CBSE – 9th Standard Science

Class 9 Chapter 4 – Structure of the Atom


  • An atom consists of a positively charged sphere and the electrons are embedded in it.



  • The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
  • Its observation could be called as aplum pudding model or a watermelon

Rutherford’s Model Of an Atom

A stream of high energy alpha particles from a radioactive source was directed at thin foil of gold metal. The thin foil had a circular fluorescent zinc sulphide screen around it. Whenever alpha particles struck the screen , a tiny flash of light was produced at a point


  • Most of the space inside the atom is empty because most of the particles passed through the gold foil without getting deflected.
  • Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.



  • Neil’s Bohr put forward the following postulates about the model of an atom:
  • Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
  • While revolving in discrete orbits the electrons do not radiate energy.
  • These orbits or shells are called energy levels.
  • The sub-atomic particle equal in magnitude but opposite in sign to that of the electron is called
  • Neutrons are present in the nucleus of all atoms. A neutron is represented as ‘n’.


  • Valency is the combining capacity of an atom.
  • The electrons present in the outermost shell of an atom are known as the valence electrons.
  • Atomic Number is the number of protons of an atom, which determines its atomic number. It is denoted by Z’.
  • The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom


  • Isotopes are the number of atoms of some elements, which have the same atomic number but different mass n
  • Atoms of different elements with different atomic numbers, which have the same mass number, are known as

Elements are defined by the number of protons they possess