Elements have been classified into metals and non-metals based on their physical and chemical properties. Around 118 elements are known to us; about 92 of them occur naturally. Of these, about 22 are non-metals and the rest are metals. Metals such as iron, copper, aluminium, gold and silver have been used by human civilisation for thousands of years, while non-metals such as oxygen, carbon and nitrogen are essential for life itself. Understanding the differences between metals and non-metals helps us decide which material is suitable for a particular use, from building bridges to manufacturing electrical wires.
Metals generally have a lustrous appearance, are good conductors of heat and electricity, are malleable and ductile, and are sonorous. They are usually solids at room temperature (mercury being the notable exception) and have high melting and boiling points. Non-metals, on the other hand, are generally brittle, poor conductors of heat and electricity, and may be solids, liquids or gases at room temperature. Carbon (graphite), however, is a non-metal that conducts electricity.
In this chapter, we will compare the physical and chemical properties of metals and non-metals, learn about the reactivity series, study how metals react with oxygen, water and acids, understand the formation of ionic compounds, and explore important processes such as extraction of metals, corrosion and the reaction of metals with acids and bases.
Metals are malleable (can be beaten into sheets), ductile (can be drawn into wires), sonorous (produce ringing sound), and lustrous (shiny when freshly cut). Aluminium foils are used to wrap food because aluminium is malleable, and copper and aluminium wires are used for electrical connections because they are good conductors of electricity.
Non-metals are brittle when solid, not malleable or ductile, not sonorous, and poor conductors of heat and electricity. For example, sulphur and carbon (coal) break into pieces when beaten. Graphite, a form of carbon, is the exception as it conducts electricity.
Some elements show properties of both metals and non-metals; these are called metalloids, for example boron, silicon and germanium. Metals generally have high densities and high melting and boiling points, while non-metals have low densities and low melting points, except for diamond (a form of carbon) which is the hardest natural substance.
Metals react with oxygen to form metal oxides. These oxides are basic in nature; when dissolved in water, they form hydroxides which turn red litmus blue.
$$4\text{Na} + \text{O}_2 \rightarrow 2\text{Na}_2\text{O}$$
$$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$$
Magnesium reacts with oxygen with a dazzling white flame to form white magnesium oxide. Magnesium oxide dissolves in water to form magnesium hydroxide, which is basic. Some metal oxides, such as aluminium oxide and zinc oxide, are amphoteric in nature, meaning they react with both acids and bases.
Metals react with water to form metal hydroxide and hydrogen gas. The vigour of the reaction depends on the reactivity of the metal. Sodium and potassium react violently with cold water:
$$2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2$$
Magnesium reacts with hot water, while aluminium, iron and zinc react with steam. Copper, silver and gold do not react with water at all.
Metals react with dilute acids to form salt and hydrogen gas. The rate of reaction depends on the position of the metal in the reactivity series. Sodium, potassium and calcium react violently, magnesium and zinc react vigorously, while copper, silver and gold do not react with dilute acids:
$$\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$$
$$\text{Mg} + \text{H}_2\text{SO}_4 \rightarrow \text{MgSO}_4 + \text{H}_2$$
A more reactive metal displaces a less reactive metal from its salt solution. This is used to establish the order of reactivity. For example:
$$\text{Cu} + 2\text{AgNO}_3 \rightarrow \text{Cu(NO}_3\text{)}_2 + 2\text{Ag}$$
The arrangement of metals in decreasing order of their reactivity is called the reactivity series. The order of some common metals is:
$$\text{K} > \text{Na} > \text{Ca} > \text{Mg} > \text{Al} > \text{Zn} > \text{Fe} > \text{Pb} > \text{H} > \text{Cu} > \text{Ag} > \text{Au}$$
Metals above hydrogen can displace hydrogen from dilute acids, while metals below hydrogen cannot. The reactivity series helps predict the products of displacement reactions and guides the choice of method for extracting metals from their ores.
Metals tend to lose electrons and form positively charged ions (cations), while non-metals tend to gain electrons and form negatively charged ions (anions). When a metal reacts with a non-metal, electrons are transferred from the metal to the non-metal, and ionic compounds are formed. For example, in sodium chloride:
$$\text{Na} \rightarrow \text{Na}^+ + e^-$$
$$\text{Cl} + e^- \rightarrow \text{Cl}^-$$
The Na+ and Cl- ions are held together by strong electrostatic forces of attraction, called ionic bonds. Ionic compounds are usually hard, brittle, crystalline solids with high melting and boiling points. They conduct electricity in the molten state and in aqueous solution (because free ions are present), but do not conduct electricity in the solid state.
Metals occur in nature as compounds called minerals. A mineral from which a metal can be profitably extracted is called an ore. Extraction of metals depends on their position in the reactivity series.
Corrosion is the slow destruction of a metal by the action of air, moisture and chemicals. Rusting of iron is the most common example. Corrosion can be prevented by painting, oiling, greasing, galvanising (coating with zinc), tin plating, or by alloying the metal to make it resistant to corrosion.
An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. Alloys are made to improve the properties of metals. For example, stainless steel (iron + chromium + nickel) is resistant to corrosion, brass (copper + zinc) is harder than copper, and solder (lead + tin) has a low melting point and is used for soldering electrical wires. Amalgam is an alloy of mercury with another metal.
| Property | Metals | Non-Metals |
|---|---|---|
| Lustre | Lustrous | Dull |
| Malleability | Malleable | Brittle (non-malleable) |
| Ductility | Ductile | Non-ductile |
| Conductivity | Good conductors | Poor conductors |
| Sonorous | Yes | No |
| State at room temp | Solid (except mercury) | Solid, liquid or gas |
| Oxide nature | Basic | Acidic |
| Examples | Fe, Cu, Al, Na | S, O, N, Cl |
| Position in Reactivity Series | Example Metals | Extraction Method |
|---|---|---|
| High reactivity | K, Na, Ca, Mg, Al | Electrolytic reduction of molten ore |
| Medium reactivity | Zn, Fe, Pb | Calcination/roasting then reduction with carbon |
| Low reactivity | Cu, Ag, Hg, Au | Found in free state, obtained by roasting/heating |
| Noble metals | Au, Pt | Found in free state, no extraction needed |
Metals and non-metals surround us in every aspect of life, and their contrasting properties explain the diversity of materials we use. The reactivity series is the single most powerful tool in this chapter, as it predicts the outcome of reactions, guides the extraction of metals, and explains corrosion and its prevention. Ionic compounds, formed by the transfer of electrons between metals and non-metals, display a distinct set of properties that set them apart from covalent compounds. Extraction of metals from their ores, corrosion, and the design of alloys are real-world applications that show how fundamental chemistry translates into engineering and industry. A clear understanding of this chapter lays the foundation for studying the periodic table, chemical bonding and metallurgy in higher classes.