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Just as words convey meaning in language, chemical formulae are the concise symbolic representations of the composition of compounds. They tell us which elements are present and the number of atoms of each combining element. Learning to write chemical formulae is like learning the grammar of chemistry, making it easy to understand and communicate about different substances.

The key to writing chemical formulae lies in understanding valency. The combining power (or capacity) of an element is known as its valency. You can think of valency as the "arms" or "hands" an atom has to bond with other atoms. For instance, if a human has two arms and an octopus has eight, and an octopus wants to hold as many humans as possible so all arms are locked, the octopus (O) would hold four humans (H), forming OH₄. The subscript '4' indicates the number of humans held.

Rules for Writing Chemical Formulae:

1. Balancing Valencies/Charges: The valencies or charges on the ions in a compound must always balance each other out, ensuring the overall compound is electrically neutral. The charges on the ions are typically not indicated in the final formula.
2. Metal First, Non-metal Second (for Metal-Non-metal Compounds):
   When a compound consists of a metal and a non-metal, the name or symbol of the metal is always written first, followed by the non-metal.
   • Example: Calcium oxide (CaO), Sodium chloride (NaCl), Iron sulphide (FeS), Copper oxide (CuO). Notice oxygen, chlorine, and sulphur (non-metals) are on the right, while calcium, sodium, iron, and copper (metals) are on the left.
3. Use of Brackets for Polyatomic Ions:
   In compounds formed with polyatomic ions (groups of atoms carrying a charge), if there is more than one polyatomic ion present in the compound, its formula is enclosed in a bracket, and the number of ions is written as a subscript outside the bracket.
   • Example: Magnesium hydroxide, Mg(OH)₂. The subscript '2' outside the bracket indicates two hydroxide (OH^-) groups.
   • If the number of the polyatomic ion is one, no bracket is required. Example: Sodium hydroxide (NaOH).

Writing Formulae for Simple Compounds (Binary Compounds):

The simplest compounds, made up of two different elements, are called binary compounds. We use the "criss-cross" method with valencies:

1. Write the symbols of the constituent elements.
2. Write their valencies (or charges for ions) below their symbols.
3. Criss-cross the valencies (swap the numerical values) of the combining atoms.

Examples:

• Hydrogen Chloride:
  • Symbol: H Cl
  • Valency: 1 1
  • Formula: HCl (valencies criss-cross to 1:1, so no subscripts needed)
• Hydrogen Sulphide:
  • Symbol: H S
  • Valency: 1 2
  • Formula: H₂S (Hydrogen gets '2' from Sulphur, Sulphur gets '1' from Hydrogen)
• Magnesium Chloride:
  • Symbol: Mg Cl
  • Charge: 2+ 1-
  • Formula: MgCl₂ (Magnesium's 2+ charge goes to Chlorine as subscript 2, Chlorine's 1- goes to Magnesium as subscript 1)
• Aluminium Oxide:
  • Symbol: Al O
  • Charge: 3+ 2-
  • Formula: Al₂O₃
• Calcium Oxide:
  • Symbol: Ca O
  • Charge: 2+ 2-
  • Formula: CaO (The valencies are both 2, so they simplify to a 1:1 ratio. We write CaO, not Ca₂O₂)
• Sodium Nitrate:
  • Symbol/Formula: Na NO₃
  • Charge: 1+ 1-
  • Formula: NaNO₃ (No brackets needed as there's only one polyatomic ion)
• Calcium Hydroxide:
  • Symbol/Formula: Ca OH
  • Charge: 2+ 1-
  • Formula: Ca(OH)₂ (Brackets are essential here as there are two hydroxide groups)
• Sodium Carbonate:
  • Symbol/Formula: Na CO₃
  • Charge: 1+ 2-
  • Formula: Na₂CO₃
• Ammonium Sulphate:
  • Symbol/Formula: NH₄ SO₄
  • Charge: 1+ 2-
  • Formula: (NH₄)₂SO₄ (Both are polyatomic, so brackets are used for NH₄⁺ as there are two of them)

Writing chemical formulae is a foundational skill in chemistry, enabling us to represent the composition of millions of known compounds in a standardized and universally understood way.

Question for You: How many atoms are present in an H₂S molecule and in a PO₄³⁻ ion?