Chemical Bonding
Metallic Bonding
5th Year · 6th Year (Leaving Cert)
- ✓By the end of this lesson students will be able to define metallic bonding.
- ✓By the end of this lesson students will be able to describe the 'sea of delocalised electrons' model for metallic bonding.
- ✓By the end of this lesson students will be able to explain the characteristic physical properties of metals based on the 'sea of delocalised electrons' model.
- ✓By the end of this lesson students will be able to relate the strength of metallic bonding to factors such as the charge on the metal ion and atomic radius.
Key concepts
Metallic bonding is the strong electrostatic force of attraction between positively charged metal ions (cations) and a 'sea' of delocalised valence electrons. These bonds are non-directional and extend throughout the entire metallic lattice.
In the 'sea of electrons' model, each metal atom donates its valence electrons to a common pool. These electrons are not associated with any particular atom but are free to move throughout the entire metallic structure, forming a 'sea' of mobile electrons. The remaining positively charged metal ions (which consist of the nucleus and inner shell electrons) are held in fixed positions within this 'sea'. This model effectively explains the unique physical properties of metals.
The 'sea of electrons' model provides a clear explanation for the characteristic properties of metals: 1. **High Electrical Conductivity:** The delocalised valence electrons are free to move throughout the metallic lattice. When a potential difference is applied, these mobile electrons can move in a directed manner, carrying electrical charge and thus conducting electricity. 2. **High Thermal Conductivity:** The delocalised electrons can rapidly absorb and transfer kinetic energy through the metal by colliding with other electrons and metal ions. This efficient transfer of energy accounts for high thermal conductivity. 3. **Malleability and Ductility:** Metals are malleable (can be hammered into sheets) and ductile (can be drawn into wires) because the metallic bond is non-directional. When a force is applied, the layers of positive metal ions can slide past one another without breaking the metallic bond. The 'sea' of delocalised electrons simply rearranges to maintain the electrostatic attraction between the positive ions and the electrons, preventing the structure from fracturing. 4. **High Melting and Boiling Points:** A large amount of energy is required to overcome the strong electrostatic forces of attraction between the positive metal ions and the 'sea' of delocalised electrons throughout the lattice. This results in high melting and boiling points. 5. **Lustre (Shiny Appearance):** The delocalised electrons can absorb and re-emit photons of light over a range of wavelengths, giving metals their characteristic shiny appearance.
Key facts to remember
- 1Metallic bonding is the electrostatic attraction between positive metal ions and a 'sea' of delocalised valence electrons.
- 2The 'sea of electrons' model explains the unique physical properties of metals.
- 3Delocalised electrons are responsible for the high electrical and thermal conductivity of metals.
- 4Metals are malleable and ductile because layers of positive ions can slide past each other without breaking the metallic bond, due to the mobile electron 'sea'.
- 5Metals generally have high melting and boiling points due to the strong electrostatic forces within the metallic lattice.
- 6The strength of metallic bonding increases with increasing charge on the metal ion and decreasing atomic radius.
- 7Metallic bonds are non-directional, meaning the attraction is not limited to specific pairs of atoms.
Worked examples
Example 1
Explain, using the 'sea of delocalised electrons' model, why metals are good conductors of electricity.
Answer
Metals are excellent conductors of electricity because they possess a 'sea' of delocalised valence electrons. These electrons are not bound to any specific atom but are free to move throughout the entire metallic lattice. When an electric potential difference is applied across the metal, these mobile electrons can move in a directed flow, effectively carrying electrical charge and thus conducting electricity.
Example 2
Account for the malleability and ductility of metals based on their bonding and structure.
Answer
Metals are malleable (can be hammered into sheets) and ductile (can be drawn into wires) due to the nature of metallic bonding. In a metallic lattice, positive metal ions are arranged in layers, all immersed within a 'sea' of delocalised valence electrons. When an external force is applied, these layers of positive ions can slide past one another. Crucially, the 'sea' of delocalised electrons can simply rearrange itself to maintain the strong electrostatic attraction between the positive ions and the electrons. This prevents the metallic bond from breaking and the metal from fracturing, allowing it to change shape without losing its structural integrity.
Example 3
Explain why magnesium (Mg) has a significantly higher melting point (650 °C) than sodium (Na) (98 °C).
Answer
Both sodium and magnesium exhibit metallic bonding. The strength of metallic bonding is influenced by the charge on the metal ion and the number of delocalised electrons contributed per atom. Sodium atoms each contribute one valence electron to the 'sea' and form Na⁺ ions. Magnesium atoms, however, each contribute two valence electrons to the 'sea' and form Mg²⁺ ions. The Mg²⁺ ion has a higher charge (+2 compared to +1 for Na⁺) and contributes more delocalised electrons per atom. This results in much stronger electrostatic forces of attraction between the Mg²⁺ ions and the 'sea' of electrons in magnesium compared to the forces in sodium. Therefore, significantly more energy is required to overcome the stronger metallic bonds in magnesium, leading to its much higher melting point compared to sodium.
Other factors like atomic radius also play a role, but the charge and number of delocalised electrons are the primary distinguishing factors here.
Common mistakes
- ✗Confusing metallic bonding with ionic bonding (where electrons are transferred) or covalent bonding (where electrons are shared between specific atoms).
- ✗Incorrectly stating that the metal atoms themselves are free to move; it is the delocalised electrons that are mobile, while the positive metal ions are in fixed positions within the lattice.
- ✗Failing to explicitly link a specific property (e.g., conductivity) directly to the presence and mobility of the 'sea' of delocalised electrons.
- ✗Not explaining *why* layers of ions can slide without the metal fracturing – the role of the delocalised electrons in maintaining attraction is crucial.
- ✗Overlooking the importance of the charge on the metal ion and the number of delocalised electrons when comparing the strength of metallic bonds between different metals.
Exam tips
- ★Always refer to the 'sea of delocalised electrons' model when explaining any property of metals in your exam answers.
- ★Use precise terminology: 'positive metal ions' (or 'cations') and 'delocalised valence electrons'. Avoid vague terms like 'atoms' or 'free electrons'.
- ★When asked to compare properties of different metals, explicitly discuss the factors affecting metallic bond strength (e.g., charge on the ion, number of delocalised electrons, atomic radius).
- ★Practise drawing simple diagrams of the 'sea of electrons' model; it helps to solidify your understanding and can be useful for visualising explanations.
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