Atomic Theory | Dalton’s, Rutherford Atomic Model, Bohr’s model, Quantum Mechanical Model

Atomic Theory | Dalton’s, Rutherford Atomic Model, Bohr’s model, Quantum Mechanical Model We knows that atoms and molecules are the fundamental building blocks of matter. In this chapter we learn that what makes the atom of one element different from the atom of another element? Are atoms really indivisible, as proposed by Dalton, or are there smaller constituents inside the atom? We also read about Dalton’s atomic theory, Rutherford model, Bohr’s model and quantum mechanical model. Mass of electron amu.

Dalton’s atomic theory, Rutherford atomic model, quantum mechanical model, Bohr’s model

Dalton’s Atomic Theory

John Dalton presented this theory in 1808 based on his experiments. Following are the main points of this theory –

  • An element is made up of many microscopic particles, it is called an atom.
  • The atom is inseparable, meaning the atom cannot be divided into any more subtle particles, Which are neither produced nor destroyed in chemical reactions.
  • Atoms of the same element are similar to each other in size, mass and all other properties, but atoms of different elements are different. The atoms of different elements are compounded by adding them in proportion to the smallest whole number.
  • The relative numbers and types of any compound atoms are fixed.
  • Chemical reactions are a rearrangement of atoms.
  • A compound is formed by the addition of an atom of two or more elements. The ratio of the number of atoms of its constituent elements in a compound is fixed.

Difference between Atoms and Molecules :

After Dalton, in 1811, an Italian chemist Amedeo Avogadro explained the difference between the atom and the molecule, saying that there are two types of microscopic particles of any substance.
1. Atom
2. Molecule

1. Atom – The atom is the basic unit that makes up matter. An atom is the smallest particle of a substance that participates in its chemical reaction, it is called an atom.
Example : H (Hydrogen), Fe (Iron), Ca (Calcium), O (oxygen) etc.

2. Molecule – Two or more atoms that form molecules by a certain ratio. That is the smallest particle of an element or compound, which has an independent existence possible in nature, is called an molecules.
There are two types of molecules :
(a) Homo atomic Molecules
(b) Hetero atomic Molecules

(a) Homo atomic Molecules – If a molecule has only one type of atom, then it is called homo atomic Molecules.
Example : N2 (Nitrogen Molecule), O2 (oxygen Molecule), Cl2 (Chlorine Molecule) etc.

(b) Hetero atomic Molecules – If a molecule contains two or more types of atoms, then it is called a hetero atomic Molecules.
Example : H2O (water), Co2 (Carbon dioxide), H2O2 (Hydro peroxide) etc.

Difference Between Atom and Molecule by Table :

S. No. Atom Molecule
1. An atom is the smallest particle of an element that participates in its chemical reaction. A molecule is the smallest particle of an element or compound that can remain in an independent state.
2. These are formed up of electrons, protons and neutrons. They are formed up of atoms.
3. Often atoms cannot remain in an independent state. Molecules can remain independent in nature.
4.  In chemical reactions, atoms participate in almost indivisible forms and their form is not clear. In chemical reactions, molecules are often divided into atoms, which form other types of molecules by chemical combination.

Fundamental Particle of Atom : Many scientists have proved that an atom is made up of some fundamental particles like electrons, protons and neutrons. Here we will go into detail about these fundamental particles.


We are know that the atom was invisible particle but contained at least one sub-atomic particle which is electron. J. J. Thomson electricized at low pressure in gases. Approximately 10–3 mm in a semi-long glass immersion tube. Some gas was taken at mercury pressure and two electrodes were placed at both ends of the drain. Using a high gap between 1000 volts to 30000 volts between the two electrodes, it was found that some rays from the cathode fall on the front wall in the glass tube. Due to which the glow of green light is produced, these rays are called J. J. Thomson called cathode rays. J. J. Thomson, by finding the ratio of the negative charge and their volume present on the particles of the cathode rays, stated that this ratio does not depend on the substance of the electrodes of the immersion tube or the nature of the gas present in the tube but remains constant in every case.

 Immersion tube

  • Electron discovered by J..J. Thomson in 1897.
  • These particles named by Stony.
  • Symbol of electron is e
  • The Mass of Electron is 9.1×10-28 grams OR 9.1×10-31 kg.
  • Electron Mass Compared to Hydrogen is 1/1840.
  • The charge on the electron was discovered by Millikan.
  • Charge on electron in C.G.S. unit is 4.8×10-10 esu.
  • Charge on electron in M.K.S. unit is 1.6×10-19 Coulomb.
    (1 esu = 3.33×10-10 Coulomb)
  • Mass of one mole electron 9.1×10-28 OR 6.023×10-23
  • Number of mass in 1 mole electron = 5.49×10-4 grams/mole.
  • Mass of electron in A.M.U. is 0.000527 amu.


Even before the electron was identified by J.J. Thomson, E. Goldstein in 1886 discovered the presence of new radiation in a gas discharge and called canal rays. These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle. This sub-atomic particle had a charge equal in magnitude but opposite in sign to that of the electron. Its mass was approximately 2000 time as that of the electron. It was given the name of proton.


  • These are positively charged particles present in the anode rays, while hydrogen (H) gas is filled in the electric immersion tube.
  • Symbol of proton is p+.
  • Charge on proton is +1 ive.
  • Mass of Proton is 1.672×10-24 grams.
  • Mass of proton in A.M.U. is 1.0072 a.m.u.
  • The mass of a proton is the same as the mass of Hydrogen.
  • Charge on Proton is C.G.S + 4.8×10-10 esu.
    = + 1.6×10-19 Coulomb.
  • The number of protons present in the nucleus is called the atomic number. It was discovered by Moseley. This is the physical property of an atom. According to Moseley, the frequency of radiation from an electron bombarding a metal is proportional to the nuclear charge.
    ν ∝ (z – b)
    ν = a (z – b)
    here a,b is constant
    z = here is a nuclear charge or atomic number.
    ν = Frequency of radiation generated.


A neutron is a changeless fundamental particle, which is found with a proton in the nucleus of an atom. A neutron is a subatomic particle found in the nucleus of an atom of all types of matter. Protons and neutrons together form the nucleus of an atom.

  • Neutron is discovered by James Chadwick in 1932.
  • Mass of Neutron particles is 1.675×10-24.
  • Mass of neutron in A.M.U is 1.0086 amu.
  • The number of electrons and protons in neutral atoms is the same as the atomic number. On this basis, the mass of protons and electrons present in an atom of an element can be determined.
  • The actual mass of the elements is about twice the mass of the protons and electrons present in them.
  • Rutherford said on this basis that the third type of particles also exists. Whose weight is approximately equal to the atomic weight of hydrogen.

Thomson’s Atomic Model

The first J.J. Thomson presented a model to determine the arrangement of electrons and protons into atoms, which is called Thomson’s atomic model. Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge, were like currants in a spherical Christmas pudding. We can also think of a watermelon, the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon. 

Thomson proposed that –

  • An atom consists of a positively charged sphere and the electrons are embedded in it.
  • The negative and positive charge are equal in magnitude. So, the atom as a whole is electricity neutral.

Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model, as we will see below.

Rutherford Atomic Model

Rutherford want to know that how the electrons are arranged within an atom. So he designed an experiment for this, which is called solar model or the planetary model.
The following are the main stages of this experiment –

  • He fast moving alpha (α) – particles were made to fall on a thin gold foil. He selected a gold foil because he wanted as thin a layer as possible. This gold foil was about 1000 atoms thick. α particles are two positively charged nuclei of helium) and he observed that –
  • Most of α-particle passed straight through the gold foil.
  • Some α-particle change their path.
  • one or two particles out of 2000 return to their path again.

Basis on the above project the concept of Rutherford’s atomic model is as follows – 

  • At the center of the atom is the nucleus, the very subtle.
  • The atom is hollow.
  • In this hollow atom, the negatively charged electrons revolve around the nucleus in a circular path.
  • The entire weight of the atom is centered in the nucleus and the size of the nucleus is 10-15 m.
  • The volume of the nucleus is 1015 times smaller than the volume of the atom.

Drawbacks of Rutherford’s model of the atom –

  • Energy must be emitted (according to Maxwell) by the electron moving around the nucleus in the form of electromagnetic radiation, so due to the decrease in energy, the electron should slowly fall near the nucleus but eventually fall into the nucleus. This does not happen.
  • Rutherford could not explain the linear spectrum of any hydrogen atom, that is, the reason for the origin of spectral lines.

Max Planck’s Quantum Theory

“The energy of a photon is proportional to its frequency.”
E ∝ ν
E = hν
here h = 6.625 × 10-27 erg-atom
ν = Photon’s frequency
E = Photon’s energy

  • According to Planck, the energy received or emitted by any element is in the form of ears, which are called quanta or photons.

Bohr’s model

Bohr’s presented the following postulate to modified the Rutherford model. According to this –

  • At the center of the atom is found the nucleus, the most positively charged part, in which the entire portion of the atom is centered.
  • Electrons revolve around this nucleus in a circular path, which is called the energy level. These are K, L, M, N, O etc. respectively.
  • They rotate in the same circular chambers that have an absolute coefficient of angular momentum h/2n.
    h = Planck constant
  • Electrons rotate towards the nucleus by being balanced by the coulombic attraction force and the centrifugal force opposite the nucleus.
  • Electrons revolve at a certain velocity in the energy level.

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