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.
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.
Edexcel GCSE Chemistry Notes
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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.
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