1 What is a metal?
To begin, have a look at the following questions.
Write down the names of as many metals as you can think of.
Some you may have thought of are iron, silver, gold, tin, lead, zinc, copper, aluminium, sodium and potassium. Slightly more exotic metals are chromium, nickel, cobalt, cadmium, titanium and manganese.
There are a couple of chemistry terms used in this course that you may not be familiar with, so we have defined them to help your study of this course. An element is a substance made up of only one type of basic building block and each element is made up of building blocks, called atoms. You might already know that an atom itself comprises many other smaller particles (e.g. electrons, protons, neutrons).
What elements can you think of that might be non-metallic?
Some non-metallic elements that you might have thought of are hydrogen, nitrogen, carbon and oxygen. Chlorine, bromine, sulfur and phosphorus are also non-metals. Living organisms are made of mainly non-metallic elements.
Scientists have tended to formalise the characteristics of metals (as distinct from non-metals) by suggesting that metals are dense, lustrous (shiny), good conductors of heat and electricity and can be shaped by physical forces.
Metals can be shaped by physical forces in two main ways:
- they can be deformed under tensile stress, e.g. by stretching – a property known as ductility
- they can be deformed under compressive stress, e.g. by hammering into thin sheets – a property known as malleability.
What properties are shown by the elements in Figure 1?
The gold is malleable (it has been hammered into thin sheets) and the copper is ductile (it can be stretched into thin wire).
At room temperature metals are solids, with the exception of mercury which is a liquid. There are also chemical criteria that help distinguish metals from non-metals as you will see later.
Table 1 includes some qualitative and quantitative data for a range of metals. It also includes the non-metallic element sulfur for comparison.
| Element | Proportion in Earth’s continental crust by mass/% | Melting temperature /°C | Density/ 103 kg m–3 | Heat conduction (1, best; 10, worst) | Electric conduction (1, best; 10, worst) |
| Aluminium | 8.2 | 660 | 2.70 | 4 | 4 |
| Chromium | 0.012 | 1857 | 7.19 | 7 | 9 |
| Copper | 0.0068 | 1083 | 8.96 | 2 | 2 |
| Gold | 0.000 0004 | 1064 | 19.3 | 3 | 3 |
| Iron | 5.6 | 1535 | 7.87 | 8 | 7 |
| Magnesium | 2.3 | 649 | 1.74 | 5 | 5 |
| Silver | 0.000 008 | 962 | 10.5 | 1 | 1 |
| Sulfur | 0.034 | 113 | 1.96 | 10 | 10 |
| Tin | 0.000 21 | 232 | 7.31 | 9 | 8 |
| Zinc | 0.0076 | 420 | 7.13 | 6 | 6 |
We often use percentages to express proportions. However, as you can see above, this is less effective when there is only a very small percentage of something. For example, the proportion of sulfur in the Earth’s crust is 0.034%, or 3.4 × 10–2%.
For such small proportions, it is better to use parts per million.
For example, the 3.4 × 10–2% of sulfur represents 3.4 × 10–2, or 0.034, parts per hundred. As a fraction of the total, this is . As with any fraction, the top and bottom terms can be multiplied by 10, and the overall value of the fraction does not change, i.e.
And as 1000 000 is a million, we can see that 0.034 parts per hundred can also be written as 340 parts per million, or 340 ppm.
Table 1 shows that the percentage of chromium in the Earth’s crust is 0.012%. Express this as a fraction, and as a proportion in ppm.
To convert 0.012% into a fraction, divide by 100, i.e.
or 0.012 parts per hundred.
To convert this fraction into ppm, multiply it by a million:
Of course, it is sensible to use the most appropriate tool for the job. So, a discussion of the proportions of aluminium and iron would favour the use of percentages. However, when discussing the proportions of the minor constituents in the Earth’s crust (such as sulfur), it is more appropriate to use parts per million. Indeed, if the proportion is very small, even parts per billion (ppb) might be more appropriate; one billion being 109. For example, the concentration of gold in the Earth’s crust is at a level of 0.000 000 4%, i.e. 4 ppb.
In Table 1, which three metals are the best conductors of electricity? Why do you think just one of these three metals is used much more than the other two?
Silver, copper and gold are the best conductors of electricity. Silver and gold are both high-cost metals. Copper is a cheaper metal so it is often used as an electrical conductor. Generally, the higher the abundance of the metal in the Earth’s crust, the lower the cost. However, this is not an exact relationship, and the cost of a metal also depends on other factors (such as the ease and cost of extraction).
The non-metal, sulfur, is the poorest heat (thermal) and electrical conductor of the elements in Table 1. In fact sulfur is not regarded as a conductor at all. It is an effective insulator, being as good as the plastic insulation that surrounds electric cables.
Looking at the data in Table 1, why do you think aluminium is used extensively in the construction of civil aeroplanes?
Aluminium has low density and this makes it ideal for the construction of aeroplanes where weight is important. Density is dependent on mass, as is weight; so the lower the density, the lower the mass (per unit volume), and the lower the weight.