Formula of Ionic Compounds

• What is the key rule for determining the formula of an ionic compound?

  Charges on the ions must cancel out to leave an overall charge of zero.
• What is an ionic compound?

  • A compound formed from ions
  • Overall charge is zero
  • Composed of metal and nonmetal ions
• Why do ionic compounds have no overall charge?

  Because the charges on the ions cancel each other out.
• What is the formula of sodium chloride?

  NaCl
• How do the charges of sodium and chloride ions relate in sodium chloride?

  One sodium ion cancels out the charge of one chloride ion.
• What is the formula of sodium oxide?

  Na₂O
• How many sodium ions are needed to balance the charge of one oxide ion?

  Two sodium ions are needed.
• What is the formula of magnesium iodide?

  MgI₂
• How do the charges of magnesium and iodide ions relate in magnesium iodide?

  Two iodide ions cancel the +2 charge of magnesium.
• What is the formula of lithium carbonate?

  Li₂CO₃
• How many lithium ions are needed to balance the charge of one carbonate ion?

  Two lithium ions are needed.
• What is the formula of calcium hydroxide?

  Ca(OH)₂
• Why do we use brackets in the formula of calcium hydroxide?

  To show that the subscript applies to the whole hydroxide ion.
• What is the formula of magnesium nitrate?

  Mg(NO₃)₂
• Why do we use brackets in the formula of magnesium nitrate?

  To indicate that the subscript applies to the entire nitrate ion.
• What is the general process for determining the formula of an ionic compound?

  1. Identify the charges of the ions.
  2. Determine how many of each ion are needed to balance the charges.
  3. Write the formula using the appropriate subscripts.