1.2.3 Formulae

• 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
• 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 (III) nitrate, the copper has a charge of 3+

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

• Elements in group 17 gain 1 electron so have a 1- charge, eg. Br^-
• Elements in group 16 gain 2 electrons so have a 2- charge, eg. O^2-
• Elements in group 15 gain 3 electrons so have a 3- charge, eg. N^3-
• There are also more complex negative ions, which are negative ions made up of more than one type of atom

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

Formulae of ionic compounds table

Worked Example: Formulae

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: Iron (III) oxide

• The Roman numeral states that iron has a charge of 3+
• Oxygen is in group 16 so has a charge of 2-
• The charges need to be equal so 2 iron to 3 oxygen atoms will balance electrically, so the formula is Fe₂O₃

Answer 3: Aluminum nitrate

• Aluminium is in group 13 so has a charge of 3+
• Nitrate is a compound ion and has a charge of 1-
• The complex ion needs to be placed in a bracket if more than 1 is needed
• The formula of aluminium nitrate is Al(NO₃)₃