Eduqas A-Level Chemistry

## Formulae of ions and ionic compounds An ionic compound is electrically neutral, so its formula has the smallest whole-number ratio of ions whose charges cancel. The charge of the common ions has to be known, and the ones below cover most of the course. | Positive ions | Formula | Negative ions | Formula | |---|---|---|---| | Hydrogen | $\ce{H+}$ | Hydroxide | $\ce{OH-}$ | | Ammonium | $\ce{NH4+}$ | Nitrate | $\ce{NO3-}$ | | Silver | $\ce{Ag+}$ | Hydrogencarbonate | $\ce{HCO3-}$ | | Zinc | $\ce{Zn^2+}$ | Carbonate | $\ce{CO3^2-}$ | | Copper(II) | $\ce{Cu^2+}$ | Sulfate | $\ce{SO4^2-}$ | | Lead(II) | $\ce{Pb^2+}$ | Sulfite | $\ce{SO3^2-}$ | | Iron(II) and iron(III) | $\ce{Fe^2+}$, $\ce{Fe^3+}$ | Phosphate | $\ce{PO4^3-}$ | | Aluminium | $\ce{Al^3+}$ | Manganate(VII) | $\ce{MnO4-}$ | | Chromium(III) | $\ce{Cr^3+}$ | Dichromate(VI) | $\ce{Cr2O7^2-}$ | Group 1 metals form $+1$ ions, Group 2 metals form $+2$ ions, aluminium forms $+3$, the halogens form $-1$ ions and oxygen and sulfur form $-2$ ions. A roman numeral in a name gives the oxidation number of the metal, so iron(III) chloride contains $\ce{Fe^3+}$ and is $\ce{FeCl3}$. To write a formula, find the lowest common multiple of the two charges. Aluminium sulfate combines $\ce{Al^3+}$ with $\ce{SO4^2-}$; the lowest common multiple of 3 and 2 is 6, so two aluminium ions balance three sulfate ions and the formula is $\ce{Al2(SO4)3}$. Brackets go round a polyatomic ion when more than one is needed, as in $\ce{Ca(OH)2}$ and…