Deducing Formulae Using Common Ions (OCR Gateway GCSE Chemistry: Combined Science)

• 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. O^2-
• Elements in group 5 gain 3 electrons so have a 3- charge, eg. N^3-
• 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

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