True/False (Bonding): In a purely covalent (valence) crystal, the bonding between atoms is accomplished by sharing valence electrons.
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ATrue
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BFalse
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CTrue only at absolute zero
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DFalse unless doped
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ETrue only for metals
Answer
Correct Answer: True
Explanation
Introduction / Context:Crystalline solids are stabilized by different bonding mechanisms: ionic, covalent, metallic, Van der Waals, or mixed. A “valence crystal” typically refers to a covalent crystal where each atom shares electrons with neighbors to fill valence shells, leading to directional bonds and characteristic properties.
Given Data / Assumptions:
- Ideal covalent solids (e.g., diamond, silicon, germanium).
- Atoms form tetrahedral or other directional bonds by sharing electrons.
- No significant ionic character for the idealized statement.
Concept / Approach:Covalent bonding arises when atoms with comparable electronegativities share pairs of electrons to achieve closed-shell configurations. In diamond (sp^3 hybridization), each carbon atom shares four electrons with neighbors, yielding a strong, directional network. This contrasts with ionic crystals (electron transfer) and metals (delocalized electron sea).
Step-by-Step Solution:
Identify the bonding mechanism for covalent crystals: sharing of valence electrons.Relate to structure: strong, directional bonds → high hardness, high melting point, low electrical conductivity at low temperature.Conclude the statement is correct.Verification / Alternative check:
Band theory: covalent solids show a valence band–conduction band gap derived from shared-electron bonds.Why Other Options Are Wrong:
Metals do not bond by localized sharing; ionic solids primarily transfer electrons.Common Pitfalls:
Assuming all real materials are purely one type; many have partial ionic/covalent character, but the statement about covalent crystals remains true.Final Answer:
True