Ionic compounds

• How do you work out the formula of an ionic compound from the formulae of its ions?

  Use the criss-cross method.

  Example 1: Mg2+ and O2- = MgO (2 + -2 = 0 so cancel out)

  Example 2: Al 3+ and S2- = Al2 S3 (swap them)

  Example 3: Al3+ and O2- = Al (O2) 3
• Why are there brackets in some formulae of ionic compounds?

  This is because some ions contain more than one atom. So if an ionic formula contains two or more of the same polyatomic ions then the formula must be written in brackets.
• What is the structure of ionic compounds?

  -giant ionic lattice structure.

  -strong electrostatic forces of attraction between opposite ions.

  -made up of tightly packed ions.
• What are the properties of ionic compounds?

  -composed of positively charged ions (cations) and negatively charged ions (anions)

  -high melting and boiling points due to the strong ionic bonds so large amount of energy needed to overcome.

  -solid at room temperature and are brittle.

  -Ionic compounds conduct electricity when dissolved in water or melted, but not in solid state.
• Why do ionic compounds conduct electricity when they are molten and in aqueous solution?

  Because the ions are free to move and carry an electric charge.
• Why do ionic compounds not conduct electricity when solid?

  Fixed in place so unable to move.
• What are two conditions that need to be met for ionic compounds to conduct electricity?


  1)It must contain charged particles

  2)These particles must be free to move around.
• What are anodes and what are they attracted to?

  They are positively charged and are attracted to anions.
• What are cathodes and what are they attracted to?

  They are negatively charged and are attracted to cations.