Ionic bonding 5

Ionic Bonds

• form between atoms of metals and non-metals with very different electronegativities

• bond formed by transfer of electron

• large numbers of cations and anions combine to form a 3D crystal lattice

• 3D crystal 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

Common Properties of Ionic Compounds

• forces between the particles are strong

• there are no free-moving electrons present

• there are charged particles present but in the solid state they are not free to move

• when an ionic compounds melts, the charged particles are free to move and then the compound will conduct electricity

Melting/Boiling Points

• strong electrostatic attraction between ions in the lattice requires a high amount of energy to overcome

  • electrostatic attraction - the strong attraction between positive and negative charges

• strong ionic bond - it is difficult to break

  • requires high input of energy to separate ions in a lattice

  • e.g. a lot of heat

• therefore:

  • ionic compounds are solids at room temperature

  • have high boiling points

  • have high melting points

• 3D crystal lattice

• magnesium oxide and other ionic compounds are used to line furnaces

• ceramics are materials that contain a mixture of strong ionic and covalent bonds and are used in some engine parts

Hardness and Brittleness

Hardness

• disrupting an ionic lattice structure requires a high amount of force, owing to the strong electrostatic attraction between the cations and anions

• due to the strong electrostatic attraction holding the ions in the crystal lattice

• calcium phosphate is a constituent of bone tissue that gives it strength

• granite, limestone and sandstone are used as a building stone

Brittleness

• an ionic lattice is highly rigid as its strength is dependent on the alternating oppositely charged ions having fixed, localised positions

  • ∴ ionic substances are highly brittle and prone to shattering when enough force is applied

• brittleness results from disruption of the orderly arrangement as force is applied on the solid

  • a layer is forced to slide against the layer next to it, the ions are displaced from their position and like-charged ions come in contact

  • results in repulsion and the crystal lattice shatters

Solubility in Water

• electrolyte - a solution/molten substance that conducts electricity by means of the movement of ions

• for the ionic compounds that are soluble in water, the ions break away from the crystal lattice and get dispersed in the aqueous medium

• vary from very soluble to insoluble in water

• whether an ionic compound is soluble or insoluble depends on the relative strength of the forces of attraction between the:

  • positive and negative ions in the lattice

  • water molecules and the ions

Electrical Conductivity

• ionic solids do not conduct electricity

• have to be in the molten state (liquid) or in an aqueous solution to conduct electricity

  • charged particles are mobile → able to carry the charge through

• the non-conducting property of ionic compounds is used in ceramic insulators, which are used to keep high-voltage power lines insulated from electricity poles and electric fence wires

• when an electric current is applied to either a molten ionic compound or a solution of the compounds in water, positive ions move towards the negatively charged electrode (cathode) and negative ions move towards the positively charged electrode (anode), resulting in an electric current

• ammonium chloride is used as an electrolyte in dry cell batteries

• potassium hydroxide is used as an electrolyte in the ‘button’ cells used in small electronic devices