Structure and properties of materials

• Element:

  • made of only one element
  • mostly naturally occurring
  • fixed melting and boiling points
• Compound:

  • made of two or more elements that are chemically combined
  • formed from a chemical reaction
  • Its constituents have a fixed ratio
  • has different properties from its constituent elements
  • Fixed melting and boiling points
• Mixture:

  • made of two or more elements and/or compounds that are not chemically combined
  • formed usually from physical mixing
  • Its constituents do not have a fixed ratio.
  • usually has similar properties to its constituent elements
  • melt and boil over a range of temperatures
• Element: a pure substance that cannot be broken down into two or more simpler substances by chemical methods.

  • can appear as atoms if not chemically bonded to each other. e.g. Helium
  • can appear as molecules if covalently bonded. e.g. Oxygen (O2), Nitrogen (N2), Hydrogen (H2), Fluorine (F2), Chlorine (Cl2), Bromine (B2), Iodine (I2)
• Compound: a pure substance containing two or more elements that are chemically combined in a fixed ratio to form either a covalent or ionic compound. E.g. Sodium Chloride (NaCl), Water (H2O), Hydrogen peroxide (H2O2), ammonia (NH2), methane (CH4)
• Difference between compounds and mixtures:

  • A compound has a fixed proportion of the different types of atoms it is made up of while mixtures e.g. salt solution, air (O2, N2, CO2, water vapour, Ar), sugar solution, crude oil do not have substances in fixed ratio.
• Separation of mixtures into their constituents:

  • In mixtures, the element and/or compounds are physically combined. So, they can be separated using techniques such as filtration, chromatography and distillation. There are no new substances formed in the separation of these mixtures.
• Melting and boiling points (ionic):

  • Ionic compounds have high melting and boiling points because a large amount of heat energy is needed to overcome strong electrostatic forces of attraction between oppositely charged ions throughout the giant ionic crystal lattice.
• Hardness (ionic):

  • The strong forces of attraction between oppositely charged ions make ionic compounds resistant to deforming. Ionic compounds tend to be hard, as these forces cause the ions to resist motion.
  • When enough forces are applied, the ions move away from their lattice positions. Ions of the same charge approach each other. The repulsive forces between ions of the same charge become larger than the attractive forces. Hence, the lattice structure shatters. Hence, ionic compounds are hard but brittle.
• Solubility (ionic):

  • Ionic compounds are often soluble in water to produce aqueous solutions in which water is the solvent but insoluble in organic solvents.
• Electronic conductivity (ionic):

  • Ionic compounds conduct electricity when molten (liquid) or in aqueous solution because of the presence of free moving ions but not in the solid state because ions are held in fixed positions and are not free to move.
• Structural properties (covalent):

  • In a simple covalent molecule, the atoms are held by strong covalent bonds. However, the molecules are held together by weak intermolecular forces of attraction. Only a small amount of energy is needed to overcome these weak intermolecular forces of attraction. Hence, simple covalent substances are usually gases or liquids at room temperature and pressure.
• Melting and boiling points (covalent):

  • Simple covalent substances have low melting and boiling points because little heat energy is needed to overcome the weak intermolecular forces of attraction between the simple covalent molecules.
  • As the molecule gets larger, the melting and boiling points are usually higher.

  
• Solubility (covalent):

  • Simple covalent substances are usually insoluble in water and soluble in organic solvents.
  • Exceptions: Hydrogen chloride (HCl) dissolves in water to form hydrochloric acid; Ammonia (NH3) dissolves in water to form aqueous ammonia.
• Electrical conductivity (covalent):

  • Simple covalent substances do not conduct electricity in any state due to the absence of mobile ions or mobile electrons.
  • Exceptions: HCl (aq)/Hydrochloric acid and NH3 (aq)/aqueous ammonia can conduct electricity due to presence of mobile ions.
• Structure of metals and alloy:

  • Alloy: a mixture of a metal with one main metal or more other elements
  • Metals are arranged very orderly. When force is applied, the layers slide past one another easily. Thus, pure metals are soft.
  • Alloys have different-sized atoms which disrupts the disorderly arrangement of atoms in a pure metal. Hence, when force is applied, layers of atoms don't slide one another easily. Hence, alloys are harder than pure metals.
• Structural properties:

  • Pure metals are malleable (can be moulded) and ductile (can be stretched easily).
• Melting and boiling point:

  • Metals and alloys usually have high melting and boiling points. They are good conductors of heat.
  • Exception: mercury
  • Unlike pure metals, alloys do not have fixed melting points because alloys are mixtures, they usually melt over a range of temperatures.
• Electrical conductivity:

  • Metals and alloys are good electrical conductors as metal atoms have valence electrons that are free to move throughout the structure
• Alloys in daily life: