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Page 1: Amorphous Solids

Definition and Characteristics

• Amorphous solids do not melt at specific temperature; instead, they gradually soften over a range of temperatures.

• Examples include rubber, plastic, asphalt, chocolate, taffy, wax, and glass.

Page 2: Crystalline Solids

Structure and Unit Cell

• Crystalline solids feature a regular repeating 3D array of atoms, ions, and/or molecules.

• The basic building block of these solids is called the unit cell.

• Translating the unit cells in 3 dimensions constructs the complete crystal structure.

• The size of the unit cell and the arrangement of atoms within it define the entire 3D crystal structure.

• Types of crystalline solid

  • iodine

  • silicon

  • iron

  • sodium chloride

Page 4: Ionic Solids

Definition and Properties

• Ionic solids are composed of ions that are held together in a three-dimensional network through strong electrostatic forces—for example, NaCl.

• Properties include:

  • High melting point

  • Hard and brittle

  • Non-conducting of electricity except when in liquid form.

Example

• Cesium Chloride: CsCl consists of Cs+ and Cl- ions.

Page 5: Covalent Network Solids

Definition and Properties

• A solid where all the atoms are held together in a large network with covalent bonds, such as diamond and SiO2.

• Properties include:

  • Very hard

  • Very high melting points

  • Non-conducting of electricity even in molten state.

Page 6: Silicon Dioxide

Structure and Polymorphs

• Silicon dioxide (SiO2) forms covalent network structures comprising Si–O single bonds.

• These structures can vary significantly; they typically involve SiO4 tetrahedra linked by bridging oxygen atoms.

• Different crystal structures of the same compound are termed polymorphs.

• Examples include:

  • Quartz crystals

  • Cristobalite crystals

Page 7: Continuation of Silicon Dioxide Structures

Variants of Silicon Dioxide

• Illustrations and data related to different crystalline forms such as Quartz and Cristobalite, reinforcing the concept of polymorphism.

Page 8: Metals

Characteristics

• Metals generally have low ionization enthalpies, allowing for easy loss of valence electrons.

• Atoms in metals are held together by a sea of delocalized electrons - referred to as the free electron model of bonding.

• Metals have varying melting points and hardness. They are good conductors of electricity due to mobile electrons.

Iron Example

• Alpha Iron (α-Fe) is ferromagnetic due to its crystalline structure and remains stable below 910°C.

Page 9: Tin

Allotropes of Tin

• Tin has multiple allotropes, including β-tin and α-tin.

  • β-tin is the metallic allotrope, while α-tin is non-metallic.

  • Example of structural changes and stability at given temperatures.

Page 10: Phase Transitions of Allotropes

Tin Pest Issue

• In medieval Europe, cold winters led to a phenomenon known as tin pest, where wart-like structures formed due to the transformation below 13°C from β-tin to α-tin.

• This phase change involves a 27% increase in volume, causing the metal to crumble.

• Modern prevention methods include alloying tin with antimony or bismuth.

Page 11: Molecular Crystals

Properties and Characteristics

• Molecular crystals consist of molecules held together by various intermolecular forces such as dipole-dipole, London dispersion forces, and hydrogen bonding.

• Properties of molecular crystals include low melting points, softness, brittleness, and non-conductivity.

Page 12: Bromine and Chlorine

Molecular Structures

• Bromine (Br2) and Chlorine (Cl2) are examples of molecules arranged regularly by London forces.

Energy Considerations

• Energy required for breaking covalent bonds versus weak non-covalent attractions in molecular structures.

Page 13: Intermolecular Forces in Organic Compounds

Bonds in Organic Compounds

• Example of pentafluorobenzoic acid showcasing intermolecular hydrogen bonding.

• Sucrose (sugar) as a representative organic molecular crystal in context.

Page 14: Allotropes of Carbon

Graphite Characteristics

• Graphite's structure features covalent network layers held together by strong covalent bonds with layers connected by weaker London dispersion forces.

• The ability of layers to slide over one another grants graphite lubricant properties.

Page 15: Controversy over Graphite as a Lubricant

Real-World Applications

• The suitability of graphite as a lubricant has been questioned due to excessive wear observed in graphite brushes under high-altitude conditions.

• The need for presence of contaminants and organic vapors is cited as a factor affecting performance.