AS Starters and Revision

Read our general notes on Risk Assessment

Chemical formulae: the formulae of ionic compounds

What is a formula
A formula conveys two pieces of information; it gives the identity of the elements present, and it also tells us the ratio in which the atoms or ions are combined together. For example, the formula for glucose, C₆H₁₂O₆, tells us that there are three elements present in the compound: carbon, hydrogen and oxygen, and that in one molecule of glucose there are 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms all linked together.
 
It is convenient to deal with the formulae of ionic and covalent compounds separately. This tutorial tells you about ionic compounds.

You can read the tutorial on screen or download from this link a file with the whole of a series of tutorials about formulae and equations.
 
Metal ions
Atoms of metals lose electrons in order to form ions, and you should recall that this often results in the formation of a noble gas structure. This is sometimes called the ‘octet’ rule.
 
From the position of many metals in the Periodic Table it is possible to deduce how many outermost electrons are lost when they form positive ions. Thus the formulae of some simple metal ions are:
 
Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, A1³⁺
 
Look at an outline Periodic Table to see how this pattern is related to the columns (groups) in the Table.
 
The transition metals also form positive ions, but the rules which govern just how many electrons are lost from their atoms are more complicated. Transition metals usually form a variety of different positive ions, for example:
 
Fe²⁺, Fe³⁺, Cu⁺, Cu²⁺
 
Non-metal ions
To balance the loss of electrons by metals, the non-metals on the right-hand side of the table gain electrons to achieve noble gas structures. From their position in the Periodic Table it is again possible to deduce how many electrons are gained when they form negative ions, for example:
 
F^–, Cl^–, Br^–, O^2–, S^2–, N^3–
 
Ionic formulae
For the electric charges to be balanced in an ionic compound, positive and negative ions are present in compounds in a ratio which gives the compound overall neutrality, and this ratio gives the formula.
 
For example, with sodium oxide, containing Na⁺ ions and O^2– ions, there must be twice as many Na⁺ ions as there are O^2– so the formula can be written Na⁺₂O^2– or, more simply, as Na₂O, with the charges omitted. Notice that the ₂ referring to the Na⁺ ions, follows the symbol, and does not precede it.
 
With magnesium nitride, containing Mg²⁺ ions and N^3– ions, the ions will combine in a 3 to 2 ratio, so the formula can be written (Mg²⁺)₃ (N^3–)₂, or more simply as Mg₃N₂ (once again, with the charges omitted). Notice as before that the 3 and the 2 follow the symbols of the elements to which these numbers refer.
 
When an element can form more than one type of ion, Roman numerals in brackets after the name of the element give us the number of charges on the ion, for example:
 

the compound containing Fe³⁺ ions and Cl^– ions is called iron(III) chloride, and the formula is FeCl₃.
 
These two iron chlorides, although they both contain the same elements, are different compounds, and therefore must be given their full names; the name iron chloride is ambiguous and should not be used.

---

Rate this page or react
Share your views on this page, 23 ratings so far 4 , rated at

---

updated: 21 October 2003