Chemical Formulae of Ionic and Covalent Compounds

Covalent Compounds

Covalent compounds are formed when non-metal atoms share electrons. For the ESAT, you must learn the following formulae for common covalent compounds:

1. Ammonia: $NH_{3}$
2. Carbon dioxide: $CO_{2}$
3. Carbon monoxide: $CO$
4. Methane: $CH_{4}$
5. Nitrogen dioxide: $NO_{2}$
6. Nitrogen monoxide: $NO$
7. Sulfur dioxide: $SO_{2}$
8. Sulfur trioxide: $SO_{3}$
9. Water: $H_{2}O$

Ionic Compounds

Ionic compounds consist of a giant lattice of positive and negative ions. The formula of an ionic compound is determined by the charges of its constituent ions. The total number of positive charges must always equal the total number of negative charges to ensure the compound is electrically neutral.

Common Positive Ions

The charge of a metal ion is often related to its Group in the Periodic Table:

• Group 1 ions: $Li^{+}$, $Na^{+}$, $K^{+}$
• Group 2 ions: $Mg^{2+}$, $Ca^{2+}$, $Ba^{2+}$
• Group 13 ions: $Al^{3+}$
• Other positive ions: $NH_{4}^{+}$ (ammonium), $Cu^{2+}$ (copper(II)), $H^{+}$ (hydrogen), $Fe^{2+}$ (iron(II)), $Fe^{3+}$ (iron(III)), $Ag^{+}$ (silver), $Zn^{2+}$ (zinc).

Common Negative Ions

Similarly, non-metal ion charges can be predicted by their Group:

• Group 16 ions: $O^{2-}$ (oxide), $S^{2-}$ (sulfide)
• Group 17 ions: $F^{-}$ (fluoride), $Cl^{-}$ (chloride), $Br^{-}$ (bromide), $I^{-}$ (iodide)
• Other negative ions: $CO_{3}^{2-}$ (carbonate), $OH^{-}$ (hydroxide), $NO_{3}^{-}$ (nitrate), $SO_{4}^{2-}$ (sulfate).

Determining Ionic Formulae: Worked Examples

To write a formula, find the ratio of ions that makes the total charge zero.

Example 1: Sodium oxide Sodium oxide contains $Na^{+}$ and $O^{2-}$ ions. To balance the $2-$ charge of the oxide ion, you need two $Na^{+}$ ions (totaling $2+$). The formula is $Na_{2}O$.

Example 2: Aluminium oxide Aluminium oxide contains $Al^{3+}$ and $O^{2-}$ ions. To balance the charges, the lowest common multiple of 3 and 2 is 6. You need two $Al^{3+}$ ions (total $6+$) and three $O^{2-}$ ions (total $6-$). The formula is $Al_{2}O_{3}$.

Example 3: Ammonium bromide Ammonium bromide contains $NH_{4}^{+}$ and $Br^{-}$ ions. One of each balances the charges perfectly. The formula is $NH_{4}Br$.

Example 4: Calcium hydroxide Calcium hydroxide contains $Ca^{2+}$ and $OH^{-}$ ions. To balance the $2+$ charge of calcium, two $OH^{-}$ ions are required. When more than one compound ion is present, we use brackets. The formula is $Ca(OH)_{2}$.

Example 5: Iron(III) nitrate Iron(III) nitrate contains $Fe^{3+}$ and $NO_{3}^{-}$ ions. Three nitrate ions are needed to balance one iron(III) ion. The formula is $Fe(NO_{3})_{3}$.

Common Acids and Covalent Elements

You must be familiar with the formulae of these common acids:

• Hydrochloric acid: $HCl$
• Nitric acid: $HNO_{3}$
• Sulfuric acid: $H_{2}SO_{4}$
• Ethanoic acid: $CH_{3}COOH$

Additionally, many non-metal elements exist as diatomic molecules or unique structures. These include:

• Bromine: $Br_{2}$
• Buckminsterfullerene: $C_{60}$
• Chlorine: $Cl_{2}$
• Fluorine: $F_{2}$
• Hydrogen: $H_{2}$
• Iodine: $I_{2}$
• Nitrogen: $N_{2}$
• Oxygen: $O_{2}$

Summary of Formula Correctness

Applying these rules helps identify incorrect formulae. For instance, magnesium oxide must be $MgO$, not $MgO_{2}$, because $Mg^{2+}$ and $O^{2-}$ balance in a 1 to 1 ratio. Similarly, sodium carbonate must be $Na_{2}CO_{3}$ rather than $NaCO_{3}$ because two $Na^{+}$ ions are required to balance one $CO_{3}^{2-}$ ion.