Identify three reactive and three non-reactive elements, including examples of metals and non-metals in

34 moDUlE 1: the chemical Earth

2.4 Metals, non-metals and semi-metals

Physical properties are those that can be determined without changing the chemical composition of the substance. Physical properties include odour, colour, taste, lustre, hardness, density, mechanical strength, malleability (ability to be beaten into sheets), ductility (ability to be drawn into wires), electrical conductivity, thermal conductivity, melting point, boiling point and solubility. The physical properties of the elements vary widely, as illustrated in Table 2.3.

Table 2.3 Physical properties of a selection of elements

Density (g cm–3) Thermal conductivity (J s–1 m–1 K–1) Electrical conductivity (MS m–1) Melting point (ºC) aluminium 2.70 237 37 660 Boron 2.34 27 10–10 2300 Bromine 3.12 0.12 10–16 –7 Carbon (graphite) 2.26 10 0.07 3974 Copper 8.96 401 58.4 1085 Germanium 5.32 6160 10–4 937 Gold 19.3 317 44 1064 hydrogen 8.2 3 10–5 0.18 – –259 Iodine 4.94 0.45 10–13 114 Iron 7.86 80 10.3 1535 mercury 13.53 8.3 1.0 –39 oxygen 1.3 3 10–3 0.026 – –219 Platinum 21.4 72 9.3 1772 silicon 2.33 142 10–3 1410 silver 10.5 429 63 962 sodium 0.97 141 21 98 sulfur 2.07 0.27 10–21 113 tungsten 19.3 174 19 3410

Using physical properties such as those in Table 2.3, elements may be classified into three groups: metals, non-metals and semi-metals.

It is usually fairly easy to experimentally distinguish between members of these three groups, although it is not always clear-cut. Semi-metals, not surprisingly, exhibit some properties characteristic of metals and other properties that are more like those of non-metals.

Metals

Because the physical properties of metals can vary greatly, it is difficult to give a single definition that can be used to distinguish metals from non-metals. However, by examining a range of properties typical of most metals, it is possible to make a

Figure2.8 Aluminium products

Figure2.9 Silver and gold are often used in jewellery and ornaments.

ChaPtEr 2: Elements of the Earth 35 list of their characteristics. Typically, metals share the following

physical properties:

• relatively high densities (although lithium, sodium and potassium are less dense than water)

• good conductors of heat and electricity

• malleable (can be beaten into sheets) and ductile (can be drawn into wires)

• have a shiny surface when freshly cut or cleaned (lustrous) • relatively high melting points (although mercury and

gallium have quite low melting points).

The uses of metals in modern society are often related to their physical properties. For example, aluminium is widely used in domestic utensils, drink cans, saucepans, cooking foil, building construction as roofing and window frames, and in boat

construction. These uses relate to aluminium’s thermal conductivity, malleability and attractive lustre. More details about the uses and properties of metals are discussed in the next module.

Non-metals

Non-metal elements, which occur as solids, liquids and gases, display a very broad range of physical properties. Typically, non-metals share the following physical properties:

• state and form is variable, for example, oxygen is a gas, bromine a liquid and sulfur a solid

• usually not lustrous

• poor conductors of heat and electricity (except carbon in the form of graphite) • not malleable or ductile, often brittle

• variable melting points as shown in Table 2.3.

Because non-metals display a very wide range of physical properties, they find many different applications. Of those that are used as pure elements, the most commonly known are:

• carbon (graphite)—used in ‘lead’ pencils and as a lubricant because of its softness and layer structure, which allows layers of atoms to slide over each other. It is also used in electrodes in batteries and electrolytic processes because of its electrical conductivity.

Figure2.10 A selection of non-metals: sulfur, bromine, phosphorous, iodine and carbon

Figure2.11 Silicon: a semi-metallic element

36 moDUlE 1: the chemical Earth

• carbon (diamond)—used in jewellery because it is transparent and has a high refractive index and dispersive power, and in drill tips because of its high melting point and hardness • sulfur—used in vulcanising rubber, and in

the manufacture of sulfuric acid, fungicides, insecticides and hydrogen sulfite bleaches due to its abundance and reactivity

• phosphorus—used in smoke bombs and the manufacture of match heads because of its reactivity

• helium—used in airships due to its very low density and lack of reactivity

• neon—used in ‘neon lights’ because of its ability to glow red and give out a great deal of light when excited by an electric discharge • chlorine—used in water treatment because

it is a bactericide and algicide, and as a bleaching agent because of its reactivity in decolourising other chemicals.

Semi-metals

Semi-metals are also known as metalloids. The physical properties of semi-metals, as the name suggests, are a combination of some of the properties of metals and non-metals. Semi-metals have a wide range of properties. Their conductivities vary considerably with temperature and can be increased dramatically if the semi-metal is mixed with very small amounts of other elements such as arsenic and boron. This property is exploited in the use of silicon and germanium as semi-conductors in transistors, silicon chips for integrated circuits and photocells. Table 2.4 summarises the properties of these three groups of elements: metals, non-metals and semi-metals.

Table 2.4 Properties of metals, non-metals and semi-metals

Property Metals Non-metals Semi-metals

melting point usually high usually low high

Boiling point usually high usually low usually high

Electrical conductivity high very low low

heat conductivity high very low low

appearance lustrous usually not lustrous variable

❉

Review exercise 2.4

ChaPtEr 2: Elements of the Earth 37

Elements and the periodic table

In the periodic table, which was introduced in Unit 2.1, the metals, non-metals and semi-metals are grouped together. On the periodic table shown in Figure 2.2, a diagonal line is marked from the top of the third column or group to the lower right corner. This diagonal separates metals from non-metals. Elements to the left and below the diagonal are metals, with the exception of hydrogen. Metals make up the majority of the known elements. Thus, potassium (K), chromium (Cr), silver (Ag) and aluminium (Al) are all metals. Elements to the right and above the diagonal are non-metals. These include carbon (C), chlorine (Cl) and xenon (Xe).

The semi-metals, notably boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb) and tellurium (Te), are grouped along the diagonal. As noted above, these elements display some properties characteristic of metals and some characteristic of non-metals. These elements are also known as metalloids.