Metals
Properties of Metal
Metal possesses a number of particular characteristics. Metal substances:
- are good conductors of electricity
- are good conductors of heat
- have lustre (shine)
- are malleable (able to be beaten into a shape)
- are ductile (able to be drawn into a wire)
- are solid at room temperature, with high melting points
- have a high density
Exceptions to note are: mercury (Hg), which is a liquid in standard laboratory conditions due to its low melting point, and chromium (Cr), which is brittle rather than malleable. These two elements, however, still have enough metallic properties for them to be considered as metals. See image 1.
Given the diversity of metals, some elements exhibit these properties at different levels from others. Copper, for example, is an extremely good conductor of electricity compared with lead.
Alloys
A pure metal rarely possesses all the desirable qualities required of useful material, though copper, which is an excellent electrical conductor, and aluminium, which is non-corrosive and a good heat conductor, are examples of elements that are useful in their purest form.
To maximise the beneficial properties of metallic substances and create improved materials, an alloy is made by combining elements. In an alloy, the metal in the majority is the parent or base metal. Usually made with two or more types of metal, alloys can also comprise metal and non-metal elements.
Alloys are created in two ways. One method combines two (or more) similar elements with atoms of comparable size, causing the atoms of the added metal to act as a substitute for some of the atoms of the parent metal. This results in a mixture where the lattice (grid-like structure) retains its structure with the combined benefits of all elements involved. A good example of this is the 'silver' of a 20c coin, which is made of 75% copper and 25% nickel. Copper and nickel atoms are interchangeable in the lattice resulting in the alloy 'cupronickel', which is durable and resistant to corrosion. See image 2.
Another method of creating an alloy is to combine the parent metal with an element or elements with much smaller atoms. The alloy forms when the smaller atoms fit in the spaces between the larger atoms. Pure iron, for example, is soft and therefore benefits from combination with smaller carbon atoms. The carbon atoms find the gaps in the iron lattice to form the alloy steel, which is harder than pure iron. See image 3.
For the most part, alloys have many advantages, which is why they are used in everyday materials. Alloys, however, are usually less malleable and poorer electrical conductors than their component metals, which can be unfavourable for some purposes.
Extraction of Metals
Although metals make up about 80% of the periodic table, they comprise only about 25% of the matter in the Earth's crust. Of this 25%, only a few metals - aluminium, iron, calcium, magnesium, sodium and potassium - occur in amounts significant enough to note. See image 4.
Very few metals are native, that is, pure, naturally occurring elements. Gold and silver are two examples of native elements, while other metals reside as compounds in rocks. Any metal found naturally in rock is a mineral. An ore is a commercially viable mineral and its waste product, after the metal has been extracted, is the gangue.
There are two main stages of extraction. The first is a physical process, separating the minerals from the gangue. This can be as simple as removing the surrounding impurities from the metal, like panning for gold. In the case of most ores, a more intense process called froth flotation is commonly used.
Froth flotation starts with crushed ore in a tank of water mixed with a foaming agent. Air pumped through the tank froths the mixture, causing minerals to float with the bubbles while the gangue sinks to the bottom. If there are still impurities, the minerals proceed to the next step in extraction.
Following froth flotation, there are two main methods of extraction, smelting or electrolysis. Smelting involves heating the metal in a blast furnace, often used for ores that are metal oxide (oxygen) compounds. An important part of smelting is the addition of carbon during the heating process. The heat causes the carbon to form carbon monoxide or carbon dioxide with the metal oxide, leaving the metal behind. The remaining metal is a reduction. See animation 1.
Electrolysis, as its names suggests, uses electricity to extract metal from the ore. It is possible to extract any metal via electrolysis, but it is an expensive process. It is used, therefore, on ores for which there is no other extraction method. A negative electrode is placed in a tank of acid with the ore. The electrode attracts the positively charged cations of the metal in the impure ore, resulting in pure metal coating the negative electrode. The impurities settle at the bottom. See image 5.
