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NCERT Solutions for Class 9th Science


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Chapter 3. Atoms and Molecules

Chapter Review




Chapter Review:

  • The smallest tiny particles of matter which cann't be divided further is called atom (Parmanu).
  • The law of conservation of mass and the law of constant proportions these two laws of chemical combination were established by Lavoisier and Joseph L. Proust.
  • Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
  • The law of constant proportions which is also known as the law of definite proportions.
  • In a chemical substance the elements are always present in definite proportions by mass, this law is called the law of constant proportions.
  • Dalton's theory was based on the laws of chemical combination.
  • According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.
  • Dalton's second postulate states "Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction".
  • Dalton's third postulate states "atoms of a given element are identical in mass and chemical properties".
  • Dalton's third postulate states "atoms of different elements have different masses and chemical properties".
  • Atomic radius is measured in nanometres.
  • IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.
  • The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase).
  • Each element has a name and a unique chemical symbol.
  • The atomic mass unit is writen as ‘u’ – unified mass.
  • Carbon-12 isotope was chosen as the standard reference for measuring atomic masses.
  • One atomic mass unit is a mass unit equal to exactly onetwelfth
    (1/12th) the mass of one atom of carbon-12.
  • Atoms form molecules and ions.
  • These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.
  • 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.
  • Atoms of the same element or of different elements can join together to form molecules.
  • A molecule of oxygen consists of two atoms of oxygen and hence it
    is known as a diatomic molecule, O2.
  • 3 atoms of oxygen unite into a molecule these form ozone e.i O3.
  • The number of atoms constituting a molecule is known as its atomicity.
  • Atoms of different elements join together in definite proportions to form molecules of compounds.
  • The charged atoms are known as ions.
  • An ion is a charged particle and can be negatively or positively
  • A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’.
  • Ions may consist of a single charged atom or a group of atoms that have a net charge on them.
  • A group of atoms carrying a charge is known as a polyatomic ion.
  • The chemical formula of a compound is a symbolic representation of its composition.
  • The combining power (or capacity) of an element is known as its valency, in other hand the number of valnce electron is known as valancy of an atom. 
  • The quantity of a substance can be characterised by its mass or the number of molecules.
  • One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.
  • The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 × 1023 ,  This number is called the Avogadro Constant or Avogadro Number.
  • The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams.
  • Mass of 1 mole of a substance is called its molar mass.
  • The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12.



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