Question

What are the rules for writing the formulae of compounds?

Answer 1

Chemical formula -

• A chemical formula is a representation of a substance's molecular composition in terms of the symbols for the elements that make up the molecule.
• Water is a substance with two Hydrogen atoms and one Oxygen atom in each molecule. ${\mathrm{H}}_2\mathrm{O}$ is the formula for water.

Rules for writing the formulae of compounds are as follows -

Step 1- Writing the chemical symbols of the elements or the ions that form the compound

Copper - $\mathrm{Cu}$

Oxygen - $\mathrm{O}$

Step 2- Writing the valency of each element or the charge of each ion below its symbol.

$$\begin{gathered} \mathrm{Cu}\mathrm{O} \\ +2-2 \end{gathered}$$

Step 3- Now interchanging the valencies or charges of the combining atoms or ions.

$$\begin{gathered} \mathrm{Cu}\mathrm{O} \\ \xcancel{} \\ +2-2 \end{gathered}$$

Step 4- Excluding the positive and negative signs, exchange numbers and write them as subscripts.

$\mathrm{CuO}$