination that is both strong and easy to use. (Add chromium and you get stainless steel. Check your kitchen pots and pans to see how many are made from stainless steel!)
Other alloys like brass (copper and zinc) and bronze (copper and tin) are easy to shape and beautiful to look at. Bronze is also used frequently in ship-building because it is resistant to corrosion from sea water. Titanium is much lighter and less dense than steel, but as strong; and although heavier than aluminum, it's also twice as strong. It's also very resistant to corrosion. All these factors make it an excellent alloy material. Titanium alloys are used in aircraft, ships, and spacecraft, as well as paints, bicycles, and even laptop computers!
Gold, as a pure metal, is so soft that it is always mixed with another metal (usually silver, copper, or zinc) when it's made into jewelry. The purity of gold is measured in karats. The purest you can get in jewelry is 24 karat, which is about 99.7% pure gold. Gold can also be mixed with other metals to change its color; white gold, which is popular for jewelry, is an alloy of gold and platinum or palladium.
Metal from Ore
Ores are rocks or minerals from which a valuable substance - usually metal - can be extracted. Some common ores include galena (lead ore), bornite and malachite (copper), cinnabar (mercury), and bauxite (aluminum). The most common iron ores are magnetite and hematite (a rusty-colored mineral formed by iron and oxygen), which both contain about 70% iron.
There are several processes for refining iron from ore. The older process is to burn iron ore with charcoal (carbon) and oxygen provided by bellows. The carbon and oxygen, including the oxygen in the ore, combine and leave the iron. However, the iron does not get hot enough to melt completely and it contains silicates left over from the ore. It can be heated and hammered out to form wrought iron.
The more modern process uses a blast furnace to heat iron ore, limestone, and coke (a coal product, not the soft drink). The resulting reactions separate out the iron from the oxygen in the ore. This 'pig iron' needs to be further mixed to create wrought iron. It can also be used for another important purpose: when heated with carbon and other elements, it becomes a stronger metal called steel.
Considering the process involved, it's not surprising that iron was not used until around 1500 BC. But some pure metals - gold, silver, and copper - were used before then, and the alloy bronze is thought to have been discovered by the Sumerians around 3500 BC. But aluminum, one of the most essential metals in modern use, wasn't discovered until AD 1825, and wasn't commonly used until the 20th century!
Corrosion: Process & Prevention
Have you ever seen a piece of silver that lost its shine, or iron with reddish-colored rust on it or even holes in it caused by corrosion? This happens when oxygen (usually from the air) reacts with a metal. Metals with a higher reactivity (such as magnesium, aluminum, iron, zinc, and tin) are much more prone to this kind of chemical destruction, or corrosion.
When oxygen reacts with a metal, it forms an oxide on the surface of the metal. In some metals, like aluminum, this is a good thing. The oxide provides a protective layer that keeps the metal from corroding further. Iron and steel, on the other hand, have serious problems if they are not treated to prevent corrosion. The reddish oxide layer that forms on iron or steel when it reacts with oxygen is called rust. The rust layer continually flakes away, exposing more of the metal to corrosion until the metal is eventually eaten through.
One common way to protect iron is to coat it with special paint that keeps oxygen from reacting with the metal underneath the paint. Another method is galvanization: in this process, steel is coated with zinc. The oxygen, water molecules, and carbon dioxide in the air react with the zinc, forming a layer of zinc carbonate that protects from corrosion. Look around your house, yard, and garage for examples of corrosion as well as galvanization and other means of protecting metal from rust.