In metallic solids, the malleability and ductility of metals are best accounted for by which feature of their internal structure and bonding?

Introduction / Context:
Malleability and ductility are characteristic mechanical properties of metals. Malleability refers to the ability of a metal to be beaten into thin sheets, while ductility describes the ability to be drawn into wires. This question checks whether you understand how the metallic bonding model and the arrangement of metal ions and electrons explain these properties at the microscopic level.

Given Data / Assumptions:

• Metals have a regular, often close-packed crystalline lattice of metal ions.
• Valence electrons in metals are delocalised, forming an electron sea or cloud.
• Mechanical deformation involves layers of ions moving relative to one another.
• The options mention crystal structure, electron interactions, electrostatic forces and sliding layers.

Concept / Approach:
In the metallic bonding model, each metal atom contributes its outer electrons to a shared pool of delocalised electrons. The positive metal ions occupy definite positions in a regular lattice, while the electrons move freely throughout the solid. When a force is applied, entire layers of metal ions can slide over each other. Because the delocalised electrons move to keep the metallic bonds intact, the structure does not shatter. Instead, the metal changes shape, which we observe as malleability and ductility. The key idea is that the sliding of layers does not disrupt the metallic bonding network, as it might in a brittle ionic or covalent crystal.

Step-by-Step Solution:
Step 1: Recall that metals are made of positive metal ions arranged in a lattice embedded in a sea of delocalised electrons.Step 2: Recognise that when a force is applied, the ions can shift positions in layers relative to each other.Step 3: Understand that the mobile electrons adjust and continue to hold the ions together, preserving metallic bonding even after the shift.Step 4: Identify that this ability for layers to slide without breaking bonds gives rise to malleability and ductility.Step 5: Note that while crystal structure and electron interactions are involved, the specific mechanism is the sliding of layers.Step 6: Conclude that the capacity of layers of metal ions to slide over one another is the most direct explanation for malleability and ductility.

Verification / Alternative check:
Compare metals with ionic crystals such as common salt. In an ionic lattice, if one layer slides over another, ions of like charge can be forced close together, causing strong repulsion and fracture. This brittleness contrasts with the behaviour of metals, where delocalised electrons shield the positive ions and allow movement without large repulsive forces. This comparison supports the explanation that sliding layers supported by metallic bonding are responsible for malleability and ductility.

Why Other Options Are Wrong:
Simply stating that metals have a crystalline structure is too general; many brittle substances are also crystalline. The interaction of electrons with metal ions in the lattice is part of metallic bonding, but the option does not clearly express the sliding mechanism that explains malleability and ductility. The presence of electrostatic forces is universal in matter, so it does not distinguish metals from other solids. These options are incomplete or too vague.

Common Pitfalls:
Students sometimes memorise that metals have a 'crystal lattice' and stop there, without linking structure to observable properties. Others confuse metallic bonding with ionic bonding and expect metals to be brittle like salts. To avoid these mistakes, always connect the ability of layers to slide and the role of delocalised electrons with the mechanical properties of metals.

Final Answer:
Malleability and ductility of metals arise mainly because layers of metal ions can slide over one another while metallic bonds stay intact, thanks to the sea of delocalised electrons.