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Structure 2. Models of bonding and structure - The metallic model

Grade 11IBChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Metallic bonding is the electrostatic attraction between a regular lattice of positive metal ions (cations) and a 'sea' of delocalized valence electrons.

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The strength of the metallic bond depends on the charge density of the metal ions. It is directly proportional to the number of valence electrons (charge) and inversely proportional to the ionic radius: Bond Strength∝ChargeRadius\text{Bond Strength} \propto \frac{\text{Charge}}{\text{Radius}}.

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Melting and boiling points of metals generally increase across a period (e.g., from NaNa to AlAl) because the number of delocalized electrons increases and the ionic radius decreases.

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Metals are excellent electrical conductors because the delocalized electrons are mobile and can move through the lattice when a potential difference is applied.

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Metals are malleable (can be hammered into sheets) and ductile (can be drawn into wires) because the metallic bond is non-directional. Layers of cations can slide over each other without breaking the bond.

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Alloys are solid solutions or mixtures of a metal with other elements (metals or non-metals). Examples include steel (iron and carbon) and brass (copper and zinc).

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The presence of atoms of different sizes in an alloy disrupts the regular lattice structure. This prevents layers of atoms from sliding over each other easily, making alloys harder and stronger than pure metals.

📐Formulae

F=kq1q2r2F = k \frac{q_1 q_2}{r^2}

Charge Density=Ionic ChargeIonic Volume/Radius\text{Charge Density} = \frac{\text{Ionic Charge}}{\text{Ionic Volume/Radius}}

💡Examples

Problem 1:

Explain why Magnesium (MgMg) has a higher melting point than Sodium (NaNa).

Solution:

Magnesium forms Mg2+Mg^{2+} ions with two delocalized electrons per atom, whereas Sodium forms Na+Na^{+} ions with only one delocalized electron per atom. The Mg2+Mg^{2+} ion has a higher charge and a smaller ionic radius compared to Na+Na^{+}, leading to a higher charge density. This results in a much stronger electrostatic attraction between the Mg2+Mg^{2+} cations and the sea of delocalized electrons, requiring more energy to overcome.

Explanation:

This demonstrates the application of the metallic model where melting point is a proxy for bond strength, influenced by ionic charge and radius.

Problem 2:

Why is an alloy like Brass harder than pure Copper (CuCu)?

Solution:

Pure Copper consists of a regular lattice of Cu2+Cu^{2+} ions of uniform size, allowing layers to slide easily when force is applied. Brass is an alloy of Copper and Zinc (ZnZn). The ZnZn atoms have a different atomic radius than CuCu atoms. These 'foreign' atoms disrupt the regular arrangement of the Copper lattice, acting as barriers that prevent the layers of atoms from sliding over one another.

Explanation:

This explains the mechanical property of hardness in alloys compared to pure metals using the structural disruption concept.