Formulae & Names of Ionic Compounds (DP IB Chemistry: SL)

• Ionic compounds are formed from a metal and a nonmetal bonded together
• Ionic compounds are electrically neutral; the positive charges equal the negative charges

Charges on positive ions

• All metals form positive ions
  • There are some non-metal positive ions such as ammonium, NH₄⁺, and hydrogen, H⁺
• The metals in Group 1, Group 2 and Group 13 have a charge of 1+ and 2+ and 3+ respectively
• The charge on the ions of the transition elements can vary which is why Roman numerals are often used to indicate their charge
• This is known as Stock notation after the German chemist Alfred Stock
• Roman numerals are used in some compounds formed from transition elements to show the charge (or oxidation state) of metal ions
  • Eg. in copper (II) oxide, the copper ion has a charge of 2+ whereas in copper (I) nitrate, the copper has a charge of 1+

Non-metal ions

• The non-metals in group 15 to 17 have a negative charge and have the suffix ‘ide’
  • Eg. nitride, chloride, bromide, iodide

The charges of simple ions depend on their position in the Periodic Table

• There are seven polyatomic ions you need to know for IB Chemistry:

Formulae of Polyatomic Ions Table

Answer:

Answer 1: Magnesium chloride

• • Magnesium is in group 2 so has a charge of 2+
  • Chlorine is in group 17 so has a charge of 1-
  • Magnesium needs two chlorine atoms for each magnesium atom to be balanced so the formula is MgCl₂

Answer 2: Aluminium oxide

• • Aluminum is in group 13 so the ion has a charge of 3+
  • Oxygen is in group 16 so has a charge of 2-
  • The charges need to be equal so 2 aluminium to 3 oxygen atoms will balance electrically, so the formula is Al₂O₃

Answer 3: Ammonium sulfate

• • Ammonium is a polyatomic ion with a charge of 1+
  • Sulfate is a polyatomic ion and has a charge of 2-
  • The polyatomic ion needs to be placed in a bracket if more than 1 is needed
  • The formula of ammonium nitrate is (NH₄)₂SO₄