NCERT Class 9 Science Solution, Notes, Atoms and molecules

Atoms and molecules

  1. One atomic mass unit is a mass unit equal to exactly one twelfth(1/12th) the mass of one atom of  carbon-12

10.Relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom is most elements are not able to exist independently. Atom form molecules and ions

  1. A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance
  2. Molecules of element are formed by the atoms of the same type
  3. Atoms of different elements join together in definite proportions to form molecules of compounds

Based upon atomicity we can classify molecules as follows.

  • Monoatomic molecules: Noble gases like helium, neon, argon, etc., exist in atomic forms, i.e., they are monoatomic. For example, helium, neon and argon exist as He Ne and Ar respectively.
  • Diatomic molecules: These molecules consist of two atom. For example, hydrogen (H2 ) oxygen (O2), nitrogen (N2), chlorine (Cl2), carbon monoxide (CO), hydrogen chloride (HCl) and sodium chloride (NaCl).
  • Triatomic molecules: These molecules consist of three atom. For example. ozone (O3) is a triatomic molecule. Similarly, carbon dioxide (CO2), nitrogen dioxide (NO2) and hydrogen sulphide (H2S) are examples of triatomic molecules.
  • Tetraatomic molecules: These molecules consist of four atoms. For example, P4 is a tetraatomic molecule of phosphorus.
  • Polyatomic molecules: Any molecule containing more than four atoms is called a polyatomic molecule. For example, sulphur (S8), methane (CH4), nitric acid (HNO3), ethanol (C2H5OH).

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