Giant covalent, polymers and representation limits
Bonding, structure, and the properties of matter • Chemical bonds
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Key concepts
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Giant covalent structure: definition and representation
A giant covalent structure contains atoms held together by a continuous network of covalent bonds forming an extended lattice. Examples include diamond and silicon dioxide. A portion of a giant covalent lattice can show covalent bonds with single straight lines between atoms; each line represents one shared pair of electrons. Drawing only part of the lattice clarifies bonding between nearby atoms while avoiding impractical full-lattice depictions.
Polymers and repeating unit representation
Polymers consist of long chains made from repeating monomer units connected by covalent single bonds. The repeating unit is shown in brackets with a subscript n to indicate polymerisation. Each covalent single bond in the repeating unit is represented by a single straight line. Showing only the repeating unit and adding n efficiently communicates chain structure without drawing the entire molecule.
Dot-and-cross diagrams: what they show
Dot-and-cross diagrams show electrons as symbols (dots and crosses) to illustrate bonding pairs and lone pairs. Cause: electrons are explicitly drawn; Effect: the diagram highlights electron sharing and bond formation. Dot-and-cross diagrams suit small molecules or simple bonding explanations because they make electron locations visible and clarify how covalent bonds form between specific atoms.
Limitations of dot-and-cross diagrams
Dot-and-cross diagrams show only electron placement and bonding pairs; cause: electrons are represented symbolically rather than as continuous clouds; effect: they fail to show relative atom sizes, accurate bond lengths, bond angles or the three-dimensional shape of molecules. For giant covalent structures or long polymers, dot-and-cross diagrams become cluttered and impractical because repeating electrons and bonds obscure large-scale structure and do not show extended lattice geometry.
Limitations of two-dimensional diagrams
Two-dimensional (2D) diagrams present atoms and bonds on a flat plane. Cause: projection onto 2D; effect: they cannot accurately convey bond angles that exist in three dimensions or the spatial arrangement of atoms. For extended networks and polymers, 2D diagrams can distort apparent connectivity and packing, causing misunderstanding of actual molecular geometry and steric relationships.
Limitations of three-dimensional diagrams
Three-dimensional perspective diagrams attempt to show bond angles and relative positions using wedges and dashed lines. Cause: use of perspective cues; effect: they improve perception of shape but still simplify electron distribution and may misrepresent relative atom sizes and true bond lengths. Three-dimensional diagrams remain schematic because they cannot show continuous electron density or realistic interatomic distances at atomic scale without specialized models.
Limitations of ball-and-stick diagrams
Ball-and-stick models show atoms as spheres and bonds as rods to emphasise geometry and connectivity. Cause: use of discrete spheres and sticks; effect: they exaggerate gaps between atoms and underrepresent atomic size and electron clouds while clearly showing bond angles. For giant structures, ball-and-stick models give clear geometry for small sections but imply unrealistic voids and are impractical for entire lattices or very long polymer chains; they do not display electron density or properties that arise from extended bonding.
Key notes
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