Deducing Formulae Using Common Ions
- If you know the charges on ions you can work out the formula of an ionic compound very easily
- The position of an element in the Periodic Table can be used to predict the most likely ion formed
The charges of simple ions depend on their position in the Periodic Table
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 3 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 Groups 5 to 7 have a negative charge and have the suffix ‘ide’
- Eg. nitride, chloride, bromide, iodide
- Elements in Group 7 gain 1 electron so have a 1- charge, eg. Br-
- Elements in group 6 gain 2 electrons so have a 2- charge, eg. O2-
- Elements in group 5 gain 3 electrons so have a 3- charge, eg. N3-
- There are also more polyatomic or compound negative ions, which are negative ions made up of more than one type of atom
Compound Ions
- There are several common compound ions which are summarised below
- Some chemists call these polyatomic ions
Formulae of common compound ions
- When you write the formula of a compound ion it is necessary to use brackets around the compound ion where more than one of that ion is needed in the formula
- For example copper(II)hydroxide is Cu(OH)2
Worked example
What is the formula of?
- sodium bromide
- aluminium fluoride
- aluminium oxide
- magnesium nitrate
- ammonium sulfate
Answer 1
| Symbol | Na Br |
| Ion charge | 1+ 1- |
| Balance the number of ions | 1 sodium ion is needed for each bromide ion |
| Ratio of ions | 1:1 |
| Formula | NaBr |
Answer 2
| Symbol | Al F |
| Ion charge | 3+ 1- |
| Balance the number of ions | 3 fluoride ions are needed for each aluminium ion |
| Ratio of ions | 1:3 |
| Formula | AlF3 |
Answer 3
| Symbol | Al O |
| Ion charge | 3+ 2- |
| Balance the number of ions | 2 aluminium ions are needed for 3 oxide ions |
| Ratio of ions | 2:3 |
| Formula | Al2O3 |
Answer 4
| Symbol | Mg2+ NO3- |
| Ion charge | 2+ 1- |
| Balance the number of ions | 2 nitrate ions are needed for each magnesium ion |
| Ratio of ions | 1:2 |
| Formula | Mg(NO3)2 |
Answer 5
| Symbol | NH4+ SO42- |
| Ion charge | 1+ 2- |
| Balance the number of ions | 2 ammonium ions are needed for each sulfate ion |
| Ratio of ions | 2:1 |
| Formula | (NH4)2SO4 |
