0620 · 8.4Transition elements

Properties and uses of transition elements

0:00An iron block, a blue copper(II) sulfate solution, and an industrial catalyst seem to have little in common. Yet all three involve transition elements. These metals have a distinctive combination of physical and chemical properties. By the end of this video, you will understand those properties and why iron can form two different types of ion. Transition elements occupy the central region of the Periodic Table. Familiar examples include iron, copper and nickel.

0:25Remember that an element contains only one type of atom, whereas a compound contains different elements chemically combined. That distinction matters because the properties of a metal can differ greatly from those of its compounds. We also need the idea of a positive ion. When a metal atom loses electrons, it becomes positively charged because it now has fewer electrons than protons. Losing two electrons produces an ion with a 2+ charge.

0:49The number lost will become important when we explain iron compounds. Transition elements are metals with high densities and high melting points. Density means mass per unit volume, so it compares mass within equal amounts of space. An iron block has much more mass than a sodium block of the same size. High density therefore means heavy for its size, not simply a large total mass. Density depends on the masses of the atoms and how closely they are packed.

1:13Transition metals generally contain relatively heavy atoms packed closely together. A tiny piece can still have a high density, even though its total mass is small. Size alone cannot tell you whether a material is dense. Their high melting points connect to the strong metallic bonding within these solids. Positive metal ions are attracted to electrons that can move throughout the structure. Melting requires enough energy to overcome the attractions holding that structure together.

1:37Stronger attractions generally mean more heating is needed before the solid becomes liquid. Keep density and melting point separate in your thinking. One describes mass packed into a volume, while the other describes the temperature needed for melting. Transition elements characteristically have high values for both, especially compared with Group I metals. These are characteristic properties, not a claim that every metal behaves identically. Another characteristic is that transition elements form coloured compounds.

2:03Copper(II) sulfate solution, for example, is blue. Notice that we are describing a compound containing copper, not the appearance of copper metal itself. A metal and its compounds are different substances, so their colours need not match. Colour comes from the interaction between light and electrons in the substance. White light contains a mixture of colours, and a coloured compound absorbs some more strongly than others. The light reaching your eye therefore has a different mixture.

2:26You do not need the detailed theory here; just remember that transition element compounds are often coloured. Transition elements often act as catalysts, both as elements and in compounds. A catalyst increases the rate of a reaction and is unchanged chemically at the end. It provides an alternative pathway with a lower activation energy. This means the energy barrier is smaller, so a greater proportion of particle collisions are successful.

2:51Iron is used as an element catalyst in the Haber process, which produces ammonia. Vanadium(V) oxide is a compound catalyst in the Contact process, which is used to manufacture sulfuric acid. The first example uses the metal itself; the second uses a compound containing a transition element. You do not need the operating conditions to understand this distinction. A catalyst does not simply supply extra heat. Instead, it changes the route by which the reaction happens.

3:17It may take part in intermediate steps, but it is regenerated rather than used up overall. The word often also matters because we are not saying every transition element catalyses every reaction.

Variable oxidation numbers in iron

3:28This next part is for the Extended paper. Transition elements have ions with variable oxidation numbers. An oxidation number describes the oxidation state of an element in a substance. For a simple ion containing just one atom, it equals the ion’s charge. Iron can form an iron(II) ion with a 2+ charge or an iron(III) ion with a 3+ charge. The first forms when an atom loses two electrons; the second forms when it loses three.

3:54Iron therefore does not have one fixed ionic charge in all its compounds. Different reactions can produce different oxidation states. The Roman numeral in a compound’s name tells you the metal’s oxidation number. Iron(II) oxide contains the metal in the plus two oxidation state. In iron(III) oxide, that value is plus three instead. The numeral does not tell you how many iron atoms appear in the formula. Let us work out the oxidation number of the metal in iron(II) oxide from its formula.

4:22There is one iron atom for every oxygen atom, and the oxide ion has a 2− charge. The compound is electrically neutral, so its positive and negative charges must balance. One iron ion must therefore have a 2+ charge, giving an oxidation number of plus two. Now consider iron(III) oxide, whose formula contains two iron atoms and three oxygen atoms. Three oxide ions contribute a total charge of 6−. The two iron ions must together provide six plus to make the compound neutral.

4:51Dividing that positive charge equally between them gives three plus each, so iron has an oxidation number of plus three. These examples show why variable oxidation numbers lead to different compounds. Changing the iron ion’s charge changes the ratio needed to balance the oxide ions. Both substances still contain the element iron, but their formulas are different. Each ion keeps the same number of protons as an iron atom, and only the number of electrons changes.

Characteristic properties recap

5:18An exam might ask you to describe four characteristic properties of transition elements. First, they have high densities. Their melting points are also high. They form coloured compounds. Finally, they often act as catalysts as elements and in compounds. Avoid vague answers such as heavy, strong or colourful. Heavy does not compare equal volumes, and strong does not describe a melting temperature. Colourful also fails to identify whether you mean the metal or its compounds.

5:45Precise wording shows that you understand which property belongs to which substance. For Extended questions, state that transition elements have ions with variable oxidation numbers, using iron(II) and iron(III) as examples. Do not confuse the Roman numeral with the number of atoms in a formula. Also remember that a catalyst is not consumed overall. These distinctions turn a memorised list into an explanation you can apply. The key picture is now connected.

6:10Transition elements are metals characterised by high densities and high melting points. Their compounds can be coloured, and both the elements and their compounds often act as catalysts. Variable oxidation numbers explain how the same metal can form differently charged ions and compounds with different formulas.