CCEA A-Level Chemistry

## Writing formulae of ionic compounds The charge on a simple ion can be predicted from the position of its element in the Periodic Table. Metals in Groups I, II and III (13) lose one, two and three electrons to form $\ce{Na+}$, $\ce{Mg^2+}$ and $\ce{Al^3+}$, and non-metals in Groups V, VI and VII gain three, two and one electrons to form $\ce{N^3-}$, $\ce{O^2-}$ and $\ce{Cl-}$. Each of these ions has the electron arrangement of a noble gas. An ionic compound is electrically neutral, so the formula is the smallest whole-number ratio of ions in which the charges cancel: magnesium and chlorine give $\ce{MgCl2}$, and aluminium and oxygen give $\ce{Al2O3}$, because two $\ce{Al^3+}$ (+6) balance three $\ce{O^2-}$ (−6). Where an ion contains more than one atom, its formula goes in brackets when more than one is needed, as in $\ce{Ca(NO3)2}$ and $\ce{(NH4)2SO4}$. The molecular ions to know, with their charges, are in the table. | Name | Formula | Name | Formula | |---|---|---|---| | sulfate | $\ce{SO4^2-}$ | hydroxide | $\ce{OH-}$ | | sulfite | $\ce{SO3^2-}$ | dichromate | $\ce{Cr2O7^2-}$ | | thiosulfate | $\ce{S2O3^2-}$ | chromate | $\ce{CrO4^2-}$ | | hydrogensulfate | $\ce{HSO4-}$ | permanganate (manganate(VII)) | $\ce{MnO4-}$ | | hydrogencarbonate | $\ce{HCO3-}$ | ammonium | $\ce{NH4+}$ | | carbonate | $\ce{CO3^2-}$ | chlorate | $\ce{ClO3-}$ | | nitrate | $\ce{NO3-}$ | hypochlorite | $\ce{ClO-}$ | | nitrite | $\ce{NO2-}$ | phosphate | $\ce{PO4^3-}$ | ## Balanced equations A…