Simple and Giant Structures
Section: Chemistry | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
Simple Molecular Structures
- Simple molecular structures are made up of small molecules held together by weak forces between the molecules
- These weak intermolecular forces give simple molecules distinctive properties
- Simple molecular structure: a structure containing small molecules held together by weak intermolecular forces
- Molecule: two or more atoms joined together by covalent bonds (see the Chemical Bonding topic for the full definition)
Examples of Simple Molecules
- Water (H₂O)
- Carbon dioxide (CO₂)
- Oxygen (O₂)
- Methane (CH₄)
Properties of Simple Molecular Structures
- Low melting and boiling points - weak forces between molecules are easy to break
- Do NOT conduct electricity - no charged particles present
- Often gases or liquids at room temperature
- Strong covalent bonds within molecules but weak forces between molecules
Common Mistakes
- Don't confuse the strong covalent bonds inside molecules with the weak forces between molecules - simple molecules have low melting points because of the weak intermolecular forces
Giant Ionic Structures
- Giant ionic structures (also called ionic lattices) are regular arrangements of millions of ions
- Strong electrostatic forces hold the ions together in all directions
- Giant ionic structure: a regular three-dimensional arrangement of ions held together by strong ionic bonds in all directions
Example: Sodium Chloride (NaCl)
- Each Na⁺ ion is surrounded by 6 Cl⁻ ions
- Each Cl⁻ ion is surrounded by 6 Na⁺ ions
- Forms a cubic crystal structure
The giant ionic lattice structure of sodium chloride (NaCl)
Properties of Giant Ionic Structures
- High melting and boiling points - strong electrostatic forces throughout the structure
- Conduct electricity when molten or dissolved - ions are free to move and carry charge
- Do NOT conduct when solid - ions are fixed in position
- Hard but brittle - layers can slide and like charges repel, causing the structure to shatter
- Often soluble in water
Giant Covalent Structures
- Giant covalent structures (also called macromolecules) consist of millions of atoms joined by strong covalent bonds
- The atoms are connected in a continuous network forming a giant lattice
- Giant covalent structure: a structure where all atoms are connected by strong covalent bonds forming a giant lattice
Examples of Giant Covalent Structures
Diamond (Carbon)
- Each carbon atom forms 4 strong covalent bonds with other carbon atoms
- Forms a tetrahedral (pyramid) structure
- Very hard - used in cutting tools and drill bits
- Very high melting point - strong covalent bonds throughout
- Does NOT conduct electricity - no free electrons
The giant covalent structure of diamond
Graphite (Carbon)
- Each carbon atom forms 3 strong covalent bonds with other carbon atoms
- Forms layers of hexagonal rings
- Layers are held together by weak forces
- Soft and slippery - layers can slide over each other (used in pencils and lubricants)
- High melting point - strong covalent bonds within layers
- Conducts electricity - has free electrons (one electron per carbon is delocalised)
The giant covalent structure of graphite
Silicon Dioxide (Silica/Sand)
- Each silicon atom bonds to 4 oxygen atoms
- Each oxygen atom bonds to 2 silicon atoms
- Very hard with a very high melting point
- Does NOT conduct electricity
Giant Metallic Structures
- Giant metallic structures are found in all metals, made up of a giant lattice of positive metal ions surrounded by a "sea" of delocalised (free-moving) electrons
- Metallic bonding: the strong electrostatic force of attraction between positive metal ions and the delocalised electrons surrounding them
- The outer shell electrons of each metal atom are not attached to one particular atom - they are free to move throughout the whole structure
Properties of Giant Metallic Structures
- Conducts electricity (and heat) well, even as a solid - the delocalised electrons are free to move and carry charge throughout the structure
- High melting and boiling points - strong electrostatic forces between the positive ions and delocalised electrons act throughout the whole structure
- Malleable - the layers of positive ions can slide over each other without breaking the metallic bonding, because the sea of electrons moves with them
Example: Iron
- Iron has a giant metallic structure, giving it a high melting point (about 1500°C) - much higher than substances with a simple molecular structure, such as oxygen (−218°C) or water (0°C)
- A substance that conducts electricity as a solid and has a high melting point is a good indicator of a giant metallic structure, unlike a substance with a simple structure, which would have low melting/boiling points and not conduct
Comparing Diamond and Graphite
- Both diamond and graphite are forms of carbon with very different properties
- The difference in structure explains their different properties and uses
| Property | Diamond | Graphite |
|---|---|---|
| Number of bonds per carbon | 4 | 3 |
| Structure | 3D tetrahedral network | Layers of hexagonal rings |
| Hardness | Very hard (hardest natural material) | Soft (layers slide) |
| Electrical conductivity | Does NOT conduct | Conducts (free electrons) |
| Uses | Cutting tools, jewellery, drill bits | Pencils, lubricants, electrodes |
- Diamond and graphite are both made of carbon but have completely different properties because of their different structures - this shows that structure determines properties
Simple and Giant Structures - Summary
- Different types of structures have distinct properties based on their bonding
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