What is Chemical Formula of a Compound?

Just as a symbol represents one atom of an element, a chemical formulate represents the number of atoms of each element in one molecule of a compound. Atomic symbols help in writing chemical formulate of compounds. Knowing the different elements present in the molecule and the number of atoms of each element in it, the chemical formulate or molecular formula of a compound can be written. The atoms are present in a molecule of a compound in whole numbers.

Chemical compound

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Hydrogen chloride consists of one atom of hydrogen (H) and one atom of chlorine (CI). A molecule of hydrogen chloride (hydrochloric acid) is represented by the formula H1CI1 or simply HCI.

Remember: The three steps to be followed while writing the chemical formula of a compound:

  • First write down the symbols for the various elements of which the compound is composed.
  • Write the number of atoms as a subscript written after the symbol of each. If only one atom of an element is present, no number is written below the symbol. This is because the symbol itself represents one atom.
  • Write the appropriate number before its molecular formula to represent a certain number of molecules of the compound. 3HCL represents three molecules of hydrogen chloride. It contains 3 atoms of hydrogen and 2 atoms of chlorine. Similarly 3O2 means 3 molecules of oxygen, contains 6 atoms of oxygen. 2C means two atoms of carbon. Similarly H2O is the molecular formulate for one molecule of water which contains 2 atoms of hydrogen and one atom of oxygen.


The capital and small letters of chemical symbols should be written very carefully. For example, the compound carbon monoxide is represented by CO, whereas element cobalt is written as Co.

Important of Chemical Formula

Consider the compound aluminum chloride One molecules of this compound contains one atom of aluminum and three atoms of chlorine. Hence the chemical formulate of aluminum chloride is ALCL3.

The formulate ALCI3 shows:

  • One molecule of aluminum chloride.
  • The elements from which it is made.
  • The proportion of aluminum and chlorine atoms in one molecule of aluminum chloride.

The mass of one molecule of aluminum chloride.

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