Question

Write down the chemical formulae of the following compounds.
A.Sodium Oxide
B.Aluminium Chloride
C.Sodium Sulphide
D.Magnesium Hydroxide.

Answer 1

Hint: All these compounds are ionic in nature, and all ionic compounds are neutral in charge thereby neutralising the charges of the Cation and the Anion.

Step-By-Step answer:
Let us first establish how the chemical formula of an ionic compound with elements C and A is written to ensure that the resultant compound is charge neutral and chemically stable.
For an ionic compound having Cation \[{{C}^{x+}}\]and Anion \[{{A}^{y-}}\], the chemical formula to ensure charge neutrality and stability of compound is :
              ${{C}_{y}}{{A}_{x}}$
Since the positive charge (+xy) is the same as the negative charge (-xy) rendering the compound neutral in charge.
Using this idea, let us now write down the charges on all the ions required for us to write down the formulae of these compounds:
$\begin{align}
  & Sodium\text{ ion}-N{{a}^{+}} \\
 & Oxide\text{ ion}-{{O}^{2-}} \\
 & Al\text{ ion}-A{{l}^{3+}} \\
 & Chloride\text{ ion}-C{{l}^{-}} \\
 & Sulphide\text{ ion}-{{S}^{2-}} \\
 & Magnesium\text{ ion}-M{{g}^{2+}} \\
 & Hydroxide\text{ ion}-O{{H}^{-}} \\
\end{align}$
Now using these ions, the following would be the chemical formula for the given compounds after the balancing of charges:
$\begin{align}
  & \text{Sodium Oxide : N}{{\text{a}}_{\text{2}}}\text{O} \\
 & \text{Aluminum Chloride : AlC}{{\text{l}}_{\text{3}}} \\
 & \text{Sodium Sulphide : N}{{\text{a}}_{\text{2}}}\text{S} \\
 & \text{Magnesium Hydroxide : Mg(OH}{{\text{)}}_{\text{2}}} \\
\end{align}$

Note:
Please ensure that all chemical formulae you write are ionically balanced, to make sure of doing so you must be very thorough with your knowledge of valences and charges of different ions. Another point of consideration is that for ions like the Hydroxide ion, always use parentheses when you need to express a multiple number of ions.