Chapter 5 Properties and structures of ionic compounds

• Ionic compounds are made by the combination of atoms, or groups of atoms, where electron(s) are transferred from one to another. In doing so, these particles become ions, hence ionic compounds are compounds formed from ions.
• These materials are very common in the natural world because the Earth's crust is largely made up of complex ionic compounds
• Most rocks, minerals and gemstones (Figure 5.1.1) are ionic compounds
• Soil contains weathered rocks mixed with decomposed organic material, so soil contains large quantities of ionic compounds
• Ceramics, kitchen crockery and bricks are made from clays
• Clays are formed by the weathering of rocks, so these materials also contain ionic compounds
• Kitchen crockery and bricks contain mixtures of different ionic compounds
• Table salt (sodium chloride)

  A pure ionic compound
• Generally, ionic compounds:

  • have high melting and boiling points and they are all solids at room temperature
  • are hard but brittle (fragile), unlike metals and therefore they are neither malleable nor ductile
  • do not conduct electricity in the solid state
  • are good conductors of electricity in the liquid state or when dissolved in water
  • vary from very soluble to insoluble in water.
• Properties of metals

  Indicate something about the structure of metals; how particles are arranged in a metallic lattice
• Properties of ionic compounds

  Provide evidence for how the particles are arranged in ionic compounds
• The properties of ionic compounds are different from those of metals
• Chemists have concluded that the particles in ionic compounds, and how they are arranged in the solid state, are different from those present in metals
• Properties of sodium chloride (table salt)

  • Listed in Table 5.1.2
• Descriptions beside each property

  Describe the nature of the particles and the types of forces between the particles, inferred from the different properties of sodium chloride
• properties apply to ionic compounds generally.

  • The forces between the particles are strong.
  • There are no free-moving electrons present, as there are in metals.
  • There are charged particles present, but in the solid state they are not free to move.
  • When an ionic compound melts, the charged particles are free to move and then the compound will conduct electricity.
• Ionic bonding model


  The solid state
• Formation of ionic compounds

  1. One set of atoms (generally metal atoms) lose electrons and become positively charged ions (cations)

  2. Electrons are transferred to another set of atoms

  3. The other set of atoms (generally non-metal atoms) become negatively charged ions (anions)
• Cations


  Positively charged ions
• Anions


  Negatively charged ions
• Atoms are at their most stable when they have a filled valence shell
• An atom's ability to form a cation or anion depends on how many electrons it needs to gain or lose to achieve this stable arrangement
• Arrangement of cations and anions

  1. Large numbers of cations and anions combine to form a three-dimensional lattice (or crystal lattice)

  2. The three-dimensional lattice is held together strongly by electrostatic forces of attraction between the oppositely charged ions
• Ionic bonding


  The electrostatic force of attraction holding the ions together
• Sodium loses one electron

  1. Full valence shell (electron configuration 2,8)

  2. Positively charged

  3. Symbol Na+
• Chlorine gains one electron

  1. Full valence shell (electron configuration 2,8,8)

  2. Negatively charged

  3. Anion Cl-
• Sodium chloride

  • Each sodium ion is surrounded by six chloride ions
  • Each chloride ion is surrounded by six sodium ions
• Attractive force between sodium and chloride ions

  Outweighs repulsive force between chloride ions
• The lattice of sodium chloride is held together quite strongly
• i lonic bonding is the strong, electrostatic attraction that exists between positively charged cations and negatively charged anions.
• Sodium chloride


  Chemical formula: NaCl
• In a solid sample of an ionic compound, such as sodium chloride, individual pairs of sodium and chloride ions do not exist
• The solid is not built up of discrete NaCl molecules
• Sodium chloride solid


  • Made up of a continuous lattice of alternating Na+ and Cl- ions
  • NaCl is the formula unit, representing the simplest whole number ratio of the ions
  • All sodium ions are an equal distance from six chloride ions
  • All chloride ions are an equal distance from six sodium ions
  • The overall ratio of sodium ions to chloride ions in the lattice is 1:1
• Ionic bonding model


  Explains properties of ionic compounds
• The metallic bonding model represents the structure of metals as a lattice of positively charged metal ions held together by delocalised electrons
• The metallic bonding model explains many properties of metals, such as why metals generally have high melting points and conduct electricity in the solid state
• High melting points of ionic solids

  • Energy is required to allow the ions to break free and move
  • Indicates strong electrostatic attraction between oppositely charged ions
  • Explains why high temperature is required to melt ionic solids
• High melting point ionic compounds

  • Sodium chloride (801°C)
• The high melting point of ionic compounds is put to use in the bricks that line furnaces and kilns and in the ceramic materials used to make brake discs for high-performance cars