Atoms and Molecules

Tutormate > CBSE Syllabus-Class 9th Chemistry > Atoms and Molecules

05 Molecules

  • A molecule is the smallest particle of a substance that is an element or a compound, and possesses properties of that particular substance.
  • They are electrically neutral group of two or more atoms chemically bonded together.
  • A molecule can exist in Free State and are either made of elements or compounds.


  • It is the amount of mass associated with a molecule, also called as molecular weight.
  • Molecular mass of elements depends upon the constituent atoms of the molecule.
  • It can be calculated by adding the mass of each atom multiplied by the number of atoms of the element present in the molecule.
  • Consider water which is made up of 2 hydrogen atoms and 1 oxygen atom.
  • Mass of water molecule is equal to the average atomic mass of hydrogen multiplied by two plus the atomic mass of oxygen.
  • The molecular weight of molecules can be determined by following methods:
    • Mass spectrometry: This method is generally used in determining the mass of small molecules. This is reported as monoisotropic mass.
    • Hydrodynamic method: The weight is determined as per Mark-Houwink relations. This method requires calibration hence it is also described as relative molecular weight determination method.
    • Static Light Scattering: Molecular weight is determined from the amount of light scattered using the Zimm method.


  • Two or more similar atoms chemically combine together to form molecule of an element.
  • For example: Hydrogen gas consists of H2 molecules and not of single H atoms
  • Atomicity: The number of atoms present in one molecule of an element is called its atomicity.
    • Noble gases (helium, neon, argon, krypton, etc.) have one atom each in their molecules such as He, Ne, Ar and Kr. And hence, the atomicity of noble gases is 1.
    • The atomicity of metal elements like sodium (Na), magnesium (Mg), aluminium (Al), copper (Cu) and iron (Fe), etc., is also taken to be 1.
    • Hydrogen (H2), nitrogen (Nsub>2), all have 2 atoms each in their molecules. So, the automaticity of hydrogen, nitrogen, oxygen, chlorine, bromine and iodine is 2 each.
    • Ozone (O3) has 3 atoms in its molecule, so the atomicity of ozone is 3.
    • Phosphorus (P4) has 4 atoms in its molecule, so the atomicity of phosphorus is 4.
    • Solid sulphur (S8) has 8 atoms in its molecule, so the atomicity of sulphur is 8.


  • Two or more different types of atoms, combine together chemically to form a molecule of a compound.
  • For example, the molecule of hydrogen chloride (HCl) contains two different types of atoms i.e. hydrogen atom (H) and chlorine atom (Cl)


A chemical formula represents the composition of a molecule of the substance in terms of the symbols of the elements present in the molecule.

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