Chemistry - Structure of the Atom



  • By 1900, it was discovered that the atom was not a simple, indivisible particle, but rather it contains sub-atomic particles.

  • J.J. Thomson discovered the sub-atomic particle namely ‘electron.’

J.J. Thomson
  • J.J. Thomson was the first person who proposed a model for the structure of an atom.

  • In 1886, E. Goldstein discovered the presence of new radiations in a gas discharge and named them canal rays.

  • Another positively charged sub-atomic particle was discovered with experiments of canal rays and named it proton.

Thomson’s Model of Atom

  • Thomson proposed that an atom consists of a positively charged sphere and the electrons (negative charge) are embedded in it (as shown in the image given below).

Thomson Atom Model
  • Further, Thomson said that the negative and positive charges are equal in magnitude. Thus, the atom as a whole is electrically neutral.

Rutherford’s Model of Atom

  • E. Rutherford is popular as the ‘Father’ of nuclear physics.

E. Rutherford
  • Rutherford is largely known for his work on radioactivity and the discovery of the nucleus of an atom with the gold foil experiment (as shown in the image given below.

  • Rutherford said that in an atom, there is a positively charged center known as the nucleus.

  • Rutherford said that nearly all the mass of an atom exists in in the nucleus.

  • According to Rutherford, the electrons revolve around the nucleus in well-defined orbits.

Bohr’s Model of Atom

  • Neils Bohr further extended Rutherford’s model and improved his drawbacks.

Neils Bohr
  • According to Bohr, only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

  • Bohr said that electrons do not radiate energy while revolving in discrete orbits.

  • Bohr named orbits or shells as energy levels (as shown in the image given below).

Energy Levels
  • Bohr represented these orbits or shells are by the letters K, L, M, N,… or the numbers, n = 1,2,3,4,….


  • In 1932, J. Chadwick discovered a new sub-atomic particle i.e. neutron.

  • Neutron has no charge and a mass nearly equal to that of a proton.

  • Neutrons are present in the nucleus of all atoms, except hydrogen.

Electrons Distributed in Different Orbits (Shells)

  • The maximum number of electrons that can be present in a shell is given by the formula 2n2.

  • ‘n’ is the orbit number or energy level index, i.e. 1, 2, 3,….

  • According to the given formula −

    • First orbit i.e. K-shell will be = 2 × 12 = 2

    • Second orbit i.e. L-shell will be = 2 × 22 = 8

    • Third orbit i.e. M-shell will be = 2 × 32 = 18

    • Fourth orbit i.e. N-shell will be = 2 × 42 = 32

  • Likewise, the maximum number of electrons that can be accommodated in the outermost orbit is 8.

  • Electrons are not filled in a given shell, unless the inner shells are filled. It means, the shells are filled in a step-wise manner; starting from inner shell to outer shell.


  • The electrons, those are present in the outermost shell of an atom, are known as the valence electrons.

  • According to Bohr-Bury model, the outermost shell of an atom can have a maximum of 8 electrons.

Atomic Number

  • The total number of protons, present in the nucleus of an atom, is known as atomic number.

  • The number of protons of an atom determines the atomic number.

  • Atomic number is denoted by ‘Z’.

  • Protons and neutrons collectively are known as nucleons.

Mass Number

  • The sum of the total number of protons and neutrons, present in the nucleus of an atom, is known as mass number.


  • The atoms of the same element, having the same atomic number but different mass numbers, is known as isotopes. E.g. Hydrogen atom has three isotopes namely protium, deuterium, and tritium.

  • The chemical properties of isotopes of an atom are similar but their physical properties are different.


  • Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars. E.g. calcium’s atomic number is 20and argon’s atomic number is 18; further, the number of electrons in these atoms is different, but the mass number of both these elements is 40.