Chemical Formula

• The chemical formula of a substance is a representation of the substance using symbols.
• Two atoms of hydrogen combine together to form a molecule of hydrogen. A molecule of hydrogen is represented by H₂.
• One atom of oxygen combines with two atoms of hydrogen to form one molecule of water. A molecule of water is represented by the chemical formula H₂O.

Combining Power (Valency)

• The ability of atoms of elements to combine with hydrogen is referred to as their combining power or valency.
• One atom of oxygen can combine with two atoms of hydrogen, H₂O. Oxygen here is said to have the combining power of two.
• One atom of nitrogen can combine with three atoms of hydrogen, NH₃. Nitrogen here is said to have the combining power of three.
• Chemical formula, like chemical symbols, are written in a particular way to obey certain rules.
• In a compound, where we have more than one type of atom chemically combined, the formula of such a substance is again written to express the type and number of atoms present.
• Chemical formulas always have both letters and numbers; a combination of letters and numbers representing symbols and the number of atoms in the molecule that are used. For example:
  • CH₄ (Methane) – This contains one carbon atom (C) combined with four atoms of hydrogen (H).
  • C₄H₁₀ (Butane) – This contains four atoms of carbon combined with ten atoms of hydrogen.

Chemical Equations

• A chemical equation is used to describe the initial substance and end products in a chemical reaction.

Rules to be Followed in Writing and Balancing a Chemical Equation

• The reactants and products must be known.
• Write a word equation.
• Write the formula of all the reactants and all the products.
• Check the number of each type of atom of the reactant(s) and the product(s).
• Balance the equation by supplying coefficients that will make the number of atoms of each element, in the reactants and products, the same. For example:
  • H₂ + O₂ → H₂O
    2H₂ + O₂ → 2H₂O
  • N₂ + H₂ → NH₃
    N₂ + 3H₂ → 2NH₃
  • KClO₃ → KCl + O₂
    2KClO₃ → 2KCl + 3O₂