Ions in Compounds

Specification Point 1.24:
  • Explain the formation of ions in ionic compounds from their atoms, limited to compounds of elements in groups 1, 2, 6 and 7.
  • Elements in Groups 1, 2, 6 and 7 readily form ions to achieve the noble gas structure.
  • Group 1 and 2 elements are metals and so form positive ions by electron loss.
  • Group 6 and 7 elements are nonmetals and so form negative ions by electron gain.
  • Elements in the same Group have the same number of outer shell electrons, therefore they lose or gain the same number of electrons.
  • This means that elements in the same Group form ions with the same charge.
  • Group 1 elements lose 1 electron so they all form ions with a 1+ charge.
  • Group 2 form ions with a 2+ charge, Group 6 form ions with a 2- and Group 7 a 1- charge.

Ions in Compounds, Edexcel GCSE Chemistry

Exception 1: Carbon and silicon in Group 4 tend to form covalent bonds by sharing electrons.

Exception 2: The elements in Group 0 do not react with other elements to form ions as they already have a complete outer shell.

Naming Ionic Compounds

Specification Point 1.25:
  • Explain the use of endings –ide and –ate in the names of compounds.
  • The ending -ide means the compound contains two elements where one is a metal and the other a nonmetal.
  • The name of the metal atom comes first and the ending of the second atom is replaced by adding –ide.
    • E.g. NaCl which contains sodium and chlorine thus becomes sodium chloride.
  • If both atoms are nonmetals and one of those is hydrogen, then hydrogen comes first.
    • E.g. hydrogen and chlorine combined is called hydrogen chloride.
  • For other combinations of nonmetals as a general rule, the element that has a lower Group number comes first in the name.
    • E.g. carbon and oxygen combine to form CO2 which is carbon dioxide since carbon is in Group 4 and oxygen in Group 6.
  • The ending -ate means the compound contains a minimum of three elements where one of them is oxygen.
  • Some common ions which form part of these compounds include the carbonate ion (CO32-), sulfate ion (SO42-) and the nitrate ion (NO3).
  • When these ions form a compound with a metal atom, the name of the metal comes first.
  • E.g. KNO3 is potassium nitrate, CaCO3 is calcium carbonate.


Specification Point 1.26:
  • Deduce the formulae of ionic compounds (including oxides, hydroxides, halides, nitrates, carbonates and sulfates) given the formulae of the constituent ions.
  • If we know the charges on the constituent ions we can work out the formula of an ionic compound.
  • Consider a compound consisting of a Group 3 element combined with a Group 7 element.
  • There will need to be three of the Group 7 ions (X) to balance the three positive charges on the Group 3 ion (M3+) so the formula will therefore be MX3 e.g. AlBr3.
  • Remember that even though ionic compounds contain ions which are charged particles, the overall charge of the compound is neutral.
  • This is because  the +ve and -ve charges cancel each other out.
  • The table below shows the formulae and charge for some of the more common ions.

  • Example: what is the formula of aluminium sulfate?
    • Write out the formulae of each ion, including their charges.
    • Al3+       SO42-
  • Balance the charges by multiplying them by the lowest whole numbers, which when added together gives a total net charge of 0.
  • Al3+ x 2 = +6 and SO42- x 3 = -6; so +6 – 6 = 0.
  • So the formula is Al2(SO4)3.

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Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.