Formulae (Cambridge (CIE) A Level Chemistry)
Revision Note
Formulae of Ionic Compounds
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. O2-
Elements in Group 15 gain 3 electrons so have a 3- charge, eg. N3-
There are also more complex negative ions, which are negative ions made up of more than one type of atom
Periodic table outlining common ion charges
The charges of simple ions depend on their position in the Periodic Table
Formulae of ionic compounds table
Ion | Formula and charge |
|---|---|
Silver(I) | Ag+ |
Ammonium | NH4+ |
Zinc(II) | Zn2+ |
Hydroxide | OH– |
Nitrate | NO3– |
Sulfate | SO42– |
Carbonate | CO32– |
Hydrogen carbonate | HCO3– |
Phosphate | PO43– |
Worked Example
Determine the formulae of the following ionic compounds:
Magnesium chloride
Iron(III) oxide
Aluminium nitrate
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: Aluminium 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
Examiner Tips and Tricks
Remember: Compound ions are ions that contain more than one type of element, such as OH–
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