Postulates of john Dalton’s atomic theory

John Dalton’s atomic theory given the primary basis of chemistry. But present-day electrons holds the key to the chemical structure. Thus for better understanding the atomic properties of an element we need to know about their electronic configuration. Dalton’s atomic theory based on the following assumption

  1. Matter made of atoms is indivisible and indestructible.
  2. All the atoms of a given element are identical mass and properties.
  3. Compounds formed by the combination of two or more same or different kinds of atoms.
  4. A chemical reaction is a rearrangement of atoms.

Limitations of Dalton’s atomic theory

The journey from John Dalton’s atomic theory to the modern structure of an atom is a long and arduous one.

Thus he considered the atom was the smallest building block of matter and nothing smaller exist. But as we turn this century many valuable pieces of information given about atomic structure.

It clearly indicated that Dalton’s atomic theory did not grant to its position. Today an atom composed further subatomic particles like electron, proton, and neutron.

Rutherford atomic structure

Thus Rutherford has remarked that it is not in the nature of things for any one man to make the sudden violet discovery.

Science goes step by step and every man depends on the work of his predecessor. Thus this century Rutherford has given valuable information about the structure of an atom.

Rutherford alpha particle scattering experiment provides the two most valuable information about the structure of an atom.

  1. All the positive charge and the almost entire mass of an atom concentrated in a very small central core. He named this part as a nucleus of an atom.
  2. The nucleus of an atom surrounded by negatively charged particles carrying negligible mass, called electrons arranged in a definite order.

John Dalton's atomic theory

Bohr model electromagnetic radiation

Rutherford’s model was not in conformity with the classical model of the electromagnetic spectrum. The moving charged particle will continuously emit radiation.

Thus it continuously loses kinetic energy or emission spectrum and eventually hits the nucleus of an atom. To resolve this Niels Bhors proposed an atomic structure based on the following postulates.

  1. An atom possesses several stable circular orbits in which an electron can stay. So long as an electron can stay in a particular orbit there is no emission or absorption of energy.
  2. An electron can jump from one hydrogen energy level to another higher level on the absorption of energy and one level to another lower orbit with the emission of energy.
  3. The angular momentum of an electron moving in an orbit or energy level is an integral multiple of h/2π.

\therefore mvr=\frac{nh}{2\pi }

Parts of an atom

Chemical Bond