0620 · 9.1Properties of metals

Metallic bonding and conductivity

0:00A metal saucepan transfers heat, a copper wire carries electrical current, and a metal strip can bend without breaking. These behaviours seem different, but they connect to the structure inside a metal. Metals also show recognisable patterns in their chemical reactions. By the end, you will understand these patterns without treating them as disconnected facts. We need to separate physical properties from chemical properties. A physical property describes a substance without changing its chemical identity.

0:26Bending a metal changes its shape, but it remains the same substance. A chemical property describes how it reacts to form new substances, such as when magnesium combines with oxygen. The word general matters throughout this topic. Metals share many typical properties, but there are exceptions. Non-metals are also a varied group, so their usual behaviour does not describe every individual element. Treat these comparisons as useful patterns, rather than rules that must apply without exception.

0:52A useful reminder from metallic bonding explains much of the physical behaviour. A metal contains positive ions surrounded by delocalised electrons. Delocalised means these electrons can move throughout the structure rather than belonging to one particular atom. Attractions between the opposite charges hold the metal together, while the mobile electrons help explain conductivity. Electrical conductivity means allowing an electric current to pass through a substance. In a solid metal, delocalised electrons can move through the structure and carry charge.

1:22The positive ions remain in their positions rather than travelling along the wire. Most non-metals lack mobile charged particles, so they are poor electrical conductors. Graphite is an important exception because it conducts electricity even though it is a form of carbon, a non-metal. Its structure also contains electrons that can move and carry charge. Electrical conductivity therefore depends on mobile charged particles, not simply whether we call something a metal.

1:46Graphite’s detailed structure belongs in the video on giant covalent structures.

Thermal properties and malleability

1:51Thermal conductivity describes how readily a substance transfers energy from a hotter region to a cooler one. Metals are generally good thermal conductors. Their mobile electrons help transfer energy through the structure, alongside vibrations of the particles. Most non-metals transfer thermal energy less readily, so they are generally poor thermal conductors. Good thermal conductivity does not mean a metal must already be hot. It describes how easily energy passes through the material when there is a temperature difference.

2:18A metal spoon can become hot quickly when one end touches something hot. The energy travels through it without turning the spoon into a different substance. Malleability is the ability to be hammered or pressed into shape without breaking. A malleable material can therefore be made into sheets. Ductility describes the ability to be drawn into wires. Both involve changing shape without the material simply snapping, but sheets and wires describe different ways of reshaping it.

2:43In a metal, layers of positive ions can slide past one another when a force is applied. The surrounding delocalised electrons continue attracting the ions, so the structure can remain held together. Most solid non-metals are brittle instead. They tend to crack or shatter when a force is applied, rather than deforming like a metal. Keep these ideas separate from hardness. A substance can be hard but brittle, resisting a scratch yet breaking under a blow.

3:08Malleability describes reshaping, not resistance to scratching. Also, the comparison with brittle non-metals applies to solids; a gas cannot meaningfully be hammered into a sheet or drawn into a wire. Metals generally have high melting points and boiling points compared with non-metals. Melting allows particles to move past one another, while boiling separates them into a gas. Strong attractions in a metallic structure generally require considerable energy to overcome. That helps explain why many metals need substantial heating before these changes happen.

3:39Many non-metals consist of small molecules with relatively weak attractions between them. Less energy is needed to overcome those attractions, giving generally lower melting points and boiling points. The bonds within the molecules are not broken during these changes. However, non-metals with giant covalent structures do not follow this simple molecular pattern. Mercury provides a clear warning against saying all metals have high melting points. It is liquid at room temperature despite being a metal.

4:05Graphite gives a similar warning against saying every non-metal is an electrical insulator. In comparisons, words such as generally and most keep your answer scientifically accurate.

Metals reacting with dilute acids

4:16Chemical properties reveal what happens when a metal forms new substances. We will consider dilute acids, cold water, steam and oxygen. Not every metal reacts with each of these, and the reaction conditions matter. The patterns describe the products when the reaction occurs; they are not a promise that every metal will react. With a suitable dilute acid, a reacting metal usually forms a salt and hydrogen. Magnesium reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen.

4:44The acid supplies the hydrogen that becomes the gas, while magnesium becomes part of the salt. Changing the acid changes which salt is produced. For example, magnesium reacts with dilute sulfuric acid to form magnesium sulfate and hydrogen. Notice that the gas remains hydrogen, but the salt has changed. These patterns work for the usual examples involving dilute hydrochloric acid or dilute sulfuric acid. Do not extend them automatically to every acid under every condition.

5:10Let us work through the magnesium and hydrochloric acid equation. First, identify the reactants and predict magnesium chloride and hydrogen as the products. Then write their correct formulae, as shown on the board. Finally, balance the equation so that each element has the same number of atoms on both sides. Magnesium chloride contains two chlorine atoms for every magnesium atom. We therefore need two units of hydrochloric acid to supply those chlorine atoms.

5:36They also supply two hydrogen atoms, which form one hydrogen molecule. The balanced symbol equation is on the board, with one magnesium atom on each side. Its state symbols show solid magnesium, aqueous hydrochloric acid and magnesium chloride, and hydrogen gas. Balancing changes the amounts of substances, not their identities. We put a number before hydrochloric acid rather than changing its formula. Altering a small number within a formula would describe a different substance.

6:04This is why predicting the products comes before balancing the equation. A reaction with acid may produce bubbles as hydrogen escapes and the metal is used up. However, bubbling alone does not identify a gas. Copper does not react with dilute hydrochloric acid under normal laboratory conditions, so the general equation cannot be applied blindly. The practical paper may ask you to describe observations; gas identification belongs in the video on identification of ions and gases.

Reactions with water, steam and oxygen

6:29Some metals react with cold water to form a metal hydroxide and hydrogen. Sodium is a named example: sodium plus water gives sodium hydroxide plus hydrogen. In the balanced equation, two sodium atoms react with two water molecules to give two units of sodium hydroxide and one hydrogen molecule. Sodium hydroxide dissolves in the water, forming an alkaline solution. This is a vigorous reaction and must only be demonstrated under appropriate laboratory safety controls.

6:55The word cold distinguishes ordinary liquid water from steam in these reaction patterns. Metals that react little or not at all with cold water may react with steam when heated. Steam is water vapour, so it contains the same substance as liquid water. The higher temperature helps some reactions happen that are extremely slow in cold water. A reacting metal with steam generally produces a metal oxide and hydrogen. For example, heated magnesium reacts with steam to form magnesium oxide and hydrogen.

7:22Compare this carefully with sodium reacting with cold water. The water reaction gives a hydroxide, while the steam example gives an oxide; both release hydrogen. Hydroxide and oxide are not interchangeable names. They describe compounds with different compositions, as their formulae on the board show. Keep the condition attached to the product pattern rather than learning water as one undifferentiated reactant. Differences between individual metals are explored further in the video on the reactivity series.

7:51Metals commonly react with oxygen to form metal oxides. Magnesium provides another example: magnesium plus oxygen gives magnesium oxide. In the balanced equation, two magnesium atoms react with one oxygen molecule to give two units of magnesium oxide. The oxygen becomes part of the new compound rather than being released as a gas. Hydrogen is not a product here because neither reactant contains that element. These oxygen reactions can happen at different rates, and some metals require heating before an obvious reaction occurs.

8:22Do not assume that every metal burns readily just because oxide formation is a general pattern. Also, making an oxide is a chemical change. The product has a different composition and different properties from the original metal.

Exam answers and recap

8:33An examination might ask you to describe two physical differences between metals and non-metals. A clear answer is that metals generally conduct electricity well, whereas non-metals are mostly poor conductors. For a second mark, state that solid non-metals are usually brittle, while metals can be hammered into shape. Each sentence makes a direct comparison rather than describing only one group. If asked to state the products when magnesium reacts with steam, answer magnesium oxide and hydrogen.

9:01An explain question on electrical conductivity needs the delocalised electrons, which move through the structure and carry charge. Explain requires a reason, not just a restatement that metals conduct. Match the detail of your answer to the command word. The central connection is between structure and behaviour. Mobile electrons help metals conduct electricity and transfer thermal energy, while their bonding allows layers to slide without breaking apart. Metals generally have higher melting points and boiling points than non-metals.

9:30These are general comparisons, with exceptions such as mercury and graphite. For chemical reactions, identify both the reactant and the conditions before predicting products. Suitable dilute acids give a salt and hydrogen; cold water gives a hydroxide and hydrogen. Steam generally gives an oxide and hydrogen, while oxygen gives an oxide without hydrogen. Understanding those distinctions is more reliable than remembering one vague rule that metals react.