Study Materials

NCERT Solutions for Class 9th Science


Page 1 of 5

Chapter 4. Structure of The Atom

Chapter Review




Chapter Review:

  • Atoms and molecules are the fundamental building blocks of matter.
  • The existence of different kinds of matter is due to different atoms constituting them.
  • The electron is identified by J.J. Thomson.
  • Electrons, protons and nutrons are sub atomic particles. 
  • E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays.
  • Electrons carry Negative (-) charge. 
  • Protons carry Positive (+) charge.
  • Nutrons carry No any charge.
  • J.J. Thomson was the first one to propose a model for the structure of an atom.
  • The electrons revolve around the nucleus in well-defined orbits.
  • The size of the nucleus is very small as compared to the size of the atom.
  • Protons and Nutrons are situated in nucleus of atom.
  • Orbits or shells of an atom are represented by the letters K, L, M, and N.
  • J. Chadwick discovered another sub atomic particles named nutron. 
  • The number of protons present in nucleus of an atom is known as atomic number. i.e Atomic mass = total number of protons in nucleus os an atom. 
  • The sum of the masses of protons and neutrons present in the nucleus is known as Atomic Mass.
  • The maximum number of electrons present in a shell is given by the formulla 2n2.
  • The electrons present in the outermost shell of an atom are known as the valence electrons.
  • The number of valence electrons present in outer most shell of an atom is known as its valency.
  • The mass number is defined as the sum of the total number of protons and
    neutrons present in the nucleus of an atom.
  • The atoms of the same element, having the same atomic number but differentmass numbers is called its isotopes.
  • Protium, Deuterium and tritium are isotopes of Hydrogen.
  • Many elements consist of a mixture of isotopes. Each isotope of an element is a pure substance.
  • The chemical properties of isotopes are similar but their physical properties are different.
  • An isotope of uranium is used as a fuel in nuclear reactors.
  • An isotope of cobalt is used in the treatment of cancer.
  • An isotope of iodine is used in the treatment of goitre.
  • Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.


Page 1 of 5


Chapter Contents: