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Discovering chemistry
Discovering chemistry

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3.3 Valency and predicting formulas in ionic compounds

In ionic compounds, valency gives an indication of the charge an ion formed from a particular element will carry.

Take for example, magnesium, Mg, this has a valency of two and tends to form ions carrying two positive charges, Mg2+. Note that the valency does not indicate whether an ion will carry a positive or negative charge: oxygen, O, also has a valency of two, and forms negative ions, O2.

However, you can predict whether an atom will form negative or positive ions from its position in the Periodic Table: elements to the left (metals) will tend to form positive ions; those on the right (non-metals) will tend to form negative ions. And recall when oppositely charged ions combine to form compounds, there must be the same number of positive and negative charges, so that the compound carries no overall charge.

Thus, when Mg2+and O2combine to form the ionic compound magnesium oxide, there have to be equal numbers of magnesium cations and oxide anions.

Magnesium oxide has the formula MgO.

This reveals another corner chemists cut when writing formulas. Note that charges do not appear in the chemical formulas of compounds: you don’t write Mg2+O2).

Similarly, when magnesium cations, Mg2+, combine with chloride anions, Cl , there must be twice as many chloride anions as magnesium cations in order for the charges to balance each other. The chemical formula of magnesium chloride is MgCl2.

  • Using Table 2 predict the ions that will be formed from the following atoms: K, Ca, Al, S, F and Br.

  • K+, Ca2+, Al3+, S2-, F- and Br-
  • Give the formulas of the compounds that will be formed when calcium forms an ionic compound with fluorine..

  • Calcium has a valency of 2, so you are dealing with Ca2+ cations. Fluorine has a valency of 1, so forms F- anions, thus an electrically neutral ionic compound will be CaF2.

  • In the case of oxygen you have O2- anions, what is the formula of the oxide it will form with aluminium?

  • This is a bit more involved. Remember you need to form an electrically neutral compound.So given you have Al3+ ions, you have to multiply Al by two and O by three. The formula will be Al2O3.

Now here’s another (slight) complication.

Ionic compounds can also contain molecules which bear charges, and these units are charged either positively or negatively. These are often referred to as ionic groups. One example is the nitrate group (NO3-) which you have met already.

  • In a single sentence describe the structure of the nitrate anion.

  • A molecule carrying a single negative charge, consisting of three oxygen atoms covalently bound to a nitrogen atom.

Table 3 gives the formulas and charges on some other ionic groups.

Table 3 Some common ionic groups
Charge Name and formula
+1 ammonium NH4+

hydroxide, OH-

nitrate, NO3-


sulfate, SO42-

carbonate, CO32-

  • Give the formulas of the ionic compounds that comprise the following pairs of ions:

    • i.sodium and hydroxide
    • ii.potassium and nitrate
    • iii.ammonium and nitrate
    • i.NaOH
    • ii.KNO3, note here, as there is only one nitrate group, the brackets around the NO3 unit tend to be left out.
    • iii.NH4NO3, as above, this could reasonably be written (NH4)(NO3), but again as these ionic groups combine in a 1:1 ratio the brackets are omitted.

So far you have seen how valency may be used to rationalise the formulas of compounds, but as was alluded to above to really get to grips with how (and why) compounds form you need to consider the electrons on atoms that get involved in bonding. This is the focus of the next session.