Writing chemical formulae is a systematic way of representing the composition of a chemical substance using symbols and numbers. It provides essential information about the types and numbers of atoms in a molecule. Understanding how to write chemical formulae is fundamental in chemistry, as it helps communicate the composition of substances clearly and concisely.

1. Elements and Symbols:

Each chemical element is represented by a unique one- or two-letter symbol, which is usually derived from its English or Latin name. For example:

• Hydrogen is represented as $\text{H}$
• Oxygen is represented as $\text{O}$
• Carbon is represented as $\text{C}$
• Sodium is represented as $\text{Na}$ (from the Latin name "Natrium")

2. Writing Chemical Formulae:

The chemical formula of a compound is written by combining the symbols of the constituent elements along with subscripts that indicate the number of atoms of each element present in the molecule. Here are some guidelines for writing chemical formulae:

• Identify the elements present in the compound.
• Determine the valency (combining capacity) of each element.
• Use the criss-cross method to balance the charges when combining elements with different valencies.
• If the compound contains polyatomic ions, enclose the ion in parentheses and indicate the number of ions with a subscript.

3. Examples of Writing Chemical Formulae:

Here are some examples to illustrate the process of writing chemical formulae:

• Water:
  • Elements: Hydrogen (H) and Oxygen (O)
  • Valency: H has a valency of +1, O has a valency of -2.
  • Formula: Since there are 2 H atoms needed to balance one O atom, the formula is $\text{H}_2\text{O}$.
• Carbon Dioxide:
  • Elements: Carbon (C) and Oxygen (O)
  • Valency: C has a valency of +4, O has a valency of -2.
  • Formula: To balance the charges, we need two O atoms for each C atom, resulting in the formula $\text{CO}_2$.
• Magnesium Chloride:
  • Elements: Magnesium (Mg) and Chlorine (Cl)
  • Valency: Mg has a valency of +2, Cl has a valency of -1.
  • Formula: Using the criss-cross method, we get $\text{MgCl}_2$ because two Cl atoms are needed to balance one Mg atom.
• Ammonium Sulfate:
  • Elements: Ammonium ($\text{NH}_4^+$) and Sulfate ($\text{SO}_4^{2-}$)
  • Formula: The ammonium ion has a charge of +1 and sulfate has a charge of -2. Therefore, to balance the charges, the formula is $\text{(NH}_4\text{)}_2\text{SO}_4$.

4. Polyatomic Ions:

Polyatomic ions are ions that consist of more than one atom. When writing the formula for compounds containing polyatomic ions, it is essential to remember to use parentheses if there is more than one of that ion in the compound. For example, in sodium sulfate, the formula is $\text{Na}_2\text{SO}_4$, where sulfate is a polyatomic ion.

5. Importance of Chemical Formulae:

Chemical formulae are essential for various reasons:

• They provide a clear representation of the molecular composition of substances.
• They are crucial for performing stoichiometric calculations in chemical reactions.
• They help in understanding the properties and behaviors of substances based on their molecular structure.

By mastering the skill of writing chemical formulae, students can better understand and communicate the complexities of chemical substances and their reactions.