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Atoms and Molecules

Dalton's Atomic Theory and Laws of Chemical Combination-Group A

Molecular Formula: A Brief Overview

Just like each atom has a unique symbol, each compound has a unique molecular formula.

The molecular formula of a compound provides information about the names and numbers of atoms of the different elements present in a molecule of that compound.

Molecular formula is a [[mn:glossary]]chemical formula[[/mn:glossary]] that indicates the kinds of atoms and the numbers of each kind of atom in a molecule of a compound.


  • The molecular formula of glucose is C6H12O6. One molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen and 6 atoms of oxygen.
  • The molecular formula of water is H2O. One molecule of water contains 2 atoms of hydrogen and 1 atom of oxygen.

Salient features of chemical formula:

  • Compounds are formed when two or more elements combine chemically. Hence, compounds can also be represented using symbols.
  • The notation used for representing any compound is called chemical formula of that compound.
  • Each compound has a unique chemical formula.
  • The chemical formula of any compound tells us about : The different elements which combine to form the compound and the number of atoms of each element present in a molecule of the compound
  • For example, H2O is the chemical formula of water. This denotes that there are two atoms of hydrogen and one atom of oxygen present in one molecule of water.


Chemical Formulae

Let us understand the information derived from chemical formulae by taking the example of carbon dioxide. The chemical formula of carbon dioxide is CO2. Using this formula, we can derive the following information about carbon dioxide.

  • Two elements are present in carbon dioxide: carbon(C) and oxygen (O).
  • CO2 represents one molecule of carbon dioxide.
  • Since one atom of carbon combines with two atoms of oxygen, the [[mn:glossary]]valency[[/mn:glossary]] of carbon is twice that of oxygen.
  • CO2 is a neutral molecule. It has no charge.
  • The relative atomic masses of carbon and oxygen are 12 u and 16 u respectively. So, the ratio by mass between carbon and oxygen is 12 : 32, i.e., 3 : 8.

Writing Chemical Formulae

To write the chemical formula of a compound, one should have prior knowledge of two things.

  • The symbols of the constituent elements.
  • The [[mn:glossary]]combining capacity[[/mn:glossary]] of the atom of each element constituting the compound.

The number of atoms of other elements with which one atom of an element combines is decided by the valency of that element.

For example, both hydrogen (H) and chlorine (Cl) have a valency of 1. Therefore, one atom of hydrogen reac...

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