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3.1.2 Deducing Formulae Using Common Ions

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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

Atoms, Molecules & Stoichiometry Ionic Charges, downloadable AS & A Level Chemistry revision notes

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

Formulae of Polyatomic Ions Table, downloadable IB Chemistry revision notes

  • 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?

  1. sodium bromide
  2. aluminium fluoride
  3. aluminium oxide
  4. magnesium nitrate
  5. 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

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