05- Structure and properties of materials

• elements

  • pure substance that cannot be broken down into 2 or more simpler substances by chemical methods
  • atoms or molecules

  compounds

  • pure substance containing 2 or more elements chemically combined together in a fixed ratio
  • do not have the same properties as their constituent elements

  mixtures

  • 2 or more elements and or compounds not chemically combined in no fixed ratio
  • have the same characteristics as their constituent elements
• separation of compounds into their constituents:

  • thermal decomposition
  • electrolysis

  separation of mixtures into their constituents:

  • separation methods (filtration, distillation..)
• Ionic substances

  structure: giant ionic crystal lattice (alternating + and -)

  • Melting & boiling point
  • high m&b p
  • mutual electrostatic forces are strong, large amount of energy needed to overcome forces of attraction
  • Hardness
  • hard but brittle
  • hard: strong forces of attraction between oppositely charged ions make ionic compounds resistant to deforming
  • brittle: when enough force is applied, the ions move away from their lattice positions and ions of the same charge approach each other, repulsive force becomes larger then the attractive forces and the lattice structure shatters
• Ionic substances

  • solubility
  • soluble in water
  • insoluble in organic solvents
  • electrical conductivity
  • can conduct electricity in both liquid/ molten and aqueous state: ions are mobile and can conduct electricity
  • cannot conduct in solid state: ions in the lattice can only vibrate around their fixed positions, not mobile
• organic solvents: compounds that contain mainly the elements hydrogen and carbon
• Ionic substances

  • The ions in a giant ionic crystal lattice are held together by strong electrostatic forces of attraction.

  • Ionic substances have high melting and boiling points.

  • Ionic substances are hard but brittle.

  • Most ionic substances are soluble in water and insoluble in organic solvents.

  • Ionic substances can conduct electricity in both the liquid/molten and aqueous states.
• Covalent substances

  simple covalent molecules

  structure: atoms held together by strong covalent bonds, weak intermolecular forces of attraction. At low temperatures, some simple covalent molecules such as ice form lattice structures.

  • melting and boiling points
  • low m&bp
  • weak intermolecular forces are easily overcome with small amount of energy
  • some of them are volatile (evaporate easily), exists as gases/ liquid at room temp
  • as the molecule gets larger, the intermolecular forces of attraction are stronger, higher m&bp
  • solubility
  • insoluble in water
  • soluble in organic solvents
• Covalent substances

  simple covalent molecules

  • electrical conductivity
  • unable to conduct electricity due to the absence of mobile charge carriers
  • However, some covalent molecules can dissociate in water to form a solution that can conduct electricity
• Covalent substances

  giant covalent structure

  • solid state, many atoms connected by strong covalent bonds
  • Allotropes are different forms of the same element with different structural arrangement of atoms
  • diamond and graphite are allotropes of carbon
  • diamond: each carbon forms bonds with 4 other atoms, tetrahedral arrangement
  • graphite: each carbon forms covalent bonds with 3 other atoms, layered structure of hexagonal rings of carbon, weak intermolecular force between layers
  • silicon dioxide (sand) is made up of silicon and oxygen atoms linked to each other via strong covalent bonds
• Covalent structures

  giant covalent structure

  • structural properties
  • numerous bonds holding many atoms together. rigid and can withstand large forces
  • graphite: layered structure, strong covalent bonds within layer, weak intermolecular forces between layers, layers can be easily separated
  • melting and boiling point
  • very high m&bp
  • very resistant to heat due to the large number of bonds in the structure
  • large amount of energy required to break the strong covalent bonds
• Covalent structures

  giant covalent structure

  • hardness
  • diamond is hard. strong covalent bonds
  • graphite is soft and slippery, layers slide over one another easily
  • solubility
  • insoluble in water and organic solvents
  • electrical conductivity
  • diamond & silicon dioxide: no mobile ions, do not conduct electricity
  • graphite: 1 unbonded electron per atom, free and mobile to conduct electricity
• Covalent substances

  macromolecules

  polymers: many covalent molecules joined together into chains of much larger molecules

  • structural properties
  • vary in flexibility and hardness due to the different combinations of atoms
  • melting and boiling points
  • not fixed m&bp
  • range of temperatures when the weak intermolecular forces of attraction are overcome by the molecular vibrations with higher kinetic energy
  • solubility
  • insoluble in water
  • soluble in organic solvents
  • electrical conductivity
  • not able to conduct electricity in any states, do not have mobile ions or electrons
• Covalent Molecules

  • Covalent molecules are held together by weak intermolecular forces of attraction

  • Atoms in covalent structures are connected to one another via strong covalent bonds.

  • Simple covalent molecules have low melting and boiling points, Giant covalent structures have high melting and boiling points.

  • Simple covalent molecules are insoluble in water and soluble in organic solvents, Giant covalent molecules are insoluble in both

  • Most covalent structures are unable to conduct electricity, except for graphite.
• Metals and alloys

  alloy: a mixture of a metal with one or more other elements

  • structural properties
  • pure metals: regular structure, layers can slide over one another easily and can be flattened (malleable), can also be pulled into a wire without breaking (ductile), sea of delocalised electrons will continue to hold the metal ions together
  • alloy: irregular lattice arrangement, larger force required to make the layers slide over one another, alloys tend to be harder and stronger than pure metals
• Metals and alloys

  • melting and boiling point
  • metals: high melting and boiling points, atoms are held together in a lattice by strong metallic bonds
  • alloys: melt over a range of temperatures
  • both are good conductors of heat as the delocalised valence electrons allow efficient transfer of thermal energy throughout the giant metallic lattice
  • electrical conductivity
  • the "sea of delocalised electrons" found in both metals and alloys make them good electrical conductors
• • An alloy is a mixture of a metal with one or more other elements

  • Alloys tend to be less malleable and less ductile than the pure metals they are made from

  • Metals usually have high melting and boiling points because the atoms are held together in a lattice by strong metallic bonds

  • The “sea of delocalised electrons” allow metals to conduct electricity
• giant metallic

  • held in a lattice by strong metallic bonds
  • high m&bp
  • good electrical conductor
  • insoluble in water, insoluble in os

  simple ionic

  • strong EFOA
  • high m&bp
  • good conductor in molten state
  • soluble in water, insoluble in os

  simple covalent

  • strong covalent bonds, weak intermolecular forces
  • low m&bp
  • poor conductor (absence of mobile charge carriers)
  • insoluble in water, soluble in os

  giant covalent

  • strong covalent bonds
  • high m&bp
  • diamond: poor, graphite: good
  • insoluble in water, insoluble in os