Chemistry (NTSE/Olympiad)  

3. Atoms & Molecules


The number of atoms present in one molecule of an element is called its atomicity.
Example. Noble gases (helium, neon, argon, krypton, etc.) have one atom each in their molecules such as He, Ne, Ar and Kr. So, the atomicity of noble gases is 1.
Example. Hydrogen (H2), nitrogen (N2), oxygen (O2), chlorine (Cl2), bromine (Br2), and iodine (I2), all have 2 atoms each in their molecules. So, the atomicity of hydrogen, nitrogen, oxygen, chlorine, bromine and iodine is 2 each.
Atomicity of some common elements.
Type of Element Name Symbol Atomicity
Non-metal Helium He Monoatomic
  Argon Ar Monoatomic
  Neon Ne Monoatomic
  Hydrogen H2 Diatomic
  Chlorine Cl2 Diatomic
  Nitrogen N2 Diatomic
  Oxygen O2 Diatomic
  Phosphorus P4 Tetratomic
  Sulphur S8 Polyatomic
Metals Sodium Na Monoatomic
  Iron Fe Monoatomic
  Aluminium Al Monoatomic
  Copper Cu Monoatomic

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