Subsection 8.3.2 Molecular Formula
Molecular formula can be written by knowing the valency of the elements. The valency of an element is the number of electrons it can donate, accept, or share during a chemical reaction. First identify the elements present in the compound then Determine the valency of each element based on its position in the periodic table or by consulting a table of valencies. Determine the number of atoms of each element required to balance the valencies. This can be done by finding the least common multiple of the valencies for each element. Write the molecular formula by indicating the number of atoms for each element as subscripts next to the element symbol. Make sure to reduce the subscripts to the lowest whole number ratio. For example, let’s determine the molecular formula for magnesium chloride. Magnesium has a valency of +2 and chlorine has a valency of -1.
\begin{equation*}
Mg^{+2} \quad Cl^{-1}
\end{equation*}
Now exchange the valency and place them to the bottom of opposite elements by dropping their signs as shown in below
\begin{equation*}
Mg_{1} \quad Cl_{2}
\end{equation*}
Hence, the molecular formula of magnesium chloride is
\begin{equation*}
MgCl_{2}
\end{equation*}
Another example is the molecular formula for calcium oxide. Calcium has a valency of +2 and oxygen has a valency of -2.
\begin{equation*}
Ca^{+2} \quad O^{-2}
\end{equation*}
\begin{equation*}
Ca_{2} \quad O_{2}
\end{equation*}
Hence, calcium oxide is written as
\begin{equation*}
CaO
\end{equation*}
