Physical Properties: Density and Melting Points
Transition metals are metals found in the central block of the periodic table, typically groups 3 to 12. One of their key physical characteristics is their significant density, meaning they have a large mass per unit volume. For example, iron has a density of about 7.9 g/cm³, which is higher than many other metals like aluminium (2.7 g/cm³).
Another notable physical property is their elevated melting points. Transition metals generally melt at much higher temperatures compared to metals in groups 1 and 2. For instance, iron melts at 1538°C, while sodium melts at only 98°C. This is due to the strong metallic bonding in transition metals, where delocalised d-electrons contribute to stronger bonds between atoms, requiring more energy (heat) to break.
These properties make transition metals suitable for applications requiring strength and heat resistance, such as construction materials and engine parts.
- High density due to closely packed atoms and heavier atomic masses.
- High melting points caused by strong metallic bonding involving d-electrons.
Formation of Coloured Compounds
Transition metals can form compounds that are often brightly coloured, unlike many compounds of s-block metals which tend to be colourless or white. This occurs because transition metals have partially filled d-orbitals. When light hits these compounds, electrons in the d-orbitals can absorb specific wavelengths of visible light to jump between energy levels (d-d electron transitions). The wavelengths absorbed correspond to certain colours being removed from white light, so the compound appears coloured to our eyes.
For example, copper(II) sulfate pentahydrate (CuSO₄·5H₂O) is blue, and potassium dichromate (K₂Cr₂O₇) is orange. The exact colour depends on the metal ion, its oxidation state, and the ligands attached to it, which affect the energy gap between d-orbitals.
This property is useful in analytical chemistry for identifying metal ions and in pigments for dyes and paints.
- Partially filled d-orbitals allow d-d electron transitions.
- Absorption of specific visible light wavelengths causes observed colours.
- Colour depends on metal ion, oxidation state, and ligands.
Catalytic Properties of Transition Metals
Transition metals often act as catalysts, substances that speed up chemical reactions without being consumed. Their catalytic ability arises from their variable oxidation states and the ability to adsorb reactant molecules onto their surfaces.
For example, iron is used as a catalyst in the Haber process to produce ammonia from nitrogen and hydrogen gases. The iron surface adsorbs nitrogen and hydrogen molecules, weakening their bonds and allowing them to react more easily.
Another example is vanadium(V) oxide (V₂O₅), a compound of a transition metal, which catalyses the contact process for producing sulfuric acid. It facilitates the oxidation of sulfur dioxide to sulfur trioxide by providing an alternative reaction pathway with lower activation energy, increasing the reaction rate without being consumed itself.
- Variable oxidation states enable electron transfer during reactions.
- Surface adsorption of reactants facilitates bond breaking and formation.
- Catalysts are not consumed and can be used repeatedly.
Distinguishing Transition Metals from Other Metals
Not all metals are transition metals. For example, group 1 metals like sodium and group 2 metals like calcium are not transition metals because they do not have partially filled d-orbitals in their elemental state.
These non-transition metals generally have lower densities and melting points, do not form coloured compounds, and rarely act as catalysts.
For instance, sodium is soft, has a low melting point (98°C), forms colourless compounds like sodium chloride, and does not act as a catalyst in industrial processes. In contrast, iron is hard, melts at 1538°C, forms coloured compounds, and is a key catalyst in industry.
- Transition metals have partially filled d-orbitals; others do not.
- Transition metals have higher density and melting points.
- Only transition metals commonly form coloured compounds and act as catalysts.
At-a-glance summary
Comparison table
| Step | Detail | Example | Exam Use |
|---|---|---|---|
| Identify physical properties | Look for high density and melting point | Iron melts at 1538°C, density 7.9 g/cm³ | State specific values to support answers |
| Explain colour formation | Describe d-d electron transitions in partially filled d-orbitals | Copper(II) sulfate is blue due to d-electron transitions | Use correct terminology: d-orbitals, electron transitions |
| Describe catalytic role | Explain adsorption and variable oxidation states | Iron catalyses Haber process by adsorbing N₂ and H₂ | Link catalyst properties to reaction mechanism |
| Distinguish from other metals | Compare with s-block metals lacking these properties | Sodium is soft, low melting point, colourless compounds | Contrast properties clearly to gain full marks |
Active recall
Quick check
1. Why do transition metals have higher melting points than group 1 metals?
Because transition metals have stronger metallic bonds involving delocalised d-electrons, requiring more energy to break, resulting in higher melting points.
2. What causes transition metal compounds to be coloured?
The partially filled d-orbitals allow electrons to absorb visible light and jump between energy levels (d-d transitions), causing the compounds to appear coloured.
3. How do transition metals act as catalysts?
They adsorb reactant molecules onto their surfaces and use their variable oxidation states to facilitate bond breaking and formation, speeding up reactions without being consumed.
4. Give an example of a transition metal catalyst and the reaction it catalyses.
Iron catalyses the Haber process, converting nitrogen and hydrogen gases into ammonia.