The Transition Metals

Introduction

Iron in bridges, copper in wiring, gold in jewellery, titanium in hip replacements: many of the metals we rely on most come from the wide central block of the periodic table. These transition metals look and behave very differently from the soft, explosive alkali metals. They are strong, hard-wearing and slow to react, and their compounds come in a rainbow of colours. They also have a chemical flexibility that Group 1 and Group 2 metals lack: the same element can form ions with different charges.

Core explanation

Position. The transition metals occupy the central block between Group 2 and Group 3, starting in Period 4. The first row runs from scandium to zinc and includes titanium, chromium, manganese, iron, cobalt, nickel and copper. (Chemists often exclude scandium and zinc from the strict definition because their ions do not show typical transition behaviour.)

Physical properties. Compared with Group 1 metals, transition metals are:

Property Group 1 (e.g. sodium) Transition (e.g. iron) --- --- --- Melting point Low (98 °C) High (1538 °C) Density Low (0.97 g/cm³) High (7.87 g/cm³) Hardness and strength Soft, cut with a knife Hard and strong Reactivity with water Very vigorous Very slow or none

They are also good conductors of heat and electricity, shiny, malleable and ductile, like other metals. Their strong metallic bonding — involving more delocalised electrons per atom — explains the high melting points, hardness and strength. Mercury, a liquid at room temperature, is a notable exception to the high melting point pattern.

Chemical features. Transition metals share three characteristic chemical properties.

1. Ions with different charges. Group 1 metals always form 1+ ions and Group 2 metals 2+ ions. Many transition metals can form more than one ion:

- iron: Fe²⁺ and Fe³⁺ - copper: Cu⁺ and Cu²⁺ - chromium: Cr²⁺ and Cr³⁺ (and higher states in compounds)

Because the charge is not fixed by the group number, the charge is shown with a Roman numeral in the name: iron(II) chloride is FeCl₂, iron(III) chloride is FeCl₃, copper(II) oxide is CuO.

2. Coloured compounds. Most transition metal compounds are coloured, whereas compounds of Group 1 and Group 2 metals are usually white or colourless. The colour often depends on the ion:

Ion Typical colour in solution --- --- Cu²⁺ Blue Fe²⁺ Pale green Fe³⁺ Yellow-brown Ni²⁺ Green Co²⁺ Pink

These colours are used in pottery glazes, stained glass and paints, and to identify ions in chemical tests.

3. Catalytic activity. Many transition metals and their compounds are good catalysts . Iron is used in making ammonia, nickel in turning vegetable oils into margarine, and platinum and palladium in car catalytic converters. This is explored in more detail on the next page.

Reactivity. Transition metals react much more slowly than Groups 1 and 2. Iron rusts over days or weeks, copper hardly reacts with water at all, and gold and platinum remain untarnished for centuries.

Step-by-step reasoning

To name or write the formula of a transition metal compound:

1. Read the Roman numeral: it gives the metal ion's charge (II means 2+, III means 3+). 2. Find the charge on the other ion. 3. Balance the charges to zero. 4. For example, iron(III) oxide: Fe³⁺ and O²⁻ combine as Fe₂O₃ (total +6 and −6).

Visual explanation

Imagine a row of test tubes each containing a solution of a different metal sulfate. Sodium, magnesium and zinc sulfates are all colourless, but copper(II) sulfate is bright blue, nickel sulfate green, cobalt sulfate pink and iron(II) sulfate pale green. The coloured tubes all come from the central block of the table.

Real-world analogy

Group 1 metals are like people with a single fixed job title, while transition metals are like people who can work in two or three different roles depending on what is needed. Iron can act as Fe²⁺ in one compound and Fe³⁺ in another.

Real-world example

The colour of rust and of many rocks and soils comes from iron(III) compounds, which are reddish-brown. Blood's red colour comes from iron in haemoglobin, and the green of emeralds comes from traces of chromium ions. Artists have used cobalt compounds for the deep blue of ceramics for centuries.

Why?

Why are transition metals used for structures while Group 1 metals are not? Iron, titanium and their alloys are strong, dense, high melting and react only slowly with air and water. Sodium is soft, melts in hot water and reacts violently with moisture, so it could never bear a load or survive the weather.

Common misconception

"All transition metal compounds are coloured." Most are, but some are white or colourless. Zinc compounds, such as zinc oxide and zinc sulfate, are white, which is one reason zinc is often not counted as a true transition metal. Scandium(III) compounds are also usually colourless.

Worked example

Question: Give the formula of copper(II) nitrate and predict its colour in solution. The nitrate ion is NO₃⁻.

Reasoning: Copper(II) means Cu²⁺. Two NO₃⁻ ions are needed to balance the 2+ charge, so brackets are used around the nitrate group. Cu²⁺ ions in solution are blue.

Answer: Cu(NO₃)₂; a blue solution.

Quick check

1. What is the charge on the iron ion in iron(III) chloride? Answer: 3+ (Fe³⁺).

Exam focus

Compare transition metals with Group 1 using specific properties: higher melting points, higher densities, harder, less reactive. Learn the three chemical features — ions with different charges , coloured compounds , catalysts — and give an example of each. Use Roman numerals correctly in names and formulas.

Advanced insight

The special behaviour of transition metals comes from their partly filled d subshell. Electrons from both the outer s subshell and the inner d subshell can be lost, which is why several ion charges are possible. In compounds, surrounding ions or molecules split the d orbitals into slightly different energy levels; electrons jumping between them absorb certain wavelengths of visible light, and we see the remaining colours.

Summary

Transition metals form the central block of the periodic table, from scandium to zinc in Period 4 and below. They are hard, strong, dense metals with high melting points and low reactivity compared with Group 1. Characteristically they form ions with different charges (named with Roman numerals), produce coloured compounds, and act as catalysts. These features make them vital in construction, industry and pigments.

Practice questions

1. Give two physical properties in which iron differs from sodium. Answer: Iron has a much higher melting point and a higher density (it is also harder and stronger). 2. Write the formulas of iron(II) oxide and iron(III) oxide. Answer: FeO and Fe₂O₃. 3. A solution is blue. Which transition metal ion is it likely to contain? Answer: Copper(II), Cu²⁺. 4. State the three characteristic chemical properties of transition metals. Answer: They form ions with different charges, form coloured compounds and act as catalysts.