Science: Atomic Structure: Thomson and Drawbacks of Rutherford՚s Model (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • Electrons and protons are two charged particles which constitute all matter.
  • Cathode ray tube A cathode ray tube consists of two metal electrodes in a partially evacuated glass tube. The negatively charged electrode is called cathode while the positively charged electrode is called anode. These electrodes are connected to a high voltage source.
  • Electron is the negatively charged particles emitted from the cathode in the cathode ray tube.

Thomson Model

Thomson concluded that there must be an equal amount of positive charge present in an atom. basis he proposed a model for the structure of atom:-

  • Atoms can be considered as a large sphere of uniform positive charge with a number of small negatively charged electrons scattered throughout it.
  • This model was called as plum pudding model.
  • The electrons represent the plums in the pudding made of negative charge.
Thomson Model

Rutherford՚s Model

  • Rutherford overturned Thomson՚s model in 1911 with his well-known gold foil experiment, in which he demonstrated that the atom has a tiny, high- mass nucleus.
  • In his experiment, Rutherford observed that many alpha particles were deflected at small angles while others were reflected back to the alpha source.
  • This highly concentrated, positively charged region is named the “nucleus” of the atom
  • alpha particle: A positively charged nucleus of a helium-4 atom (consisting of two protons and two neutrons) , emitted as a consequence of radioactivity; α-particle.
Rutherford՚s Model

Drawbacks of Rutherford՚s Model

  • It is possible to have infinite number of orbits. In practice it is not the case.
  • The moving electron must continuously lose energy and fall into the nucleus. Actually, it is not the case.

Bohr՚s Atomic Model

  • Postulate 1: The electrons move in definite circular paths of fixed energy around a central nucleus; just like our solar system in which different planets revolve around the Sun in definite trajectory.
  • Postulate 2: The electron can change its shells or energy level by absorbing or releasing energy. An electron at a lower state of energy Ei can go to a final higher state of energy Ef by absorbing a single photon of energy given by:

E = hν = EfEi


  • Neutrons are present in the nucleus of all atoms, except hydrogen.
  • Subatomic particle present in the nucleus which is neutral but have mass.
  • A neutron is represented as ‘n’ and is found to have a mass slightly higher than that of a proton.

Characteristics of the Fundamental Subatomic Particles

Characteristics of the Fundamental Subatomic Particles
ParticleSymbolMass (in kg)Actual Charge (in Coulombs)Relative charge
Electrone9.109 389 × 10 – 311.602 177 × 10 – 19– 1
Protonp1.672 623 × 10 – 271.602 177 × 10 – 191
Neutronn1.674 928 × 10 – 2700

Atomic Number

  • the numbers of protons present in the nucleus of the element. In other words, different elements differ in terms of their atomic number.
  • Atomic number = number of protons = number of electrons

Mass Number

  • The number of nucleons in the nucleus of an atom is called its mass number. The presence of two heavy particles namely protons and neutrons in the nucleus. These particles are called nucleons.
  • Mass number (A) = number of protons (Z) + number of neutrons (n)

The Electron Distribution in Orbitals

  • The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury and is known as electronic configuration.
  • An orbit can have a maximum of electrons, where ‘n’ is the orbit number therefore, maximum number of electrons in different shells are
The Electron Distribution in Orbitals
  • Orbit are filled from inside to outside. First, n = 1 shell is filled, then n = 2, and so on.
  • The outermost occupied shell of an atom can have a maximum of 8 electrons even if it can accommodate more electrons.

For example – Sulphur - It has 16 electrons. Therefore, the electronic configuration is 2,8, 6.

n = 1 or K shell: 2 electrons n = 2 or L shell: 8 electrons n = 3 or M shell: 6 electrons


Valency of an element is the number of electrons that its atoms should give away or take to attain stable electronic configuration i.e.. the atom should accommodate 8 electrons in the outermost shells or valence shells except the K shell which can accommodate 2 electrons to the maximum. The electrons present in the outermost orbit of an atom are known as valence electrons.

Three ways to obtain stable electronic configuration:

  • By losing electrons
  • By Gaining electrons
  • By sharing electrons

For example,

Hydrogen molecule: Hydrogen has only one electron in its outermost orbit thus requires one more electron to complete its outermost orbit (K shell) . For this, hydrogen atom shares one electron with another Hydrogen atom and forms H2 (Hydrogen molecule) .

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